DIGESTION OF CARBOHYDRATES
The principal sites of dietary carbohydrate digestion are the mouth
and intestinal lumen . This digestion is rapid and is generally completed
by the time the stomach contents reach the junction of the duodenum and
jejunum . There is little monosaccharides present in deits of mi8xed
animal and plant origin ; thus , the enzymes needed for degradation of
most dietary carbohydrates are primarily disaccharides and
endoglycoidases ( for breaking down oligosaccharides and
polysaccharides ) . Hydrolysis of glycosidases that degrade carbohydrates
into their reducing sugar components .
These enzymes are usually specific for the structure and configuration of
the glycosyl to be removed as well as the type of bond to be broken .
A. Digestion of carbohydrates begins in the mouth
The major dietary polysaccharides can be of animal ( glycogen ) or
plant origin ( starch , composed of amylose and amylopectin ) .
During mastication , salivary α- amylase ( ptyalin ) acts briefly on dietary
starch in random manner , breking some α- ( 1-4 ) bonds .
[Note : carbohydrates are the only dietary component for which
degradation begins in the mouth ]
1. There are both α- and β- ( 1-4 ) – endoglucosidases in nature , but
humans do not produced and secrete the latter in digestive juices .
Therefore , they are unable to digest cellulose , a carbohydrate of
plant origin containing β- ( 1-4 ) glycosidic bonds between glucose
residues
2. Because amylopectin and glycogen also contain α- ( 1-6 ) bonds ,
the digest resulting from the action of α- amyloase contains a
mixture of smaller , branched oligosaccharides molecules .
3. Carbohydrates digestion halts temporarily in the stomach , because the
high acidity inactivates the salivary a–amylase .
B- Further digestion of carbohydrates by pancreatic enzymes
Occurs in the small contents . When the acidic stomach contents
reach the small intestine , they are neutralized by bicarbonate secreted by
the pancreas , and pancreatic α-amylase continues the process of starch
digestion .
C. Final carbohydrate digestion by enzymes by the intestinal mucosal
cells .
The final digestion processes occur at the mucosal lining of the
upper jejunum , declining as they proceed down the small intestine , and
include the action of several disaccharides and oligosaccharides . These
1
enzymes are secreted through , and remain associated with the luminal
side of the brush border membranes of the intestinal mucosal cells .
Absorption of monosaccharides by intestinal mucosal cells
The duodenum and upper jejunm absorb the bulk of the dietary
sugars .
Insulin is not required for the p ** of glucose by intestinal cells .
However different sugars have different mechanisms of absorption .
Two mechanisms are responsible for the absorption of
monosaccharides :
Active transport against concentration gradient , and passive diffusion .
1F Active transport : galactose and glucose are transported into the
mucosal cells by an active , **** requiring that involves a specific
transport protein ( carrier ) and requires a concurrent uptake of sodium
ions .
The carrier binds glucose and sodium at separate sites and
transports them through the membrane of the intestinal cell . Both glucose
and sodium are released into the cytosol , allowing the carrier to take
more “cargo” . In this way , sodium is transported down its concentration
gradient and at the same time causes the carrier to transport glucose
against its concentration gradient .
The free energy required for the active transport is obtained from
the hydrolysis of ATP linked to a sodium pump that expels sodium from
the cell in exchange for k+
2] Simple diffusion : fructose and glucose also are transported by
facilitated glucose transporters by diffusion according to concentration
gradient .
Abnormal degradation of disaccharides
The overall process of carbohydrate digestion and absorption is so
efficient in healthy individuals that ordinarily all digestible dietary
carbohydrate is absorbed by the time the ingested material reaches the
lower jejunum .
However , because predominantly monosaccharides are absorbed ,
any defect in a specific disaccharides activity of the intestinal mucosa .
As a consequence of the presence of this osmotically active material ,
water is drawn from the mucosa into the large intestine , causing osmotic
diarrhea.
This is reforced by the bacterial fermentation of the remaining
carbohydrate of two- and three-carbon compounds ( which are also
osmotically active ) plus large volumes of Co2 and H2 gas , causing
flatulence .
2
1. Lactase intolerance :
More than half of the world’s adults are lactose intolerant .
This is particularly manifested in certain races : adult blacks and Asians .
The mechanism by which the enzyme is lost is not clear , but is
determined genetically and represent a reduction in the amount of enzyme
protein rather than a modified enzyme protein rather than a modified
inactive enzyme . Treatment for this disorder is simply to remove lactose
from the diet .
2. Isomaltose-sucrase deficiency :
This enzyme deficiency results in **** of ingested sucrose . This
disorder is found in about 10% of Greenland’s Eskiros, whereas 2% of
North . treatment is to **** dietary sucrose .
Diagnosis : Identification of the specific enzyme deficiencies can be
obtained by performing oral tolerance tests with the individual
disaccharides . Measurment of hydrogen gas in the breath is a suitable
test for determining the amount of gested carbohydrate not absorbed by
body but rather metabolized by the intestinal flora .
Out line of Glucose Metabolism
Glucose is the main carbohydrate present in blood .
The metabolism of glucose involves many metabolic pathways :
1- Oxidative pathways :
After glucose is transported into cells it is phosphorylated to glucose
6-Phospate ( G6-p ) which is metabolized to pyruvate and lactate in all
mammalian cells by the pathway of glycolysis . Glucose is a unique
substrate because glycolysis can occur in the absence of oxygen
(anaerobic) , when the end product is lactate only . However , tissues that
can utilize oxygen ( aerobic ) ar able to metabolize pyruvate to acetylCoA, which can enter the citric acid cycle for complete oxidation to CO2
and H2O , with liberation of much free energy as ATP in the process of
oxidative phosphorylation . Thus , glucose is a major fuel of many
tissues.
2- The pentose phosphate pathway :
G6-P is also oxidized in the pentose phosphate pathway . Th is a
source of reducing equivalents ( 2H ) used for biosynthesis e . g . of fatty
acids , and it is also the source of ribose , which is important for
nucleotide and nucleic acid formation .
3-Storage as glycogen :
3
G6-P is converted to its storage polymer , glycogen that occurs
particularly in skeletal muscle and liver .
4- Fat biosynthesis :
Triose phospate ( from glycolysis ) gives rise to the glycerol moiety of
Acyl glycerola ( fat ) .
3- Pyruvate and intermediates of the citric acid cycle provide the
carbon skeletons for the synthesis of amino acids , and acetyl –
COA in the building block for long-chain fatty acids and
cholesterol , the precursor of all steroids synthesized in the body .
4- Gluconeogenesis is the process that produces glucose from non
carbohydrate precursors , e . g. . lactate , amino acids , and glycerol
.
5- Fructose and galactose in the tissues are converted to intermediates
glucose metabolism , thus the fate of these sugars parallels that of
glucose .
Glycolysis ( Embeden-Meyerhof pathway )
And the Oxidation of pyruvate
Over view
The glycolytic pathway is employed by all tissues for the break
down of glucose to provide energy ( in the form of ATP ) and
intermediates for other metabolic pathways . Glycolysis is at thehub of
carbohydrate metabolism because virtually all sugars ( whether arising
from the diet or from catabolic reactions in the body ) ultimately can be
converted to glucose . Pyruvate is the end product of glycolysis in cells
with mitochondria and an adequate supply of oxygen . This series of ten
reactions is called aerobic glycolysis because oxygen is required to
reoxidize the NADH formed during the oxidation of glyceraldehyde 3phospate . Aerobic glycolysis sets the stage for the oxidative
decarboxylation of pyruvate to acetyl CoA , a major fuel of the citric acid
cycle .
Alternatively , glucose can be converted to pyruvate , which is
reduced by NADH to form lactate . This conversion of glucose to lactate
is called anaerobic glycolysis because there is ni net formation of NADH
, and therefore , it can occur in the absence of oxygen . Anaerobic
glycolysis allows the continued production of ATP in tissues that lack
mitochondria ( e . g . red blood cells ) or in cells deprived of sufficient
oxygen ( e . g . skeletal muscles ) .
4
TRANSPORT OF GLUCOSE INTO CELLS
The transport of glucose into cells occur by one of two mechanisms :
A. Facilitated transport :
This is mediated by a family of at least five glucose transporters
(GLUT) in the cell membrane , designated GLUT – 1 to GLUT – 5
Extracellular glucose binds to the transport , which then alters its
conformation , discharging glucose inside the cell . In facilitated diffusion
glucose movement is “with” a concentration gradient , that is , from a
high glucose concentration outside the cell to a lower concentration
within the cell .
B. Cotransport
The second mechanism for glucose entry into cells is contransport ,
an energy-requiring process that transports glucose “against” a
concentration gradient , that is , from low glucose concentration outside
the cell to higher concentrations within the cell . Cotransport is a carriermediated process in which the movement of glucose is coupled to the
concentration gradient of Na+ , which is transported into the cell at the
same time . This type of transport occurs in the epithelial cells of the
intestine , and renal tubules .
REACTIONS OF GLYCOLYSIS
All the glycolytic reactions occur in the cytosol .
The conversion of glucose to pyruvate ( under aerobic is transferred to
lactate .
( Stage IV ) :
1- Stage I : This involves the first three reactions of glycolysis
correspond to an energy investment phase where phosporylated
forms of glucose and fructose are synthesized at the expense of
ATP , which is converted to ADP .
2- Stage II : This involves the splitting of fructose 1,6 bisphosphate in
2 triose phosphates .
3- Stage III : This involves the subsequent reactions of glycolysis
that constitute an energy generation phase where a net of two
molecules of ATP are formed per glucose molecule metabolized .
Two molecules of NADH are formed when pyruvate is produced (
aerobic ) .
4- Stage IV : Under anaerobic conditions , NADH is reconverted to
NAD+ , and lactate is the end product .
Stage I
A] Phosphorylation of glucose :
5
The irreversible phosphorylation reaction effectively traps glucose
as glucose 5-phosphate , which does not diffuse out of the cell .
Phosphorylated sugar molecules do not readily penetrate cell membranes
because there are no specific carriers for these compounds . This commits
glucose to further metabolism in the celol . Mammals have several
isoenzymes of hexokinase that catalyze the phosphorylation of glucose to
glucose 6-phosphate .
1. Hexokinase
 It is present in most tissues .
 It has a broad specificity and is able to phosphorylate several
hexoses in addition to glucose .
 It is inhibited by the reaction product , glucose 6-phosphate , that
accumulates when further metabolism of this hexose phosphate is
reduced , for example , by a high ATP / ADP ratio . Hexokinase
has a low Km ( and therefore a high affinity ) for glucose . This
permits the efficient phosphorylation and subsequent metabolism
of glucose even when tissue concentrations of glucose are low .
 Hexokinase , however , has a low V max for glucose and therefore
cannot phosphorylate large quantities of glucose .
2. Glucokinase :
 It is present in liver ( and the *-cells of the pancreas ) , it is the
predominant enzyme for the phosphorylation of glucose .
 Glucokinase differs from hexokinase in :
a- Requiring a much higher glucose concentration for half saturation .
Thus , glucokinase functions only when the intracellular
concentration of glucose * the hepatocyte is elevated , such as
during the brief period following consumption of a carbohydraterich meal , when high level of glucose are delivered to the liver via
the portal vein ( blood glucose equilibrates rapidly across the
membrane of the hepatocyte ) .
b- Glucokinase has high ** allowing the liver to effectively remove
this flood of glucose from the portal blood . This prevents large
amounts of glucose from entering the systemic circulation
following a carbohydrate rich meal , and thus minimizes
hyperglycemia during the absorptive period .
c- Glucokinase levels are increased by carbohydrate-rich diets and bu
insulin .
d- Glucokinase is not inhibited by glucose 6-phosphate .
6
Glucokinase
1- Only in liver and pancreatic
islets .
2- Acts on glucose .
3- Low affinity to glucose ( Km ↑ )
and high V max .
4- Not inhibited by by G-6-p .
5- Level depends on glucose conc.
Hexokinase
1- In all tissues except liver and
pancreatic islets .
2- Glucose , fructose and galactose.
3- High affinity to glucose ( Km ↓ )
and low V max .
4- Inhibited by G-6-p .
5- No change on fasting or high
CHO diet .
6- No change in diabetes .
7- Not induced by insulin .
6- Level decreased in diabetes .
7- Synthesis induced by insulin .
B. Isomerization of glucose 6-phosphate :
It is catalyzed by phosphoglucose isomerase . The reaction in readily
reversible and is not a rate-limiting or regulate step .
C- Phosphorylation of fructose 6-phosphate :
This irreversible phosphorylation reaction catalyzed by
phosphofructokinase I ( PEK –1 ) . PEK-1 reaction is the rate – limiting
step in glycolysis .
1- PEK-1 is inhibited allosterically by elevated levels of ATP , which
act as as “energy rich “ signal indicating an abundance of highenergy compounds .
2- Elevated levels of citrate ( starting of citric acid cycle ) also inhibit
PEK-I .
3- Conversely , PEK-1 is activated allosterically by high
concentration of AMP , which signal that the cell’s energy stores
are depleted .
4- Fructose 2 , 6 –bis-phosphate is the most potent activator of PEK
the reciprocal actions of fructose 2,6-bisphosphate on glycolysis
and gluconeogenesis ensure that both pathways are not fully active
at the same time .
Stage II
A- Cleqavage of fructose 1 , 6-biphosphate :
Aldolase A cleaves fructose 1,6-bisphosphate to dihydroxyacetone
phosphate and glyceraldehyde 3-phosphate . The reaction is reversible
and is not regulated .
B- Isomerization dihydroxyacetone phosphate
Triose phosphate isomerase interconverts dihydroxyacetone phosphate
and glyceraldehyde 3-phosphate . Dihydroxyacetone phosphate must be
isomerized to glyceraldehyde 3-phosphate for further metabolism in the
glycolic sequence . This isomerization results in the production of two
7
molecules of glyceraldehyde 3-phosphate from the cleavage products of
fructose 1,6-bisphosphate .
Stage III
[A] Oxidation of glyceraldehyde 3-phosphate
The conversion of two moles of glyceraldehyde 3-phosphate to
1,3-bisphosphoglycerate by glyceraldehyde 3-phosphate dehydrogenase
is the first oxidation – reduction reaction of glycolysis .
[ Note : Because there is only a limited amount of NAD+ in the cell , the
NADH formed must be reoxidized to NAD+ for glycolysis to continue ] .
Two major mechanisms for oxidizing NADH :
[1] The NAD-linked conversion of pyruvate to lactate ( anaerobic ) .
[2] Oxidation via the respiratory chain ( generating 2 x 3 ATP ) .
The oxidation of the aldehyde group of glyceraldehyde 3-phosphate to a
carboxyl group is coupled to the attachment of pi to the carboxyl group .
The high-energy phosphate group at carbon 1 of 1,3 bisphosphoglycerate
conserves much of the free energy produced by the oxidation of
glyceraldehyde 3-phosphate .
[B] i- Substrate-level phosphorylation : The high-energy phosphate
group in 1,3-bisphosphoglycerate ( two moles ) is used to synthesis 2
ATP from 2 ADP in a reaction catalyzed by phosphoglycerate kinase .
This reaction is reversible ( unlike other reactions of kinase ) . This is an
example of substrate –level phosphorylation in which the production of a
high energy phosphate is coupled directly to the oxidation of a substrate ,
instead of resulting from oxidative phosphorylation via the electron
transport chain .
ii- 1,3-Bisphosphoglycerate is converted to 2,3-bisphosphoglycerate (
2,3-BpG ) by the action of bisphosphoglycerate mutase . 2,3-BPG , which
is found in only trace amounts in most ceolls , is present at high
concentration in red blood cells . 2,3-BPG is hydrolyzed by a phosphate
to 3-phpsphoglycerate , which is also an intermediate in glycolysis . In
the red blood cell , glycolysis is modified of these “shunt” reactions .
[C] I- Shift of the phosphate group from carbon 3 to carbon 2 :
This reaction is catalysed by phosphglycerate mutasde which is freely
reversible .
ii- Dehydration of 2-phosphoglycerate :
The dehydration of 2-phosphoglycerate by enolase redistributes the
energy within the 2-phosphoglycerate molecules , resulting in the
formation of phosphoenolpyruvate ( PEP ) , which contains a high energy
enol phosphate , The reaction is reversible despite the high energy nature
of the product .
8
iii- Formation of pyruvate ( substrate-level phosphorylation ) :
The conversion of PEP to pyruvate is catalyzed by pyruvate kinase
, the third irriversible reaction of glycolysis . The equilibrium of the
pyruvate kinase reaction favors the formation of ATP ( 2 ATP from 2
mols ) . [ This is a second example of substrate-level phosphorylation ] .
In liver, Pyruvate kinase is activated by fructose 1,6-bisphosphate the
product of the phosphofructokinase reaction . This feed-forward ( instead
of the more usual feed back ) regulation has the effect of linking the two
kinase activities : increased phosphofructokinase activity , resulting in
elevated levels of fructose 1,6-bisphosphate activities pyruvate kinase .
Stage IV
Reduction of pyruvate to lactate ( anaerobic ) :
Lactate , formed by the action of lactate dehydrogenase ( LHD ) is
the final product of anaerobic glycolysis in eukaryotic cells . The
formation of lactate is the major fate for pyruvate in red blood cells , lens
and cornea of the eye , Kidney medulla , testes , and leukocytes .
The significance of pyruvate to lactate transfer under an aerobic
conditions :
Under aerobic condition NADHH produced from the reaction of
glyceraldehyde 3-p dehydrogenase ( stage III ) is back transferred to
NAD+ by losing its 2 H to the mitochondria respiratory chain ( forming 3
ATP ) .
However under anaerobic conditions NADHH , instead , loses its 2H
to pyruvate via LDH transferring it to lactate , thus oxidized NAD+ is
regenerated . This permits glycolysis to proceed even under anaerobic
conditions .
1. Lactate formation in muscle : In exercising skeletal muscle ,
NADH production ( by glyceraldehyde 3-phosphate dehydrogenase
and by the three NAD+ -linked dehydrogenase of the citric acid
cycle ) exceeds the oxidative capacity of the respiratory chain .
This results in an elevated NADH/NAD+ ratio favoring reduction
of pyruvate to lactate . Therefore, during intense exercise , lactate
accumulates in muscle , causing a drop in the intracellular PH,
potentially resulting in cramps . Much of this lactate eventually
diffuses into the blood stream .
2. Lactate consumption : In liver and heart , the ratio of
NADH/NAD+ is lower than in exercising muscle . These tissues
oxidize lactate ( obtained from the blood ) to pyruvate . In the liver
, pyruvate is either converted to glucose . by gluconeogensis or
oxidized in the TCA cycle . Heart muscle exclusively oxidize
lactate to CO2 and H2o via the citric acid cycle being highly
supplied with oxygen .
9
3. In RBCs : Even under aerobic conditions , glycolysis in RBCs
usually terminates with lactate , since RBCs has no mitochondria
that contains the enzymes responsuble of aerobic oxidation of
pyruvate ( citric acid cycle enzymes ) .
The next figure ( page 31 ) shows the collective reactions of glycolysis :
ALTERNATE FATES OF PYRUVATE
A. Oxidative decarboxylation of pyruvate
Pyruvate resulted from aerobic glycolysis in cytosol enters the
mitochondria where it is oxidatively decarboxylation by pyruvate
dehydrogenase complex . This is an important path way in tissue with
high oxidative capacity , such as cardiac muscle , pyruvate
dehydrogenase irreversibly converts pyruvate , the end product of
glycolysis , into acetyl COA , a major fuel for the citric acid cycle .
B- Carboxylation of pyruvate to oxaloacetate
Carboxylation of pyruvate to oxaloacetate (OAA) by pyruvate
carboxylase is a biotin-dependent reaction . This reaction is important
because it replenishes the citric acid cycle intermediates and provides
substrate for gluconeogenesis ( see p. 99 )
ENERGY YIELD OF GLYCOLYSIS
Reaction
ATP (*p) formed per mole glucose
1- Hexokinase ( Glucokinase )
-1 ATP
2- PFK-1
-1ATP
3- Glyceraldehyde 3p dehydrogenase ( aerobic ) +2 x 3 ATP
4- Phosphoglycerate kinase ( substrate level )
+2 x 1 ATP
5- Pyruvate kinase ( substrate level )
+2 x 1 ATP
TOTAL
Aerobic =
8 ATP
Anaerobic =
2ATP
REGULATION OF GLYCOLYSIS
ENZYME
Activators
1- Hexokinase
Insulin ( inducer )
2- PEK-1
AMP , F2,6bisphosphate
3- Pyruvate kinase
F1,6-bisphosphate
Inhibitors
G 6-P
High ATP , Citrate
ATP
ATP
HORMONAL REGULATION OF GLYCOLYSIS
Glycolytic enzymes
- Consumption of a meal rich in carbohydrate or administration of
insuliun initiates an increase in the amount of glucokinase .
- These changes reflect an increase in gene transcription , resulting
in increased enzyme synthesis .
10
- High activity of these three enzymes favors the conversion of
glucose to pyruvate , a characteristic of the well-fed state .
- Conversely , gene transcription and synthesis of glucokinase ,
phosphofructokinase , and pyruvate kinase are decreased when
plasma glucagon is high and insulin is low , for example , as seen
in starvation or diabetes .
CITRIC ACID CYCLE
KREBS CYCLE
TRICARBOYLIC ACID CYCLE ( TCA )
The citric acid cycle plays several roles in metabolism :
*Its central function is the oxidation of acetyl COA to CO2 and H2o .
Acetyl CoA is derived from the metabolism of fuel molecules such as
amino acids , fatty acids , and crbohydrates . This oxidation accounts for
about two thirds of the total oxygen consumption and ATP production in
most animals , including humans .
* The citric acid cycle also participate in a number of important synthetic
reactions . For example , the cycle functions in the formation of glucose
from the carbon skeleton of amino acids and provides building blocks for
heme synthesis .
* The cycle occurs totally in the mitochondrial matrix and is therefore in
close proximity to reactions of oxidative phosphorylation .
REACTIONS OF THE CITRIC ACID CYCLE
In THE CITRIC ACID CYCLE ** is first condensed with acetone
(acetyl COA) , then regenerated as the cycle is completed , these
reactions should not be viewed only as a closed circle , but rather like a
traffic circle with compounds entering and leaving as required .
1] Oxidative decarboxylation of pyruvate
pyruvate dehydrogenase complex ( PDH ) is a multienzyme
complex located in the mitochondrial matrix . It converts pyruvate , the
end product of aerobic glycolysis into acetyl COA , a major fuel for the
citric acid cycle . the irreversibility of the reaction precludes the
formation of the pyruvate from acetyl COA and explains why glucose
cannot be formed from acetyl COA ( derived from fatty acids oxidation )
in gluconeogenesis .
A- Component enzymes : The pyruvate dehydrogenase complex is a
multimolecular aggregate of three enzymes : pyruvate
Decarboxylase , dihydrolipoyl trans acetylase , and dihydrolipoyl
dehydrogenase . Each catalyzes a part of the overall reaction . their
11
physical association links the reaction in proper sequences without the
release of intermediates .
B- Coenzymes : The PDH complex contains five coenzymes : thiamine
pyrophosphate ( TTP ) , lipoic acid , COASH , FAD and NAD . They act
as a carriers or oxidants for the intermediates of the reaction :
C- Regulation : PDH is inhibited by product ( acetyl CoA, NADHH and
ATP ) . it is activated by insulin ( in adipose tissue ) .
2- Synthesis of citrate from acetyl CoA and oxaloacetate
The condensation of acetyl CoA and oxaloacetate is catalyzed by
citrate synthase . this indol condensation has an equilibrium far in
direction of citrate synthase ( irreversible ) .
Citrate synthase is inhibited by ATP , NADH , succinyl CoA , and acetyl
CoA derrivatives of fatty acids .
3- Isomerization of citrate
Citrate is isomerized to isocitrate by aconitase . This conversion
takes place in two steps : dehydration to cis-aconitate , and rehydration to
isocitrate This reaction is inhibited by fluoroacetate .
4- Oxidation and decarboxylation of isocitrate
Isocitrate dehydrogenase catalyses the oxidative decarboxylation of
isocitrate to α-ketoglutarate ( oxalosuccinate is formed as intermediate ) ,
yielding the first of three NADH molecules produced by the cycle , and
the first release of CO2 . This is one of the rate-limiting steps of the citric
acid cycle . The enzyme is activated by ADP . Elevated levels of
mitochondrial ADP signal a need for generation of more high-energy
phosphate ( ATP ) .
The enzyme is inhibited by ATP and NADH , whose levels are
elevated when the cell has abundant energy stores . Another form of
isocitrate dehydrogenase is also present in cytosol and is NADP –linked
enzyme instead of NAD .
5- Oxidative decarboxylation of α-ketoglutarate
Conversion of α-ketoglutarate to succinyl CoA is catalysed by the
α-ketoglutarate dehydrogenase complex . The mechanism of the
oxidation decarboxylation is similar to that used for the conversion of
pyruvate to acetyl CoA . The reaction release the second Co2 and
produces the second NADH of the cycle . This reaction is irreversible .
Coenzyme : The coenzymes required are thiamine pyrophosphate , lipoic
acid , FAD , NAD+ , and CoA . Each functions in the cataalytic
mechanism in a way analogous to that described for pyruvate
dehydrogenase complex .
12
6- Cleavage of succinyl CoA
Succinate thiokinase cleaves the high-energy thioester bond of
succinyl CoA . This reaction is coupled to phosphorylation of GDP to
GTP .
The energy content of GTP is the same as that of ATP , and the
two nucleotides are interconvertible by the nucleoside diphosphate kinase
reaction :
Thus ATO formed from GTP in this reaction is an example of
substrate level phosphorylation .
N.B. succinyl CoA also formed from fatty acids with an old number of
carbon atoms and from propionyl CoA derived from the mechanism of
branched-chain amino acids . Succinyl CoA is used in the biosynthesis of
heme .
7- Oxidation of succinate
Succinate is oxidized to fumarate by succinate dehydrogenase ,
producing the reduced coenzyme FADH2 . FAD , rather than NAD+ , is
the electron acceptor because the reducing power of succinate is not
sufficient to reduce NAD+ .
Malonate , a dicarboxylic acid that is a structural analog of succinate ,
competitively inhibits succinate dehydrogenase .
8- Hydration of fumarate
Fumarate is hydrated to malate in a freely reversible reaction
catalysed by fumarase .
9- Oxidation of malate
Malate is oxidized to oxaloacetate by malate dehydrogenase . This
reaction produces the third and final NADH of the cycle .
Summary of reactions
1. Two carbon atoms enter the cycle as acetyl CoA and leave as Co2 .
2. The cycle does not involve net consumption or production of
oxaloacetate or of any other intermediate .
3. Four pairs of electrons are transferred during one turn of the cycle :
three pairs of electrons reducing NAD+ to NADH and one pair
reducing FAD to FADH2 .
Yield of ATP
Oxidation of one NADH by the electron transport chain leads to
formation of three ATP , whereas oxidation of FADH2 yields two ATP .
13
Energy yield from oxidation of one mole of acetyl CoA via TCA cycle :
Enzyme
No of ATP produced
Isocitrate dehydrogenase ( NADH )
3 ATP
3 ATP
a-ketoglutarate
dehydrogenase
(NADH)
Succinate thiokinase (substrate
1 ATP
level )
Succinate dehydrogenase (FADH2)
2 ATP
Malate dehydrogenase (NADH)
3 ATP
12 ATP
TOTAL
Oxidation of one mole of glucose under aerobic condition *****
Thus one mole of glucose oxidized completely under aerobic condition
gives 38 ATP while under anaerobic it is converted to lactate yielding 2
ATP .
Pasteur Effect : Pasteur observed that Yeast cells consumed glucose
much more slowly under aerobic than under anaerobic conditions . This is
explained by that , under anaerobic condition . The large production of
ATP ( via citric acid cycle ) inhibits glycolysis by inhibiting PEK-1
pyruvate kinase , this leads to accumulation of G6 –p which in turn ,
inhibits further uptake of glucose by cells due to allosteric inhibition of
hexokinase .”
VITAMINS PLAY KEY ROLES IN THE CITRIC ACID CYCLE
***********
functioning of the citric acid cycle . They are :
[1] Riboflavin , in the form of flavin adenine dinucleotide ( FAD ) , a
cofactor in the α-ketoglutarate dehydrogenase complex and in succinate
dehydrogenase .
[2] Niacin , in the form of nicotinamide adenine dinucleotide ( NAD ) ,
the coenzyme for three dehydrogenases in the cycle , isocitrate
dehydrogenase , α-ketoglutarate dehydrogenase , and malate
dehydrogenase .
[3] Thiamin , ( vitamin B1 ) , as thiamin diphosphate , the coenzyme for
decarboxylation in the α-ketoglutarate dehydrogenase reaction .
14
[4] Pantothenic acid , as part of coenzyme A, the cofactor attached to
“active” carboxylic acid residues such as acetyl-CoA and succinyl-CoA .
THE CITRIC ACID CYCLE PLAYS A PIVOTAL ROLE IN
METABOLISM
The amphibolic Role Of Citric Acid Cycle
Some metabolic pathways end in a constituent of the citric acid cycle
while other pathways originate from the cycle . These pathways concern
the processes of gluconeogenesis , transmination , deamination , and fatty
acid synthesis . Therefore , the citric acid cycle plays roles in both
oxidative synthetic processes ; i.e, it is amphibolic . These roles are
summarized
[1] Role of the Cyclic in Gluconeogenesis :
All major members of the cycle , from citrate to oxaloacetate , are
potentially glucongenic , since they can give rise to anet production of
glucose in the liver or kidney , the organs that contain a complete set of
enzymes necessary for gluconeogenesis . The key enzyme that facilitates
the net transfer out of the cycle into the main pathway of gluconeogenesis
is phosphoenolpyruvate carboxykinase ( PEP carboxykinase ) , which
catalyzes the decarboxylation of oxaloacetate to phosphoenolpyruvate ,
GTP acting as the source of high-energy phosphate ( will be described in
gluconeogenesis ) .
[2] Role in Transamination :
i- Pyruvate
Alanine
ii- Oxaloacetate
Aspartate
ii- α-ketoglutarate
Glutamate
[3] Final pathway for the oxidation
acids e.g.
i- Glucose
pyruvate
ii- Fatty acids
β-oxidation
iii- Serine , Cystine , Glycine
iv- Tryosine , Phenylalanine
v- Methionine , valine
[4] Synthetic pathways e.g.
i- Succinyl CoA + Glycine
ii- Citrate
Acetyl CoA
of Glucose , Fatty acids & Amino
Acetyl CoA .
Acetyl CoA .
Acetyl CoA .
pyruvate
Fumarate .
Succinyl CoA .
Heme .
Fatty acids .
Hexose Monophosphate Shunt ( HMP )
“ Pentose Phosphate Pathway ( PPP ) “
*****
Phosphate pathway or phosphogpuconate pathway ) :
15
 It contains of two irreversible oxidative reactions , followed by a
series of reversible sugar-phosphate interconversion .
 No ATP is directly consumed or produced in the cycle .
 Carbon 1 of glucose 6-phosphate is released as Co2 , and two
NADH are produced for each glucose 6-phosphate entering the
oxidative part of the pathway .
 Unlike glycolysis or the citric acid cycle in which the direction of
the reaction is well defined , the interconversion reactions of the
HMP can function in several different directions . The rate and
direction of the reactions at any given time are determined by the
supply of and demand far intermediates in the cycle .
 The HMP occurs in the cytosol of the cell .
Metabolic significance of HMP shunt ( functions ) :
 The pathway provides a major portion of the cell’s NADPH ,
which functions as a biochemical reactant .
 It is particularly important in liver and mammary glands , which
are active in the biosynth
 esis of fatty acids , and in the adrenal cortex which is active in the
NADPH-dependent synthesis of steroids .
 The HMP also produces ribose-phosphate , required for biosythesis
of nucleotides and provides a mechanism for the metabolic
utilization of five-carbon sugars ingested as food .
Reactions of HMP
A] OXIDATIVE REACTIONS
The oxidative portion of the HMP consists of three reactions that lead
to the formation of ribulose 50phosphate , Co2 , and two molecules of
NADH for each molecule of glucose 6-phosphate oxidized :
1- Dehydrogenation of glucose 6-phosphate :
Glucose 6-phosphate dehydrogenase ( G6PD ) catalyzes an
irreversible
oxidation
of
glucose
6-phosphate
tp
6phosphogluconolactone in a reaction that is specific for NADP as
coenzyme .
2- Hydrolysis of 6-phosphogluconolactone and formation of
ribulose 5-phosphate
 6-Phosphogluconolactone is hydrolyzed by
Phosphogluconolactone hydrolase to 6-phosphogluconate .
The reaction is irreversible and is not rate-limiting .
16
 The subsequent oxidative decarboxylation of 6-phosphogluconate
is catalyzed 6-phosphogluconate dehydrogenase . This irreversible
reaction produces a pentose sugar phosphate ( ribulose 5-phosphate
) , Co2 ( from carbon 1 of glucose ) , and a second molecule of
NADPH .
B] NONOXIDATIVE REACTIONS
`The nonoxidative reactions of the pentose phosphate pathway
catalze the interconversion of three- , four- , five- and seven-carbon
sugars . These reactions permit ribulose 5-phosphate ( produced by the
oxidative portion of the pathway ) to be converted either to ribose 5phosphate ( needed for nucleotide synthesis ) or to intermediates of
glycolysis , such as fructose 6-phosphate and glyceraldehyde 3-phosphate
. Thus the HMP is not an isolated repetive cycle , but is integrated with
glycolysis . The only coenzyme required in the nonoxidative pathway is
thiamine pyrophosphate ( TPP ) in the transketolase reaction .
Interconversion between HMP and Glycolysis
1- Conversion of pentose phosphate to intermediates of glycolysis
Many cells that carry out reductive biosynthetic reactions have a greate
need for NADPH than ribose 5-phosphate . In this case , transketolase
and transaldolase convert the ribose 5-phosphate produced as an end
product of the oxidative reaction to glyceraldehyde 3-phosphate and
fructose 6-phosphate , which are intermediates of glycolysis .
2- Formation of ribose 5-phosphate from intermediates of glycolysis
Under conditions where the demand for pentoses for incorporation into
nucleotides and nucleic acids is greater than the need for NADPH, the
nonoxidative reactions can provide the biosynthesis of ribose 5-phosphate
from fructose 6-phosphate in the absence of the oxidative steps .
Metabolic roles of reduced NADP ( NADPH ) produced from HMP
A. Reductive biosynthesis
NADPH can be thought of as a high-energy molecule , much in the same
way as NADH . However , the electron of NADPH are designed for use
in reductive biosynthesis , rather than for transfer to oxygen as in the case
wit NADH . Thus in the metabolic transformation of HMP , part of the
energy of glucose 6-phosphate is conserved in NADPH , a molecule that
can be used in reactions requiring a high electron-potential electron donor
For example , NADPH is used as a source of electrons for the
biosynthesis of fatty acids and steroids as cholestrol and steroid
hormones.
*** Reduction of hydrogen peroxide
17
 Hydrogen peroxide is one of a family of reactive oxygen
intermediates that are formed from the partial reduction of
molecular oxygen .
 These compounds are formed continuously as by-products of
aerobic metabolism and through reactions with drugs and
environmental toxins .
 They are highly reactive and can cause serious chemical damage to
DNA , proteins , and unsaturated lipids .
 Reactive oxygen intermediates have been implicated in a number
of pathogenic processes , including reperfusion injury , cancer ,
inflammatory disease , and aging .
The cell has several protective mechanisms that serve to minimize the
toxic potential of these compounds .
Enzymes that catalyze antioxidant reactions :
Reduced
glutathione
,
a
tripeptide-thiol
(
**glutamylcysteinylglycine ) present in most cells , can chemically detoxify
hydrogen peroxide . This reaction , catalyzed by glutathione peroxidase ,
forms oxidized glutathione , which no longer has protective properties .
The cell regenerates reduced glutathione in a reaction catalyzed by
glutathione .
Reductase using NADPH as a source of reducing electrons . Thus
NADPH indirectly provides electrons for the reduction of hydrogen
peroxide .
GLUCOSE 6-P DEHYDROGENASE DEFICIENCY
G6PD deficiency is an inhereted disease characterized by
hemolytic anemia caused by the inability to detoxify oxidizing agents ,
G6PD .
Deficiency is the most common disease – producing enzyme abnormality
in humans , affecting more than 200 milion individuals world wide . This
X-linked enzyme deficiency is , in fact , a family of deficiencies caused
by over 400 different mutations in the gene coding for G6PD . only some
of these mutations cause clinical symptoms . The life span of many
individuals with G6PD deficiency is some what shortened as a result of
complications arising from chronic hemolysis .
Role of G6Pd in red blood cells
Diminished G6PD activity impairs the ability to form NADPH that
is essential in the detoxification of free radicals and peroxides formed
within the cell ( figure 10.9 ) . Although the deficiency occurs in all cells
of the affected individual , it is most severe in erythrocytes where the
HMP provides the only means of generating NADPH . other tissues have
alternative sources for NADPH production .
18
The erythrocyte has no nucleus or ribosomes and cannot renew its supply
of the enzyme . Thus , red blood cells are particularly vulnerable to
enzyme variants with diminished stability .
Precipitating factors in G6PD deficiency
Most individuals who have inherited one of the many G6PD mutations
do not show clinical manifestations . However , some patients with G6PD
deficiency develop hemolytic anemia if they are treated with n oxidant
drug , ingest fava beans , or contact a severe infection .
1. Oxidant drugs : Commonly used drugs that produce hemolytic
anemia in patients with G6PD deficiency are best remembered
from the mnemonic AAA -= Antibiotics ( for example
suifamethoxazole ) , Antimalarials ( for example , primaquine ) and
Antipyretics ( for example , acetanilid , but not aspirin or
acetaminophen ) .
2. Flavin : Some forms of G6PD deficiency , for example the
Mediterranean variant , are particularly susceptible to the
hemolytic effect of the fava bean , a dietary staple in the
Mediterranean region .
3. Infection : Infection is the most common precipetating factor of
hemolysis in G6PD deficiency . The inflammatory response to
infection results in the generation of free radicals in macrophages ,
which can diffuse into the red blood cells and cause oxidative
damage.
4. Neonatal jaundice : Individuals with G6PD deficiency may
experience neinatal jaundice , which may result from impaired
hepatic catabolism or increased production of bilirubin .
GLYCOGEN METABOLISM
Glycogenesis & Glycogenolysis
 A constant source of blood glucose is an absolute requirement for
human life .
 Glucose is the preferred energy source of the brain and the required
energy source for cells with few or no mitochondria , such as
mature erythrocytes .
 Glucose is also essential as an energy source in exercising muscle ,
where it is the substrate for anaerobic glycolysis .
 Blood glucose can be obtained from three primary sources : diet ,
degradation of glycogen , and gluconeogenesis .
The body has developed mechanisms for storing a supply of glucose
in a rapidly mobilizable form – glycogen . In the absence of a dietary
source of glucose , glucose is rapidly released from liver glycogen .
19
Similarly , muscle glycogen is extensively degraded in working muscle .
when glycogen stores are depleted , specify tissues synthesize glucose do
novo , using amino acids from the body’s proteins as the primary source
of carbons for the gluconeogenic pathway .
Functions of Glycogen :
The main stores of glycogen in the body are found in skeletal muscle
and liver , although most other cells may store minute amounts .
 The function of muscle glycogen is to serve as a fuel reserve for
the synthesis of ATP during muscle contraction .
 That of liver glycogen is to maintain the blood glucose
concentration , particularly during the early stages of a fast .
Amounts of liver and muscle glycogen
Approximately 400 g of glycogen makes up 1% to 2% of the fresh
weight of resting muscle , and approximately 100 g of glycogen makes
up 6% to 8% of the fresh weight of well-fed adult liver .
Thus glycogen conce3ntration in liver is greater than muscle . However ,
the total amount of glycogen in muscle is greater than liver ( since the
total muscle mass is much greater than liver )
Structure of glycogen
Glycogen exists in discrete cytoplasmic granules that contain most
of the enzymes necessary for glycogen synthesis and degradation .
Glycogen is a branched-chain homopolysaccharides made exclusively
from α-D-glucose .
The primary glycosidic bond in an **-1,4 linkage . After every
eight to ten glycosyl residues , there is a branch containing an **-1,6
linkage .
GLYCOGENESIS ( Synthesis of Glycogen )
- Glycogen is synthesized from molecules of α -D-glucose .
- The process occurs in the cytosol , and requires energy supplied by
ATP ( for the phosphorylation of glucose ) and uridine triphosphate
(UTP) .
A. Synthesis of UDP –glucose
α-D-Glucose attached to uridine diphosphate ( UDP ) is the source of
all the glucosyl residues that are added to the growing glycogen molecule
UDP –glucose is synthesized from glucose 1-phosphate and UTP by
UDP-glucose pyrophosphorylase .
Note : Glucose 6-phosphate is converted to glucose 1-phosphate by
phosphoglucomutase .
20
B. Synthesis of a primer to initiate glycogen synthesis
Glycogen synthase is responsible for making the α-1,4 linkage in
glycogen . This enzyme cannot initiate chain synthesis using free glucose
as an acceptor of a molecule of glucose from UDP –glucose . Instead , it
can only elongate already existing chains of glucose . Therefore , a
fragment of glycogen can serve as a primer in cells whose glycogen
stores are not totally depleted .
In the absence of a glycogen fragment , a specific protein , called
glycogenin , can serve as an acceptor of glucose residues . The hydroxyl
group of a specific tyrosine side chain serves as the site at which the
initial glucosyl unit is attached . Transfer of the first molecule of glucose
from UDP –glucose to glycogennin in catalyzed by glycogen initiator
synthase .
C. Elongation of glycogen chains by glycogen synthase
Elongation of a glycogen chain involves the transfer of glucose
from UDP-glucose to the non reducing end of the growing chain ,
forming a new glycosidic bond between the anomeric hydroxyl of carbon
1 of the activated glucose and carbon 4 of the accepting glucosyl residue .
The enzyme responsible for making the **-1,4 linkage in glycogen is
glycogen synthase .
D. Formation of branches in glycogen
Glycogen is a highly branched , tree like structure , that i8s far
more soluble than its unbranched amylose cousin . Branching also
increases the number of non reducing ends to which new glucosyl
residues can be added , thereby greatly accelerating the rate at which
glycogen synthesis and degradation can occur , and dramatically
increasing the size of the molecule .
1. Amylo –( 1,4- 1,6 ) –transglycosylase ( branching enzyme )
transfers a chain of five to eight glucosyl residues from the non reducing
end of the glycogen chain to another residue on the chain and attaches it
by an α -1,6 linkage . The resulting new , non reducing end can now be
further elongated by glycogen synthase .
2. Synthesis of additional branches : After elongation of these two ends
has been accomplished by glycogen synthase , their terminal five to eight
glucosyl residues can be removed and utilized to make further branches .
GLYCOGENOLYSIS ( Degradation of Glycogen )
The degradation pathway that mobilizes stored glycogen in liver and
skeletal muscle is not a reversed of the synthetic reactions . Instead an
independent set of enzymes is required . When glycogen is degraded , the
primary product is glucose 1-phosphate , obtained by breaking α -1,4
21
glycosidic bonds . In addition , free glucose is released from each α - 1,6linked glucosyl residue .
Shortening of chains
Glycogen phosphorylase cleaves the *-1,4 glycosidic bonds between
the glucosyl residues at the non-reducing ends of the glycogen chains by
simple phosphorolysis . Glycogen phosphorylase is a phosphotransferase
that sequentially degrades the glycogen chains at their non-reducing ends
until four glucosyl units remain on each chain before a branch point . The
resulting structure is called a limit dextrin , and phosphorylase cannot
degrade it any further ( Figure 13.7 ) .
B. Removal of branches
Branches are removed debranching enzyme ( amylo-**- ( 1,6 ) –
glucosidase
Conversion of glucose 1-phosphate to glucose 6-phosphate
Glucose 1-phosphate , produced by glycogen phosphorylase , is
converted to glucose 6-phosphate by phosphoglucomutase .
In liver and kidney : The enzyme glucose 6-phosphate is oresent , which
trasfers glucose 6-p tp glucose that can diffuse from the cell to the blood
to maintain blood glucose level . Thus the end product of glycogenolysis
in liver is glucose .
In Muscle : glucose 6-phosphatase is absent , thus G6P resulted from
glycogenolysis in mu8scle is oxidized to lactate via glycolysis .
REGULATION OF GLUCOGENSIS AND GLYCOGENOLYSIS
 Because of the important of maintaining blood glucose levels , the
synthesis and degradation og its storage form , glycogen , are
tightly regulated .
 In the liver , glycogen synthesis accelerates during periods when
the body has been well fed , whereas glycogen degradation
accelerates during periods of fasting .
 In skeletal muscle , glycogen degradation occurs during active
exercise , and accumulation begins as soon as muscle is again at
rest .
Both synthesis and degradation of glycogen are controlled through a
complex mechanism involving insulin , glucagon and epinephrine . These
hormones initiate processes that result in the control; of several sets of
enzymes .
1. The binding of glucagon to liver cells stimulates glycogenolysis
and inhibits glycogenesis , As blood glucose levels drop in the
hours after a meal , glucagon ensures that glucose will be released
intl blood stream .
22
2. As a result of glucagon’s binding to its receptor , adenylate cyclase
( a cell membrane enzyme ) is stimulated to convert ATP to the
second messenger cAMP . cAMP then initiates a reaction cascade
that amplifies the original signal .
3. Within seconds a few glucagon molecules cause the release of
thousands of glucose molecules .
4. The binding of insulin to receptors on the surface of several cell
types stimulate glycogenesis and inhibits glycogenolysis .
5. The mechanism of insulin’s action is poorly understood . In
addition to increasing the rate of glucose uptake into several types
of cell ( but not liver or brain cells ) , insulin inhibits several
enzymes that are activated by the cAMP reaction cascade .
6. Emotional or physical stress results in the release of epinephrine
from the adrenal medulla .
7. Epinephrine affects glycagon metabolism in liver and muscle ,
while only glucagon effect is on liver . This is because of the
presence of epinephrine receptors in both liver and muscle while
glucagon receptors are present only in liver ceels .
8. Epinephrine promotes glycogenolysis and inhibits glycogenesis . In
emergency situations when epinephrine is released in relatively
large quantities , the resulting massive production of glucose
provides the energy required to manage the situation .
9. This phenomena is referred to as the “ flight – or – fight “ response
.
10.Epinephrine initiates the process by activating adenylate cyclase in
liver and muscle cells .
11.Two other second messengers , calcium ions and inositol
triphosphate are also belived to be involved in epinephrine’s action
.
Glycogen synthase and glycogen phosphorylase exist in
Active and inactive conformations . The interconversion between
these forms is effected by covalent modification .
1. The active form of glycogen synthase , known as the ( independent
) 1 form is converted to the inactive or D ( dependent ) form by
phosphorylation .
2. In contrast , the inactive form of glycogen phosphorylase
(phosphorylase b) is converted to the active form (phosphorylase a)
by the phosphorylation of a specific serine residue . The
phosphorylating enzyme is called phosphorylase kinase .
3. Phosphorylation of both glycogen synthase and phosphorylase
kinase is catalyzed by a protein kinase , which in turn is activated
by cAMP .
23
4. Glycogen synthesis occurs when glycogen synthase and glycogen
phosphorylase have been dephosphorylated . This conversion is
catalyzed by phosphoprotein phosphatase .
5. Phosphoprotein phosphatase also inactivates phosphorylase kinase
.
GLUCONEOGENESIS
It is the pathway responsible for the conversion of non carbohydrates to
glucose or glucogen .
The major substrates for gluconeogenesis are :
1- pyruvate & lactate ( the product of anaerobic glycolysis in muscles
andRBCs ) .
2- Gluconeogenic amino acids .
3- Glycerol resulted from lipolysis of triacetyl glycerols ( fat ) in
adipose tissue .
4- Propionate ( in ruminants only ) .
Gluconeogenesis occurs mainly in liver and kidney .
Some tissues , such as the brain , red blood cells , kidney medulla ,
lens and cornea of the eye , testes , and exercising muscle , require a
continous supply of glucose as a metabolic fuel . Liver glycogen can
meet these needs for only 10 to 18 hours in the absence of dietary intake
of carbohydrate . During a prolonged fast , hepatic glycogen stores are
depleted and glucose is formed from precursors such as lactate , pyruvate
, glycerol ( derived from the backbone of triacylglycerols ) and *** )
derived from amino acid catabolism ) .
The formation of glucose does not occur by a simple reversal of
glycolysis , because the overall equilibrium of glycolysis strongly favors
pyruvate formation . Instead , glucose is synthesized by a special pathway
, gluconeogenesis . Approximately 90% of gluconeogenesis occurs in ths
liver . whereas kidneys provide 10% of newly synthesized glucose
molecules . The Kidneys thus play a minor role except during prolonged
starvation , when they become major glucose producing organs .
SUBSTRATES FOR GLUCONEOGENESIS
Gluconeogenesis precursors are molecules that can give rise to a
net synthesis of glucose . They include all the intermediates of glycolysis
and the citric acid cycle . Glycerol , lactate , and the ***keto acid
obtained from deamination of glucogenic amino acids are the most
important gluconeogenic precursors .
REACTIONS UNIQUE TO GLUCONEOGENESIS
24
Seven of the reactions of glycolysis are reversible and are used in
the synthesis of glucose from lactate or pyruvate . However , three of the
reactions are irreversible and must be circumstances and must be
circumvented by four alternate reactions that energetically favor the
synthesis of glucose .
These irreversible reactions ( energy barriers or exergonic reactions )
that obstruct a simple reversal of glycolysis are :
1- From PEP to pyruvate ( pyruvate kinase ) .
2- From fructose 6-phosphate to fructose 1,6-bisphosphate ( PEK-1 ) .
3- From glucose 6-phosphate to glucose ( Hexokinase or Glucokinase
).
4- From glucose 1-phosphate to glycogen .
I- Gluconeogenesis from Lactate and Pyruvate :
1- Conversion of Pyruvate to phosphoenol pyruvate ( PEP ) :
A. Carboxylation of pyruvate
The first “roadblock” to overcome in the synthesis of glucose from
pyruvate is the irreversible conversion of pyruvate to PEP by pyruvate
kinase . In gluconeogenesis , pyruvate is first carboxylated by pyruvate
carboxylase to oxaloacetate ( OAA ) , which is then converted to PEP by
the action of PEP-carboxykinase .
Note : pyruvate carboxylase is found in the mitochondria of liver and
kidney cells , but not of muscle . ] pyruvate carboxylase contains biotin as
co-carboxylase . pyruvate carboxylase is allosterically activated by acetyl
CoA . Elevated levels of acetyl CoA may signal one of several metabolic
states in which the increased synthesis of oxaloacetate is required . For
example , this may occur during starvation where OAA is used for the
synthesis of glucose by gluconeogenesis .
B. Transport of oxaloacetate to the cytosol
Oxaloacetate , formed in mitochondria , must enter the cytosol where
the other enzymes of gluconeogenesis are located . However ,
oxaloacetate is unable to cross the inner mitochondrial membrane directly
; but it can be transported by either :
1- Reduction to malate ( by malate deyhydrogenase ) , which can be
transported to oxaloacetate .
2- Trasferred to citrate ( by citrate synthase ) , then citrate transported
to cytoplasm and again transferred to oxaloacetate by enzyme ATP
–citrate lyase .
3- Transamination to aspartate .
C. Decarboxylation of cytosolic oxaloacetate
25
Oxaloacetate , in cytosol , is decarboxylated and phosphorylated to
PEP by PEP-carboxykinase . High energy phosphate in the form of GTP
or ITP is required in the reaction , and Co2 is liberated . PEP can be
transferred easily to fructose 1,6-bisphosphate by the reversal of glycolic
reactions .
2- Dephosphorylation of fructose 1.6-bisphosphate :
Hydrolysis of fructose 1.6-bisphosphate by fructose
1.6bisphosphate by passes the irreversible phosphofructokinase-1
reaction and provides an energetically favorable pathway for the
formation of fructose -6phosphate
This reaction is an important regulatory site of gluconeogenesis . fructose
1.6bisphosphate occurs in liver and kidney .
3- Dephosphorylation of glucose 6-phosphate
Hydrolysis of glucose 6-phosphate by glucose 6-phosphatase by
passes the irreversible hexokinase reaction and provides an energetically
favorable pathway for the formation of free glucose .
Glucose 6-phosphatase , like pyruvate carboxylase , occurs in liver and
kidney , but not in muscle . Thus , muscle cannot provide blood glucose
by glyconeogenesis .
II- Glyconeogenesis from glycerol :
Glycerol is produced from fat in adipose tissue . Glycerol is then
transported , via blood , to liver and kidney where it is transferred to
glycerol 3-p by glycerokinase enzyme . Glycerol 3-p is then oxidized to
dihydroxyacetone-p by glycerol 3-p dehydrogenase and NAD+ .
Then by the reversal of glycolysis it is transferred to glucose .
III- Gluconeogenesis from amino acids :
Glucogenic amino acids , by transamination ( TA ) or deamination ( DA )
, can be transferred to pyruvate or other intermediates of citric acid cycle
so can be transferred to glucose e.g :
Alanine , Cystine , Serine
TA or DA
Pyruvate .
Aspartate
TA
Oxaloacetate
Glutamine
TA α-ketpglutarate
TCA
oxaloacetate
N.B
Acetyl CoA and compounds that give rise to acetyl CoA ( for
example . fatty acids , acetoacetate and ketogenic amino acids ) cannot
give rise to a net synthesis of glucose . This is due to irreversible nature
of the pyruvate dehydrogenase reaction , which converts pyruvate to
26
acetyl CoA . These compounds give rise instead to ketone bodies and are
therefore termed “ketogenic” .
VI- Gluconeogenesis from propionate :
Fatty acids with odd number of carbon atoms ( obtained from
vegetables in diet ) are oxidized in animal tissue to propionate ( 3-carbon
acid ) propionate is a minor precursor of glucose in humans but it is a
major source of glucose in ruminants . it is converted to glucose as seen
below :
Metabolism of propionate
Cori cycle
Lactate is released into the blood by cells that lack mitochondria ,
such as red blood cells , and by exercising skeletal muscle .
In the Cori cycle , blood-borne glucose is converted by exercising muscle
tp lactate , which diffuses into the blood . This lactate is taken up by liver
and converted to glucose , which is released back into the circulation .
Thus the four KEY ENZYMES of gluconeogenesis are :
1- Pyruvate carboxylase .
2- PEP carboxylase .
3- fructose 1.6bisphosphate .
4- G 6-phosphatase .
REGULATION OG GLUCONEOGENESIS
I- Hormonal regulation :
The moment –to- moment regulation of gluconeogenesis is
determined primarily by circulating level of hirmones as glucagon ,
epinephrine , cortisol , insulin , and by the availability of gluconeogenic
substrates . In addition , slow adaptive changes in the amount of enzyme
activity result from an alternation in the rate of enzyme or degradation ,
or both .
A) Glucagon : This pancreatic islet hormone stimulates gluconeogenesis
by two mechanisms :
1.Glucagon ( stimulated by hypoglycemia ) lowers the level of fructose
2,6-bisphosphate , resulting in activation of fructose 1,6-bisphosphate and
inhibition of phosphofructokinase-1 .
N.B. Fructose 2,6-bisphosphate is synthesized stimulates fructose 6-p by
the enzyme PFK-2 Fructose 2,6-bisphosphate stimulates PFK-1( thus
stimulates glycolysis ) and inhibits fructose 1,6-bisphosphase ( thus
inhibits gluconeogenesis ) .
Thus glucagon stimulates gluconeogenesis and inhibitd glycolysis .
2. Glucagon , via an elevation in cAMP level and cAMP-dependent
protein kinase activity , stimulates the conversion of pyruvate kinase to its
27
inactive ( phosphorylated ) form . This decreases the conversion of PEP
to pyruvate , which has the effect of diverting PEP to the synthesis of
glucose .
B) Insulin : inhibits gluconeogenesis by decreasing the rate of
transcription of the genes coding for the 4 key enzymes of
gluconeogenesis ( pyruvate carboxylase , PEP carboxykinase , F 1,6bisphosphatase , G 6-phosphatase ) .
C) Cortisol , epinephrine and glucagon ( also diabetes and starvation )
cause induction of the key enzymes of gluconeogenesis .
II- ALLOSTERIC REGULATION :
Allosteric activation of hepatic pyruvate carboxylase by acetyl
CoA occurs during starvation . As result of excessive lipolysis in adipose
tissue , the liver is flooded with fatty acids . The rate of formation of
acetyl CoA by **-oxidation of these fatty acids exceeds the capacity of
the liver to oxidize it to Co2 and H2o . As a result , acetyl CoA
accumulates and lead to activation of pyruvate carboxylase .
III- Substrate availability :
The availability of gluconeogenic precursors , particularly
glucogenic amino acids , marked influences the rate of hepatic glucose
synthesis .
Decreased levels of insulin favor mobilization of amino acids from
muscle protein and provide the carbon skeletons for gluconeogenesis .
Energy required for gluconeogenesis :
During the gluconeogenesis reactions , 6 moles of high-energy bond
are utilized :
1. Two moles of pyruvate are required for the synthesis of 1 mole of
glucose . As 2 moles of pyruvate are carboxylated by pyruvate
carboxylase , 2 moles of ATP are required .
2. Two moles of GTP ( the equivalent of 2 moles of ATP ) are
required to convert 2 moles of oxaloacetate to form 2 moles of PEP
.
3. An additional 2 moles of ATP are used when 2 moles of
phosphoglycerate are phosphorylated , forming 2 moles of 1,3biphosphoglycerate .
4. Energy in the form of reducing equivalents ( NADH ) is required
for the conversion of 1,3-biphosphoglycerate to glyceraldehyde 3phosphate .
28
Under fasting condition the energy required for gluconeogenesis is
obtained from **-oxidation of fatty acids .
Metabolism of Fructose and Galactose
Although many monosaccharides have been identified in nature ,
only a few sugar appears as metabolic intermediates or as structural
components in mammals . Glucose is the most common monosaccharides
consumed by humans , and its metabolism has been discussed extensively
. However , two other monosaccharides , fructose and galactose , occur in
significant amounts in the diet and make important contribution to energy
metabolism .
Galactose is an important component of cell structural carbohydrates .
I. FRUCTOSE METABOLISM
Dietary sources of fructose
About 15% to 20% of the calories contained in the western diet are
supplied by fructose ( approximately 100 g/day ) . the major source of
fructose is the disaccharide sucrose , which , when cleaved , releases
equimolar amounts of fructose and glucose . Fructose is also found as a
free monosaccharides in many fruits and vegetables and in honey . Entry
of fructose into cells is not insulin dependent ( unlike that of glucose into
certain tissues ) .
[ Note : In contrast to glucose , fructose is also an extremely poor elicitor
of insulin secretion . ]
Phosphorylation of fructose
 For fructose to enter the pathways of intermediary metabolism , it
must first be phosphorylated . This can be accomplished by either
hexokinase or fructokinase ( see figure ) .
 Hexokinase phosphorylates glucose in all cells of the body and
several additional hexoses can serve as a substrate for this enzyme .
However , it has a low affinity ( that is , high K, ) for fructose .
 Fructokinase provides the primary mechanism for fructose
phosphorylation . It is found in the liver ( which processes most of
the dietary fructose ) , Kidney , and the small intestine , and
converts fructose to fructose 1-phosphate using ATP as the
phosphate donor >
 Fructokinase activity is insulin independent ( while glucokinase is
insulin dependent ) thus fructose can be metabolized independent
on insulin . This shows the advantage of fructose over glucose in
the diet of diabetic patients ( insulin deficiency ) .
Cleavage of fructose 1-phosphate
29
 Fructose 1-phosphate is not converted to fructose 1,6-bisphosphate
as is fructose 6-phosphate , but is cleaved by aldolase B into
dihydroxyacetone phosphate ( DHAP ) and D-glyceraldehyde .
 DHAP can directly enter glycolysis or gluconeogenesis whereas
glyceraldehyde can be metabolized by transferring to
glyceraldehyde 3-phosphat by a triokinase then enter glycolysis . (
Aldolase A primarily cleaves fructose 1,6-bisphosphate produced
during glycolysis to DHAP and glyceraldehyde 3-phosphate ) .
High fructose ( sucrose ) in diet increases liver lipogenesis :
The rate of fructose metabolism is more rapid than that of glucose
because the trioses formed from fructose 1-phosphate by pass
phosphofructokinase , the major rat-limiting step in glycolysis . Elevated
levels of dietary fructose significantly elevate the rate of lipogenesis in
the liver , owing to the rapid production of acety; CoA .
Conversion of glucose to fructose by way of sorbitol
1- Synthesis of sorbitol :
 Aldolase reductase reduces glucose to produce sorbitol ( glucitol ) .
it is found in many tissues such as the lens , retina , kidney ,
placenta , red blood cells , and in cells of the ovaries and seminal
vesicles .
 In the liver , ovaries , sperm , and seminal vesicle cells there is a
second enzyme , sorbitol dehydrogenase , that can oxidize the
sorbitol to produce fructose .
 The two-reaction pathway from glucose to fructose in the seminal
vesicles is for sperm cells , for which fructose is a preferred
carbohydrate energy source .
 The pathway from sorbitol to fructose in the liver provides a
mechanism by which dietary sorbitol is converted into a substrate
that can enter glycolysis or gluconeogenesis .
2- The effect of hyperglycemia on sorbitol metabolism :
Because insulin is not required for entry of glucose into the cells
losted above , large amounts of glucose may enter these cells during
times of hyperglycemia , for example , in uncontrolled diabetes . Elevated
intracellular glucose concentration , and an adequate supply of NADPH ,
cause aldose reductase to produce a significant increase in the amount of
sorbitol , which , unlike glucose , cannot pass efficiently through cell
membranes and therefore remains trapped inside the cell . As result ,
sorbitol accumulates in these cells , causing strong osmotic effects and
therefore cell swelling due to water retention .
30
II. GALACTOSE METABOLISM
The major dietary source of galactose is lactose ( galactosyl β-1-4glucose ) . obtained from milk and milk products . Lactose is digested by
**-galactosidase ( Lactase ) of the intestinal mucosal cell membrane .
Loke fructose , entry of galactose into cells is not insulin dependent .
Phosphorylation of galactose
Most tissues have a specific enzyme enter for this purpose , galactokinase
which produces galactose 1-phosphate . ATP is the phosphate donor .
Formation of UDP –galactose
 Galactose 1-phosphate cannot enter the glyclytic pathway unless it
is first converted to UDP –galactose .
 This occurs in an exchange reaction , in which UDP –glucose
reacts with galactose 1-phosphate , producing UDP –galactose and
glucose 1-phosphate by the enzyme galactose 1-phosphate
uridyltransferase .
 This enzyme is missing in individuals with classical galactosemia .
This leads to accumulation of galactose 1-phosphate and therefore ,
galactose in cells . The accumulated galactose is shunted into side
pathways such as that of galacitol production catalyzed by the
same enzyme , aldose reductase that converts glucose to sorbitol .
 The accumulated galacitol in eye , nerve tissue and liver leads to
catract , sever mental retardation and liver damage .
 Non-classical galactosemia is due to deficiency of galatokinase .
 Treatment : involves removal of galactose ( and consequently
lactose ) from the diet , and the diseased infants are kept on lactose
free milk formula .
Use of UDP –galactose as a carbon source for glycolysis or
gluconeogenesis
In order for UDP –galactose to enter the mainstream of glucose
metabolism , it must first be converted to its C-4 epimer , UDP –glucose ,
by 4-epimers . This “new” UDP –glucose can then participate in many
biosynthetic reactions , as well as being utilized in the uridyltransferase
reaction described above , converting another galactose 1-phosphate into
UDP –galactose , and releasing glucose 1-phosphate .
Role of UDP –galactose in biosynthetic reactions
UDP –galactose can serve as the donor of galactose units in a
number of synthetic pathways , including synthesis of :
31
a- Lactose .
b- Glycoproteins .
c- Glycolipids .
d- Glycosaminoglycans .
Note : If galactose is not provided by the diet , all tissues requirements for
UDP –galactose can be met by the action of 4 –epimerase on UDP –
glucose which is efficiently produced from glucose 1-phosphate .
Functions of Uronic pathways :
Formation of UDP –Glucuronic acid involved in :
a- Formation of mucopolysaccharides e.g. ***** sulfate .
b- Conjugation with compounds e.g. bilirubin and steroid hormones .
c- In most mammals , other than human , it can be transferred to
aerobic acid ( vit C. )
BLOOD GLUCOSE
Under normal condition , glucose is the only sugar present in blood
. In man , glucose is equally distibuted between red cells and plasma . It is
freely diffusible into Extracellular fluids .
Concentration of blood Glucose :
1- Fasting blood glucose : ( FBG ) : FBG is measured after an
overnight fast ( at least 8-10 hours ) . It normal value is 70-100
mg/dl ( 3.89-5.83 mmol/l ) .
2- After a carbohydrate meal , blood glucose reaches to 120-140
mg/dl within a period of ½ to one hour .
3- The blood glucose then begins to decrease returning to fasting level
after 2 hours .
Regulation and maintenance of Blood Glucose levels ( BGL ) :
The BGL at any given time is determined by the balance between
1) The amount of glucose entering the blood stream , and
2) The amount leaving it .
Some tissues of the body , such as brain and red blood cells , depend on
glucose for energy . Most other tissues require glucose for synthetic
reactions , e.g. ribose moiety of nucleotides or the carbohydrate portion of
glucoproteins .
Therefore , in order to survive , humans must have mechanisms for
maintaining BGL .
1) After a meal containing carboyhydrates , blood glucose levels rise .
Some of the glucose from the diet is stored in the liver as glycogen
( 5% ) or is converted to fat ( 30-40% ) . The remainder is
metabolized in muscle and other tissues e.g. for energy production .
2) After 2 or 3 hours of fasting , this glycogen begins to be degraded
by the process of glycogenolysis , and glucose is released into the
blood .
32
3) As glycogen stores decrease , adipose triacylglycerate ( fat ) are
also degraded , providing fatty acids as an alternative fuel and
glycerol for the synthesis of glucose by gluconeogenesis . Amino
acids are also released from muscle to serve as gluconeogenesis
precursor .
4) During an overnight fast , blood glucose levels are maintained by
both glycogenolysis and glyconeogenesis . However after 18-30
hours fasting , liver glycogen stores are completely depleted and
gluconeogenesis is the only source of blood glucose .
5) Blood glucose levels are maintained not only during fasting , but
also during exercise when muscle cells take up glucose from the
blood and oxidize it for energy . During exercise , the liver supplies
glucose to the blood by process of glycogenolysis and
gluconogenesis .
6) Muscle , although it stores glycogen , does not contribute glucose
to the blood , because of the absence of glucose 6-phosphatase in
muscle.
Thus , the liver plays an important role in the regulation of blood glucose
, because it functions both in the removal of glucose from the blood , and
in the addition of glucose to the blood .
The activity of liver in maintaining normal blood glucose is influenced by
various hormones :
ROLE OF HORMONES IN THE REGULATION OF BLOOD
GLUCOSE :
Insulin : is the principal hormone affecting blood glucose levels , and an
understanding of its actions is an important prerequisite to the study of
diabetes mellitus . Insulin is small protein synthesized in the beta cells of
the islels of langerhans of the pancreas . It acts through membrane
receptors and its main target tissues are liver , muscle and adipose tissue .
The overall effect of insulin is to promote cellular uptake and sstorage of
metabolic fuels and these actions are shown in the next figure .
1. Insulin increases the uptake of glucose in muscle and adipose
tissue . In contrast , there is no effect of insulin on glucose
penetration of liver cells . However , insulin activates the enzymes
of glycolysis and glycogenesis in the liver .
2. It activates hexokinase .
3. Insulin stimulates glycogenesis , and lipogenesis .
4. It inhibits hepatic production of glucose ( glucogenolysis ) .
Insulin , therefore , promotes the reduction of blood glucose by
increasing the rate of glucose utilization And decreasing the rate of
delivery of glucose to blood by the liver .
33
Glucagon : The hormone of the **cells of pancreas . Its secretion is
stimulated by hypoglycemia . It increases the blood glucose through the
following action :
1, glucagon accelerates glycigenolysis in the liver by activating
phosphorylase . It has no effect on muscle glycogen .
1. Glucagon stimulates gluconeogenesis from amino acids and
lactate .
Epinephrine : is secreted by the adrenal medulla as a result of stressful
stimuli ( hypoglycemia , fear , excitement , hypoxia , etc ) and leads to
glycogenolysis in liver and muscle owing to stimulation of
phosphorylase.
1. In muscle , the end product of glyconeogenolysis is lactate , because
glucose 6-phosphatase is absent .
2. In liver , glucose is the main product of glycogenolysis , leading to
increase in blood glucose .
Glucocorticoids : ( as cortisol and cortisone ) are secreted by adrenal
cortex .
1. They increase gluconeogenolysis . This is a result of increased
protein catabolism in tissues , increased hepatic uptake o amino
acids and increased activity of transaminases and other enzymes
concerned with gluconeogenesis in the liver .
2. Tissues . Therefore , glucocorticoids are antagonistic to insulin .
The renal Glucose Threshold ( RGT ) :
When the blood sugar rises to high levels , the kidney exerts a
regulatory effect .
1. Normally , glucose is filtered by the glomeruli , but it is reabsorbed
and retured to the blood from the tubules . The tubular reabsorption
of glucose is affected by phosphorylation using ATP .
2. Since phosphorylation is enzymatic , therefore , the capacity of the
tubular system to reabsorb glucose is limited by its content of the
responsible enzymes .
3. When the blood glucose level is elevated , the glomerular filtrate
contains more glucose than the tubular capacity to handle , and it
passes into the urine producing glycosuria .
4. In normal individuals , glycosuria occurs when the level of blood
glucos exceeds 170-180 mg/dl , . This is termed the renal
Threshold for glucose .
Glycosuria may be produced in experimental animal with phlohizin ,
which inhibits glucose reabsorption in the tubules . The presence of
glucosuria is frequently an indication of diabetes mellitus .
34
Types of Glycosuria :
Normal urine cntains practically no glucose , Glycosuria is the
excretion of glucose in urine . Thgere are different types of glycosuria :
A) Hyperglycemia glycosuria : This is due to an increase in blood
glucose concentration above the renal threshold level i.e. , 180 mg/dl .
Therefore any thing which causes elevation of the blood glucose can
cause glycosuria e.g.
1. Diabetes mellitus : which is the most frequent cause of this type .
2. Increased secretion of epinephrine , during periods of excessive
pain and emotional excetement ( fear , anxiety , anger ) .
3. Alimentary glycosuria : when large amounts of carbohydrates are
absorbed in the gastrointestinal tract more rapidly than can be
assimilated . In this case the blood glucose rises above the renal
threshold .
B) Renal glycosuria :
1. It is not associated with hyperglycemia and occurs even when
blood glucose levels are normal . It is caused by defect in the renal
tubules , which may be inherited and thus , it is known as “bengin
glycosuria” or “diabetes innocence” .
2. Kidney disease e.g. , nephritis . In this case , the tubular capacity to
reabsorb glucose is below the rate of golmerular filtration .
N.B. : Diabetes Insipidus :
It characterized by severe polyuria ( so wrongly diagnosed as DM )
. The volume of urine may reach 30 liter/day . This results from the
deficiency of antidiuretic hormone ( ADH ) also known as arginine
vasopressin ( AVP ) responsible for water reabsorption from renal tubules
of kidney .
The urine is highly diluted and this accampanied by drinking of large
amounts of water ( polydispia ) .
DIABETES MELLITUS ( DM )
Diabetes mellitus is the commonest endocrine disorder encountered
in clinical practice . It may be defined as a syndrome characterized by
hyperglycemia due to an absolute or relative lack of insulin and/or insulin
resistance .
The most obvious symptom of diabetes . hyberglycemia , is caused by
indequate uptake of glucose from the blood . Because the kidney’s
capacity to reabsorb glucose is limited , excessive amounts of blood
glucose ( <180 mg/dl or –11 mmol/1 ) results in glucosuria ( glucose in
urine ) . High urine glucose concentration produces an osmotic diuresis
and therefore polyuria .
35
Hyperosmolarity due to water loss causes polydispia ( thirst felling ) ,
Since glucose utilization is inhibited , there is an increase in appetite and
food consumption ( polyphagia ) .
The metabolic disturbances in Diabetes mellitus :
The partial or absolute lack of insulin may lead to :
1- Disturbances in Carbohydrate Metabolism :
a- Decreased uptake of glucose by adipose tissue and muscles .
b- Inhibition of glucose oxidation as result of inhibition of the key
glycolytic enzymes .
c- Increased liver gluconeogenesis due to :
i- Activation of the key enzymes of gluconeogenesis .
ii- Increased protein catabolism
increased circulating amino
acids which provide the fuel for gluconeogenesis .
d- Increased liver gluconeogenolysis and decreased glycogenesis .
2- Disturbance in Lipid Membrane :
a- Decreased lipogenesis & increased lipolysis in adipose tissue
increased circulating free fatty acids ( FFA ) in blood .
b- FFA
livertriglycerides
blood
hypertriglyceridemia .
c- Decreased activity of lipoprotein lipase
elevated levels of
lipoprotein fractions reach in triglycerides ( chylomicrons & VLDL ) .
d- Excess FFA in liver
oxidation to acetyl CoA that accumulates
( due to decreased oxaloacetate synthesized from glucose via pyruvate )
excessive formation of ketone bodies ( acetone , acetoacetic acid ,
and hydroxybutyric )
decrease PH of blood ( Ketoacidase )
Ketonuria ( ketone bodies in urine ) .
3- Disturbances in protein Metabolism
**************************
decreased protein synthesis , and increased gluconeogenesis from amino
acids . This results in :
a- Wasting and weakening of muscles and decrease in body weight .
b- Decreasing resistance to infection .
c- Delayed wound healing .
Diabetes mellitus is generally subclassified into :
a) Insulin dependent iabetes mellitus ( IDDM ) .
b) Non-insulin dependent diabetes mellitus ( NIDDM ) .
1- Insulin dependent diabetes mellitus ( IDDM ) or juvenile onset
diabetes
36
IDDM accounts for approximately 15% of all diabetes . It can occur at
any age but is most common in the young , with a peak incidence
between 9 and 14 years of age . The absolute lack of insulin is a
consequence of the autoimmune destruction of insulin-producing beta
cells . There may be an environmental precipitating factor such as a viral
infection . The presence of islets cell antibodies in serum predicts future
development of diabetes , It now appears that **-cells destruction is
caused by an inflammatory process that occurs over several years & the
symptoms are not obvious until all insulin –producing capacity is
destroyed .
Destruction of **-cells is initiated by the binding of an atibody to a cell
surface antigen .
The most serious acute symptom of IDDM is ketoacidosis . The odour of
acetone on patient’s breath is a characteristic of ketoacidosis . Elevated
concentrations of ketones in the blood ( ketosis ) and a lowered blood PH
along with hyperglycemia cause excessive water loss . ketoacidosis &
dehydration , it left untreated , can lead to coma & death .
IDDM patients are treated with injections of insulin obtained from
animals or from recombinant DNA technology .
II- Non insulin dependent diabetes mellitus ( NIDDM )
NIDDM ACCOUNTS FOR 85 % OF all diabetes and can occur at any
age . It is most common between 40 and 80 years . In this condition there
is resistance of peripheral tissues to the actions of insulin , so that the
insulin level may be normal or even high . The most common cause of
insulin resistance is the down-regulation or defect of insulin receptors .
Obesity is the most commonly associated clinical feature . Approximately
85% of NIDDM are obeses . Obesity promotes tissue intensitivity to
insulin .
When the failure of NIDDM to control hyperglycemia is accampanied by
other serious medical conditions ( e.g. renal insufficiency , myocardial
infraction , or infection ) , a serious metabolic state referred to as
Hyperosmolar hyperglycemia non ketosis can result . [ The level of
insulin is sufficient to prevent ketosis but does not prevent hyperglycemia
and osmotic diuresis ] . The high blood glucose level accompanied by
severe dehydration in life-threatening .
Treatment of NIDDM usually consists of diet control & exercise . Often ,
obese patients become more sensitive to insulin when they lose weight .
Because sustained muscular activity increases the uptake of glucose
without requiring insulin , exercise also decreases hyperglycemia . In
some cases , oral hypoglycemia drugs are used .
37
The contrasting features of IDDM and NIDDM are shown in the
following table :
COMPARISON OF TWO TYPES OF DIABETES MELLITUS
Insulin-dependent diabetes
mellitus (IDDM)
Synonym
Age of onsel
Prevalence
Genetic predisposition
Defect or deficiency
Ketosis
Plasma insulin
Acute complications
Oral hypoglycemia drugs .
Treatment with insulin
Non-insulin-dependent
diabetes mellitus
(NIDDM)
Type I ; juvenile-onsel Type II ; adult-onsel
diabetes
diabetes
Usually dueing childhood Frequently after age 35 .
or puberly
10%-20% of diagnosed 80%-90% of dignosed
diabetics .
diabetes .
Moderate
Very strong
**-cells
destroyed
, Inability of **-cells to
eliminating production of produce
appropriate
insulin .
quantities of insulin ;
insulin resistance .
Common
Rare
Low to absent
Normal to high
Ketoacidosis
Hyperosmolar coma
Unresponsive
Responsive
Always necessary
Usually not required
DIAGNOSIS OF DIABETES MELLITUS :
DM should not be digested unless hih plasma glucose
concentrations have been found in at least two different occasions . The
patient is said to be diabetic if he has a fasting venous plasma glucose
concentration <140 mg/dl ( 7.8 mmol/l ) or < 200 mg/dl ( 11.1 mmol/l )
two hours afte cho meal or after the oral ingestion of 75 g of glucose ,
even the fasting concentration is normal .
Blood samples may be taken according to any of the following ways :
i- Random blood glucose ( RBG ) :
RBG is the only test required in an emergency . An RBG of less than
140 mg/dl ( 8mmol/l ) should be expected in non-diabetics . RBG higher
than 200 mg/dl ( 11 mmol/l ) usually indicates diabetes mellitus .
2- Fasting blood glucose ( FBG ) :
FBG is measured after an overnight fast ( at least 10 hours ) . An FBG
is better than RBG for diagnostic purposes . In non-diabetes it is usually
lower than 6 mmol/l . fasting values of 6-8 mmol/l should be interpreted
as borderline . FBG equal to or above 8 mmol/l on two occasions in
diagnostic for diabetes mellitus .
3- Postprandial : 2 hours after a mixed meal ( 120-140 mg/dl ) .
38
4- Oral glucose tolerance test ( OGTT ) :
Classically , the diagnosis of diabetes is made on the basis of a
patient’s response to an oral glucose load . A baseline bloodsample is first
taken after an overnight fast . The patient is then given 75 g of glucose
orally , in about 300 ml of water , to be drunk within 5 minutes . Plasma
glucose levels are measured every 30 minutes for 2 hours . Urine may
also be tested for glucose at time 0 and after 2 hours . The patient should
be sitting comfortably throughout the test , should not smoke or exercise ,
no alcohol or drugs and should have been on a normal diet for at least 3
days prior to the test .
Normal and diabetic responses o an oral glucose load are shown in
the next figure :
Indications
Many OGTTS are performed unnecessarily . There are relatively few
indications for the test . These include :
1- Borderline fasting or postprandial blood glucose .
2- Presistant glucsouria .
3- Glucsouria in pregnant women .
4- Pregnant women with a family history of diabetes mellitus and
those who previously had large babies or unexplained fetal loss .
39
PROTEIN METABOLISM
Proteins are organic nitrogenous compounds formed of alpha amino
acids. Most of the nitrogen in the diet is consumed in the form of protein,
typically amounting 70 to 100 g/ day.
Digestion of protein
Protein are too large to be absorbed by the intestine and therefore must be
hydrolyzed to yield their basic unit amino acids which can be absorbed.
The proteolytic enzymes responsible for protein digestion are produced
by three different organs: stomach, pancrease and small intestine
1.
In the mouth
No protein digestion.
2.
In the stomach:
The digestion of the proteins started in the stomach, which secretes
gastric juice containing HCl and the proenzyme pepsinogen
APepsins:
There are 3 types of pepsin having the same function: Pepsin I, II and
III
*
Pepsins are secreted in the form of pro-enzymes (inactive
form).They are called: pepsinogens. Pepsinogen contain extra amino
acids in their sequence which prevent them from being catalytically
active
*
Pepsinogens are firstly activated by gastric HCl into pepsin. Then
pepsin itself activates pepsinogen ( autoactivation)
HCl
Pepsinogen----------------------------Pepsin
Pepsinogen---------------------------- Pepsin
*
Action of pepsins
They are endopeptidases i.e. act on the amino acids in the middle of the
polypeptide chain, hydrolysing the bonds between aromatic amino acids :
phenylalanine, tyrosine and tryptophan.
So, the products of pepsins digestion are polypeptidechains of variable
sizes:Protein-------------------Proteoses--------------Peptone---------------Polypeptides.
BRennin (chymosin, Rennet)
Activated by calcium ions at pH 4
Action of rennin
40
Rennin acts on the casein, the main milk protein, converting it in the
presence of calcium ions into insoluble calcium caseinate (milk clot).
Rennin
Ca++
Casein---------------------Paracasein------------------Insoluble
paracaseinate.
calcium
(milk clot).
Then digestion of calcium paracaseinate is completed by pepsin enzyme.
Rennin is important for infants, as the formation of milk clot prevents the
rapid passage of milk from the stomach. This gives a sense of fullness.
Rennin is absent in the stomach of adults.
3.
In the small intestine
On entering the small intestine, large polypeptide produced in the
stomach by the action of pepsin are further cleaved to oligopeptide and
amino acid
A. Pancreatic Secretions:
(pH:8)
1Trypsin and chymotrypsin
Enteropeptidase formerly called trypsin. It is secreted in the form of
inactive precursor which is called
trypsinogen.
Trypsinogen is converted to trypsin first by enterokinase (an enzyme
synthesized by and present on the luminal surface of the intestinal
mucosal cells of the brush boarder membrane) by removal of hexapeptide
from the NH2-terminus of trypsinogen, then trypsin itself activates
trypsinogen (autoactivation)
Trypsinogen
Enterokinase
Trypsinogen------------------------------Trypsin
Trypsinogen------------------------------Trypsin
Chymotrypsinogen------------------Chymotrypsin
Procarboxypeptidase-------------carboxypeptidase.
*
Action of trypsin
1.
It is an endopeptidase hydrolyzing the bonds between basic amino
acids as arginine and lysine. So, the products of trypsin digestion are
polypeptide chains
Protein------Proteoses------Peptone---------Polypeptid.
2Trypsin acts as an activator for chymotrypsinogen and
procarboxypeptidase
converting them into chymotrypsin and
carboxypeptidase.
41
*
Action of chymotrypsin :
It acts on peptide bonds of uncharged amino acids as aromatic amino
acids.
2Carboxypeptidase
* It is an exopeptidase i.e. acts on the periphery of polypeptide chains
produced by the action of endopeptidases.
It acts on the peptide bond at the free-COOH of the polypeptide chain
liberating single amino acid.
3Collagenase
It is an enzyme that catalyzes the, hydrolysis of collagen.
B. Intestinal Secretion
1.
Aminopeptidase
It is an exopeptidase, acting on the peptide bond at the free -NH2 of the
polypeptide chain liberating single amino acid.
Hormones that stimulate gastrointestinal secretion
1. Gastrin
*
This hormone is produced by gastric cells.
Secretion of this hormone is stimulated by the presence of protein in the
stomach.
It stimulates pepsin secretion and the release of intrinsic factor from
gastric mucosa.
2. Cholecystokinin - pancreozymin ( CCK-PZ ):
*
This hormone is produced by the mucosa of small intestine.
Secretion of this hormone is stimulated by the presence of protein, fat and
their digestion products in the intestine.
It stimulates the secretion of pancreatic enzymes and contraction of the
gall bladder.
3.Secretin
*
This hormone is produced by the duodenurn and jejunum.
Secretion of this hormone is stimulated by the low pH in the duodenum.
It stimulates the secretion of pancreatic bicarbonate (HC03 ) and helps the
action of CCK-PZ hormone.
42
Digesti0n of Nucleoproteins
Nucleoproteins are digested by a group of enzymes produced by intestinal
mucosa hydrase, nuclease, phosphatase and nucleosidase.
Hydrase
Nucleoproteins--------------------------------------- Proteins + Nucleic acid
Nuclease
Nucleic acid -----------------------------------------Nucleotides
Phosphatase
Nucleotides------------------------------------------Nucleosides + Phosphate
Nucleosidase
Nucleosides -------------------------------------------------Nitrogenous base+
Sugar
ABSORPTION
Under normal conditions the dietary proteins are almost completely
digested into amino acids, which are then rapidly absorbed from the
intestine into the portal blood.
Site of Absorption : Jejunum and ileum
Mechanism of amino acids absorption
1.
Active transport
L-Amino acids are actively absorbed i.e. energy is required, which is
derived from sodium pump.
They are absorbed by specific carrier protein present in small intestine by
mechanism similar to that of glucose absorption i.e. the carrier has a site
for an amino acid and another site for sodium
There are 5 or more different amino acid carrier systems, each of which
can transport a group of closely related amino acids.
These groups are
1Small neutral amino acids.
2Large neutral
amino acids.
3Basic amino acids.
4Acidic amino
acids.
5Imino acids.
There is a competition for absorption between the amino acids of each
43
group i.e. amino acid fed in excess can retard the absorption of other
amino acids of the same group.
44
Pyridoxal (vit. B6) and Mn++ play a role in amino acids absorption.
*
There is another hypothesis for the mechanism of transport of
amino acids into the cells in which glutathione is shared. Clutathione acts
as a carrier for amino acids. absorption of amino acids by this mechanism
utilizes 3 ATP molecules.
*
PLASMA AMINO ACIDS
Fed state (after meal)
*
After the ingestion of a protein rich meal, the amino acids are
absorbed from the intestine via the portal circulation to the liver. The
branched chain amino acids account for about 20% of the total amino
acids present in dietary protein and absorbed to the liver.
*
Amino acids are then released from the liver to the systemic
circulation causing an increase in the. plasma amino acids level. The
released branched amino acids account for about 60% of the total amino
acids entering the systemic circulation. This means that liver utilizes most
of the non branched amino acids.
*
Muscles extract. the amino acids (mainly the branched chain amino
acids) where they undergo
1-Oxidation for energy production
2-Transamination with pyruvate to give alanine, Alanine then may be
released to be converted to glucose in liver.
CIRCADIAN CHANGES
The plasma level of most amino acids does not remain constant
throughout 24 hours day. It varies from 4 to 8 mg/100 ml plasma. This
depends upon the nutritional state whether it is post-absorption or fed
state. This is called a circadian changes of amino acids.
Postabsorptive level
i.e. 12 hours after last meal.
The plasma amino acids tend to be decreased. Amino acids are released
from endogenous protein stores into plasma. These stores are mainly
muscles and kidneys
Muscle : alanine and glutamine account for more than 50% of the
total amino acids released from muscle tissue, valine is also released
kidney is the major source of release of serine. In addition, the
kidney release small, but significant amount of alanine.
The released amino acids are taken up
45
Valine is taken up mainly by the brain, where it is utilized.
Glutamine is taken up by: gut and kidney: In the gut it is converted
into glutamate or free ammonia. In the kidney glutamine may give rise to
ammonia.
Serine: is taken up by the liver.
Alanine: is taken up by the liver. It is the main glucogenic amino
acid i.e. amino acid that can be converted to glucose..
Fate of Absorbed Amino Acids
The absorbed amino acids may undergo one of the following fates:
1.
Protein biosynthesis:
The amino acids are incorporated into proteins as plasma proteins, tissue
proteins, enzymes and polypeptide hormones.
2.
Small peptide synthesis
As synthesis of the glutathione tripeptide.
3.
Synthesis of specialized products
Each amino acid can synthesize certain substances in the body e.g. phenyl
alanine gives epinephrine, tryptophan gives serotonine.
4.
Excretion of amino acids as urea
Conversion of amino acids into urea includes the following processes
a-Transamination
bDeamination
C-Reactions of urea cycle.
Excretion of Amino Acids as Urea
The first step in the catabolism of amino acids involves the removal of α–
amino acids. Once removed, this nitrogen can be incorporated into other
compound and excreted
This occurs by:I.
Removal of -NH2 group as NH3 (ammonia) by:
Transamination
- Deamination ( Oxidative or Non-Oxidative)
- Transamination followed by deamination
II.
Transport of the produced ammonia.
III.
Urea formation (by reactions of urea cycle)
I. Removal of –NH2 group
1. Transamination
46
The first step in the catabolism of most amino acids is the transfer of their
–amino group to –ketoglutarate. The products are –keto acid derived
from the original –amino acids and glutamate. This transfer is catalyzed
by family of enzymes called aminotransferase.
Mechanism
Transamination is catalyzed by enzymes termed transaminase or
aminotransferase and Pyridoxal phosphate (vitamin-B6) is the coenzyme
of the transaminases enzymes.
All amino acids may undergo transamination except lysine, therionine,
proline and hydroxy proline
All transamination reactions are reversible so during amino acid
catabolism, this enzyme functions in the direction of glutamate synthesis.
Thus glutamate acts as collector of nitrogen from amino acids.
Among all transaminases :- Two are present in most mammalian tissues
and they are of clinical importance Glutamate transaminase and alanine
transaminase.
1Alanine transaminase
This enzyme catalyzes the transfer of amino groups from -aminoacids to
pyruvate to form alanine-e.g.
--ketoglutarate+ Alanine ----------------------- Glutamate
+
Pyruvate
This transaminase has 4 names
1.
Alanine transaminase. or
2.
Alanine pyruvic transaminase. or
3.
Alanine aminotransferase. or
4.
Glutamate puryvic transaminase (GPT)
2-
Glutamate transaminase :
This enzyme catalyzes the transfer of amino group from most amino
acids to --ketoglutarate to form glutamate: e.g.
-ketoglutarate+
Aspartate------------------------------Glutamate
+Oxalacetate
This transaminase has also 4 names
1.
Glutamate transaminase or
2.
Glutamate -ketoglutarate transaminase
or
3.
Glutamate amino transferase or
4.
Glutamate oxaloacetate transaminase (GOT).
GOT and GPT are intracellular enzymes. Their level in blood is increased
47
in Myocardial infarction (ischemic heart disease) mostly GOT.
Acute hepatitis
(liver infection) mostly GPT.
Role of pyridoxal phosphate in transamination
Pyridoxal phosphate which is covalently linked to the έ–amino group of
a specific lysine residue at the active site of the enzyme.
Aminotransferases act as an intermediate carrier of the amino group
Glutamic acid + Pyrodoxal Phosphate--------------ketoglutarate+
Pyrodoxamine
Then
Pyrodoxamine +Pyruvic-------------------------Pyrodoxal Phosphate +
Alanine
Equilibrium of transamination reaction:- For most transamination
reactions, the equilibrium constant is near 1, allowing the reaction to
function in both directions: amino acid degradation through removal of
-amino group ( After high protein diet) and biosynthesis through
addition of amino group to -keto acids ( when amino acids are required)
2-
Deamination
Definition
It is. the removal of amino group from an amino acid.
Site : The liver and the kidney.
Non Oxidative Deamination
The enzymes of this mechanism catalyze the removal of the amino
groups of the hydroxy amino acids serine, homoserine and therionine.
e.g.
Oxidative Deamination
In contrast to transamination reactions. L-Amino acids which are
present in the proteins utilized by human are deaminated by 2
mechanisms:1.
All L-amino acids (except glutamate) are deaminated by L-amino
acid oxidase. This enzyme is present in a minimal amount in the liver and
kidney, thus this mechanism is of little value. The coenzyme is FMN
L-amino acid Oxidase
Water
Amino acid------------------------------------Imino acid--------------------Keto acid
NH3
48
2.
Glutamate is deaminated by the enzyme : L-glutamate
dehydrogenase, which is a highly active enzyme present in most
mammalian tissues. NAD or NADP is the co-enzyme
L-Glutamate Dehydrogenas
Glutamate----------------------------------------------Ketoglutarate + NH3
NADP
NADPH+H
The amino groups of most amino acids are funneled to -ketoglutarate by
means of transamination. Glutamate is unique in that, as it is the only
amino acids that undergo rapid oxidative deamination. Therefore,
sequential action of transamination ( collection of amino group from
amino acids to -ketoglutarate forming glutamate) and subsequent
oxidative deamination of that glutamate provide pathway whereby most
amino acids can be released as ammonia.
Functions of L-glutamate dehydrogenase enzyme
1Removal of -NH2 group as ammonia(NH3) of most amino acids
Removal of-NH2 group from most amino acids to form NH3 which is then
converted to urea is not done by direct deamination of amino acids, but
through
aTransamination with -ketoglutarate to form glutamate.
bThe product glutamate then undergoes deamination by glutamate
dehydrogenase to produce NH3
2Formation of non essential amino acids
The reaction catalyzed by glutamate dehydrogenase is reversible i.e. ketoglutarate can be reaminated by free NH2 to form glutamate. Then
glutamate can be also transaminated with any -ketoacid to form the
corresponding amino acid.
Regulation of L-glutamate dehydrogenase
*
ATP, GTP- and NADH+H+ allosterically inhibits glutamate
dehydro-genase.glutamate
*
ADP allosterically stimulates glutamate dehydrogenase
Sources and Transport of Ammonia
Sources of ammonia
1-Deamination of amino acids by different tissues.
2- Ammonia produced by the action of the intestinal bacteria on
a- Dietary protein.
bUrea present in fluids secreted into the
gastrointestinal
tract.
49
This ammonia is absorbed to the liver via the portal veins. In the liver it
is converted to urea. Thus portal blood contains high levels of ammonia
than systemic blood.
-Kidneys produce ammonia by renal tubular cells in cases of acidosis a mechanism for reg
B1ood Ammonia
Blood contains traces of ammonia ( 10 – 20 µg/l00 ml ) because the
ammonia which is constantly produced in the tissues is rapidly removed
from the circulation by the liver.
Fate of Ammonia
1-
Formation of glutamate
Glutamate Dehydrogenase
NH3 + -ketoglutarate----------------------------------Glutamate + ketoglutarate
Transamination
------------------------------------------Non-essential amino acid
2Glutamine formation
Glutamine synthetase is a mitochondrial enzyme present in the kidney
and the brain.
In the kidney
Glutamine is stored in tubular cells. Deamination of glutamine is the main
source of NH3 in kidney. This deamination reaction is catalyzed by
glutaminase enzyme.
Ammonia produced shares in the mechanism for regulation of acid-base
balance.
In the brain
Glutamine formation is the major mechanism for removal of ammonia in
the brain but this must be proceeded by the synthesis of glutamate. This is
because the supply of blood glutamate is inadequate in the presence of
high levels of blood ammonia.
-ketoglutarate is an intermediate of citric acid cycle, thus in high levels
of blood ammonia, this intermediate will be depleted in the brain 
Inhibition of citric acid cycle  No energy in the brain  ammonia
intoxication.
50
3Urea formation
Urea is the major disposal form of amino group derived from amino acids
and account for about 90% of nitrogen containing compound of the urine.
Therefore, it is the main end product of protein (amino acids)
metabolism.
Site : Liver
Blood urea : 20 - 40 mg/l00 ml.
Blood urea increased in
1)
High dietary protein.
2)
Kidney disease : failure to excrete urea.
3)
Gastro-intestinal haemorrhage as in peptic ulcer
Blood is a favorable medium for the growth and multiplication of bacteria
in the intestine.
Action of intestinal bacteria on proteins (plasma proteins + globin of Hb
leads to excessive NH3 production-------------------------- excessive urea
production by the liver more than capacity of the kidney to remove. This
leads to increase of urea in the blood.
Blood urea decreased in:
1)
In liver diseases.
Steps
This occurs by a series of reactions called urea Krebs cycle
NH3
Carbamoyl phosphate synthetases
+--------------------------------------------------------Carbamoyl Phosphate
CO2
2 ATP
2 ADP
Ornithine---------------------Citrulline
Aspartate
----------------------------Arginosuccinate
Arginine------Urea
51
Fumarate
---------------------------
52
One nitrogen of the urea is supplied by free NH3 and the other nitrogen
by aspartate. Glutamate is the intermediate precursor of both ammonia
through oxidative deamination ( Glutamate dehydrogenase) and aspartate
nitrogen through transamination of oxalacetate by aspartate
53
aminotransferase. The carbon and oxygen are supplied by CO2
54
Reactions of the urea cycle
The first two reactions leading to the synthesis of urea occur in the
mitochondria, whereas remaining cycle enzymes are located in the
cytosol . Glutamate dehydrogenase occurs in the mitochondria, providing
NH3 for incorporation into carbamoyl phosphate
1. Formation of carbamoyl phosphate
Ammonia incorporated into carbamoyl phosphate is provided by
oxidative deamination of glutamate. In addition, carbamoyl phosphate
synthetase 1 requires N-acetylglutamate and two molecule of ATP for
the activity.
2. Formation of citrulline
55
Ornithine is regenerated with each turn of the urea cycle is reacted with
carbamoyl phosphate forming citrulline. The release of high energy
phosphate of carbamoyl phosphate drives the reactions in the forward
direction.. Citrulline is transported to the cytosol.
3. Synthesis of argininosuccinate
Citrulline condenses with aspartate to form argininosuccinate. The –
amino group of aspartate provide the second nitrogen of the urea. This
reaction is driven by the cleavage of ATP to AMP and PP. This is the
third and final molecule of ATP consumed in urea formation.
4. Cleavage of the argininosuccinate
Argininosuccinate is cleaved to yield arginine and fumarate. Arginine
serves as precursor of urea. Fumarate is hydrated to malate and
transported to mitochondria re-enters tricarboxylic acid cycle or cytosolic
malate can be oxidized to oxalacetate which can be converted to aspartate
or glucose.
5. Cleavage of arginine
Arginase cleaves arginine to ornithine and urea. Arginase enzyme occurs
exclusively in the liver. Thus, other tissues can synthesize arginine, can
not produce urea, only liver can cleaves arginine, thereby synthesize urea.
Carbamoyl phosphate synthetases:1.
Carbamoyl phosphate synthetase I
Present in the mitochondria of the liver cells. Use ammonia NH 3 for the
formation of carbamoyl phosphate. It is activated by N-acetylglutamate
and Mg++ ions and Biotin are the coenzyme which carry CO2. 2
molecules of ATP are used in this reaction.
2.
Carbamoyl phosphate synthetase II
Present in the cytoplasm of the cells of all tissues.
Use glutamine as a source of NH2 for the formation of carbamoyl
phosphate.
This enzyme is used in pyrimidine biosynthesis.
Regulation of urea cycle:
N-acetyl glutamate is an essential activator for carbamoyl phosphate
synthetase 1 which is the rate limiting step in urea cycle. It is synthesized
by acetyl CoA + glutamate. The ammonia produced from glutamic acid
and hence from all amino acids by glutamate dehydrogenase is used by
carbamoyl phosphate synthetase to form urea
Transamination
Glut. Dehydro
Carbamoyl P
Amino acids-----------------------Glutamate----------------NH3----------------Urea
The intrahepatic concentrations of N-acetyl glutamate increases after
56
ingestion of high protein diet. Carbamoyl phosphate synthetase 1 acts
with mitochondrial glutamate dehydrogenase.
Fate of urea.
Urea diffuses from the liver and is transported in the blood to the kidney
where it is filtered and excreted in the urine. In patients with kidney
failure, plasma urea levels are increased Therefore, promoting a greater
transfer of urea from the blood to gut. The intestinal urease action on the
urea becomes important source of NH3 contributing to hyperammonia
seen in this patients.
Sources of ammonia
Ammonia is produced from metabolism of a variety of compounds.
Amino acids are quantitatively the most important source of ammonia
1. Amino acids : most tissues especially liver form ammonia from amino
acids by aminotransferase and glutamate dehydrogenase
2. From Glutamine The kidney forms ammonia from glutamine by the
action of renal glutaminase. Most of these ammonia are excreted as
ammonium
3. From Purine and pyrimidine Catabolism of purine and pyrimidine give
rise ammonia as the amino group attached are released as ammonia
4. From bacterial action in the intestine Ammonia is formed by the
bacterial degradation of urea in the lumen of the intestine
Causes of increased serum ammonia:
1.
Acquired hyperammonia
Cirrhosis of the liver caused by alcoholism, hepatitis or biliary tract
obstruction. Therefore NH3 can not be converted to urea by the diseased
liver cells.
2. Hereditory Hyperammonia
Result from deficiency of one of 5 enzymes of urea cycle especially
carbamoyl phosphate synthetase and ornithin transcarbamoylase. In each
case the failure to synthesize urea leads to hyperammonia during the first
week following birth
Mechanism of ammonia intoxication
The mechanism of ammonia toxicity may result from shift in the
equilibrium of glutamate dehydrogenase reaction
High blood ammonia will deplete -ketoglutarate in the brain 
Inhibition of citric acid cycle  No energy in the brain  Ammonia
intoxication.
Symptoms of ammonia intoxication
1Flapping tremors.
57
2Slurring of speech.
3Blurring of vision.
4Vomiting in infancy.
5Coma.
Relation between tricarboxylic acid cycle and urea cycle
1.
CO2 needed for urea formation is mainly produced in the course of
tricarboxylic acid cycle.
2.
Aspartic acid can give oxaloacetate and vice versa
(Transamination). Oxaloacetate is the key of Krebs cycle.
3.
Fumaric acid produced in urea cycle can be oxidized in
tricarboxylic acid cycle.
4.
ATP needed for urea cycle are derived from tricarboxylic acid
cycle.
Fate of -Ketoacids
The -ketoacids (the carbon skeleton) remaining after the removal of NH2 group by transamination or deamination of amino acids may be
1Reaminated by NH3 to form .again amino acids (glutamate
dehydrogenase).
2Converted either to carbohydrate intermediates. (13 amino acids)
or fat intermediates (2 amino acid) or both (5 amino acids.).
Those which are converted to carbohydrates are called glycogenic amino
acids. Those which are converted to fat are called ketogentic amino
acids.
Glycogenic
Clycogenic&ketogenic
Alanine
Arginine
Aspartic
Phenyl alanine
Cysteine
Tryptophan
Glutamic
Tyrosine
Glycine
Histidine
Hydroxyproline
Methionine
Proline
Serine
Threonine
Valine
.
Ketogenic
Leucine
Lysine
Isoleucine
Lysine
Amino acids: Metabolism of carbon skeleton
The catabolism of the 20 amino acids found in the proteins involves the
removal of –amino group followed by breakdown of the resulting
carbon skeleton.
58
The catabolism of the carbon skeleton converges to form seven products:
oxalacetate, -ketoglutarate, pyruvate, fumarate, acetyl CoA, acetoacetyl
CoA and succinyl CoA.
These products enter in the pathways of intermediary metabolism,
resulting either synthesis of glucose or lipids or in the production of
energy through their oxidation to CO2 and H2O by citric acid cycle.
Glucogenic and ketogenic amino acids:Amino acids can be classified as glucogenc or ketogenic according to
nature of their metabolic end products
1) Ketogenic:- Amino acids whose catabolism yields either actoacetate or
one of its precursor acetyl CoA or acetoacetyl CoA. ( Acetone,
Acetoacetate and β-hydroxybutyrate). Leucine and lysine are the only
59
exclusively ketogenic amino acid
2) Glucogenic amino acids: Amino acids whose catabolism yields
pyruvate or one of the intermediate citric acid cycle. These substrates are
substrate in gluconeogenesis and therefore can give rise to glycogen in
liver and muscle.
A. Amino acids that form oxalacetate
Asparagine is hydrolyzed by asparaginase, liberating ammonia and
aspartate. By transamination aspartate loses its -amino group forming
oxalacetate
B. Amino acids that form -ketoglutarate
1.
Glutamine is converted to glutamate and ammonia by glutaminase
enzyme. Glutamate is converted to -ketoglutarate by transamination or
by oxidative deamination by glutamate dehydrogenase
2. Arginine is cleaved by arginase to produce ornithine and urea (final
step in urea cycle). Ornithine subsequently undergoes transamination to
yield glutamate γ-semialdhyde which is converted to -ketoglutarate
3.
Proline is oxidized to –pyrroline 5-carboxylate which is then
oxidized to glutamate. Glutamate is converted to -ketoglutarate by
transamination or by oxidative deamination by glutamate dehydrogenase
3.
Histidine is deaminated and hydrolysed to form N-formiminoglutamate which donates its formimino group to tetrahydrofolate leaving
glutamate. Deficiency of folic acid lead to excretion of increased amount
of N-forminino glutamate
C.
Amino acid that form pyruvate
1.
Alanine loses its aminogroup by transamination
2.
Serine: can be converted to pyruvate by serine dehydratase
3.
Glycine can either be converted to serine by addition of a
methylene group from N5, N10 methylenetetrahydrofolate or oxidized to
CO2 and NH4
4. Cysteine is reduced to cysteine using NADPH+H which can
undergo desulfuration to yield pyruvate
5. Threonine can be converted either to pyruvate or -ketobutyrate
60
61
D.
Amino acids that form fumarate
1.
Phenylalanine and tyrosine. The first step in the catabolism of
phenylalanine is hydroxylation forming tyrosine. Then phenylalanine and
tyrosine merge leading to fumarate and acetoacetate. Therefore
phenylalanine and tyrosine are glucogenic and ketogenic.
E.
Amino acids form succinyl CoA
Degradation of methionine, valine, isoleucine and threonine results in the
production of succinyl CoA (TCA cycle intermediate )
1.
Methionine: this sulfur-containing aminoacid is converted to 5adenosylmethionine (SAM) major methyl group donor
Synthesis of SAM: Methionine condenses with ATP forming
62
SAM which is high energy containing compound
Activated methyl group: the methyl group attached to the
tertiary sulfur in SAM is activated and can be transfer to a variety of
acceptor molecule e.g methylguanidoacetic acid and converted to Sadenosylhomocysteine
Hydrolysis of S-adenosylhomocysteine: after donation of
methyl group S-adenosylhomocysteine is hydrolyzed into homocysteine
and adenosine
Homocysteine can combine with serine, forming cystathionine which is
hydrolyzed into -ketobutyrate and cysteine. This interaction has the net
effect of converting serine into cysteine
2. Valine and isoleucine are branched chain amino acids that yields
succinyl CoA
3. Threonine is dehydrated into -ketobutyrate which is oxidatively
decarboxylated into propionyl CoA and Succinyl CoA
63
F.
Amino acids that form acetyl CoA, AcetoacetylCoA
The following four amino acids form acetyl CoA or acetoacetylCoa
directly with -out pyruvate serving as an intermediate
1.
Leucine: It is exclusively ketogenic in its catabolism forming
acetyl CoA and acetoacetylCoA
2.
Isoleucine: It is both ketogenic and glucogenic because its
metabolite yields acety CoA and propionyl CoA
3.
Lysine : an exclusively ketogenic amino acids. It is unusual in that,
neither of its amino group can undergoes transamination as the first step
in the catabolism
4.
Tryptophan: It is converted to acetoacetyl CoA
5. Phenylalanine and tyrosine
Catabolism of the branched chain amino acid
The branched chain amino acids isoleucine, leucine and valine are
64
essential amino acids. They are metabolized primarily by peripheral
tissues (Muscle) rather than by the liver
A. Transamination
Removal of the amino groups of all three amino acids is catalyzed by
single enzyme branched chain -amino acid aminotransferase
B. Oxidative deamination
Removal of the carboxyl group from -ketoacid derived from leucine,
isoleucine and valine is catalyzed by single enzyme complex, branchedchain-ketoacid dehydrogenase.
An inherited deficiency of branched-chain -ketoacid dehydrogenase
results in accumulation of the branched chain -ketoacid substrate in
urine. Their sweet odour prompted the name maple-syrup urine disease
C. Dehydrogenation
Oxidation of the acyl CoA products yields either -β unsaturated acyl
CoA derivative
The catabolism of
isoleucine yields acetyl CoA and succinyl CoA rendering it both
ketogenic and glucogenic amino acids
Valine yields succinyl CoA
Leucine yields acetoacetate
65
Biosynthesis of non essential amino acid
Non essential amino acid can be synthesized in sufficient amounts from
the intermediates of metabolism or from essential amino acids as in
tyrosine
A. Synthesis from -ketoacid
Alanine, aspartate and glutamate are synthesized by transfer of an
aminogroup to -ketoacid, pyruvate, oxalacetate and -ketoglutarate
respectively
Glutamate is unusual in that it can also be synthesized by the reverse of
the oxidative deamination by glutamate dehydrogenase
B. Synthesis by amidation
Glutamine contains an amide linkage with ammonia at carboxyl group. It
is formed from glutamate by glutamine synthetase. This reaction is driven
by hydrolysis of ATP
ATP
ADP
-ketoglutarate----------glutamate-------------------------Glutamine
NH3
This reaction can be considered as major mechanism for detoxification of
NH3 in brain and liver
66
Asparagine: this amino acid contains an amide linkage with NH3 at βcarboxyl. It is formed from aspartate by asparagines synthetase
C. Tyrosine
Tyrosine is formed from phenylalanine by phenylalanine hydroxylase.
The reaction requires molecular oxygen and the co-enzyme
tetrahydrobiopterin. During the reaction, tetrahydrobiopterin is oxidized
to dihydrobiopterin. Tetrahydrobiopterin is regenerated from
dihydrobiopterin in separate reaction requiring NADPH
D. Proline
Glutamate is converted to proline by the intermediate glutamate γ–
semialdhyde which cyclize to form –pyrroline 5 caboxylate by reduction
yields proline
E. Serine, glycine and cysteine
Serine arises from 3-phosphoglycerate which is first oxidize to 3phosphopyruvate and then transaminated to 3-phosphoserine. Serine is
formed by hydrolysis of phosphoester. It is also formed from glycine by
transfer of hydroxymethyl group
Glycine It is synthesized from serine by removal of hydroxymethyl group
Cysteine: It is synthesized by two consecutive reactions in which
homocysteine combine with serine forming cysthionine which in turn
hydrolyzed to α–ketobutyrate and cysteine
Conversion of
Amino Acids to Specialized Products
In addition to serving as building blocks for proteins, amino acids are
precursors of many nitrogen containing compound that have important
physiological functions
I. Glycine
Glycine is a glycogenic and non essential amino acid.
In the body, it may be enter in one of the following pathways:
1.
Synthesis of purine bases:
Carbon 4, carbon 5 and N 7 are derived from glycine.
2Synthesis of glutathione
Glutathione is a tripeptide. Formed of 3 amino acids: glycine, cystein and
glutamic
It is sometimes called : glutamyl , cysteinyl glycine
Steps of synthesis
2 ATP molecules are utilized , one for each peptide bond formed.
67
Forms of glutathione
Reduced form G-SH
Oxidized form G-S-S-G
G-S-S-G is converted to G-SH by the enzyme glutathion reductase and
NADPH produced in the first step of HMP shunt.
*
Functions of glutathione
aProtection of cell membranes against oxidizing agent e.g.
glutathione protects RBCs from haemolysis
bGlutathione acts as a component of an amino acid transport system.
e.g. it has a role in absorption of amino acids.
cIt acts as an activator for certain enzymes.
dGlutathione inactivates insulin hormone in the presence of insulinglutathione transhydrogenase
3Synthesis of creatine :
Creatine is a methylguanidoacetic acid. It is formed from 3 amino acids :
glycine , arginine and methionine.
68
*
Creatine is formed in both the kidney (transamidation),and the liver
(transamethylation) to be stored in the muscles in the form of creatine
phosphate. Creatine phosphate is high energy compound used in muscle
contraction.
69
Creatine phosphokinase CPK CK
This is an enzyme which catalyzes the conversion of creatine to
creatine phosphate
Serum CPK level increases in muscle and heart diseases.
4-
Bile salts formation
Glycine is conjugated to cholic acid to form glycholic acid (one of
bile acids).
5Synthesis of haem
Haem is the pigment which together with globin form the haemoglobin
These reactions occur in the mitochondria. Then AmLev passes to cytosol
to give haem.
2 molecules of ( AmLev )  Propobilinogen  Haem.
AmLev synthetase is an enzyme present in the mitochondria of liver cell.
It is the key enzyme in the synthesis of haem.
Pyridoxal phosphate PO4) is the co-enzyme for decarboxylation and also
to activate glycine.
6Synthesis of serine
The tetrahydroxy folic acid (FH4) acts as a one carbon atom donnor
7- Synthesis of glyoxylic acid
Glycine may be converted to glyoxylic acid by
aDeamination of glycine
70
bTransamination with α-ketoglutarate
The glyoxylic acid either reaminated again to glycine or:oxidized to
formic acid.
Failure of these 2 reactions leads to the oxidation of glyoxylic acid to
oxalate.
Excretion of excess amounts of oxalate in the urine is called primary
hyperoxaluria. It may leads to the formation of calcium oxalate stone.
8Detoxication reactions
Glycine is used in detoxication. It is conjugated in the liver with benzoic
acid (which is toxic) giving hippuric acid which is excreted in the urine.
Glycine catabolism
Breakdown of glycine is mainly done by the action of the enzyme glycine
cleavage system.
II. Phenyl Alanine and Tyrosine
Phenyl alanine is essential amino acid.
Tyrosine is non essential amino acid because it is formed in the body
from phenyl alanine.
Both amino acids are glycogenic and ketogenic amino acids.
Phenyl alanine may enter in one of the following metabolic pathways
1Protien biosynthesis.
2Tyrosine formation
Phenyl alanine is converted to tyrosine by the enzyme phenyl alanine
hydroxylase
Tyrosine may give the following compounds
1Epinephrine and nor epinephrine.
2. Melanine pigments.
3. Thyroxine.
71
1*
a.
bcd*
abcde-
Epinephrine and nor epinephrine
These are 2 hormones secreted by adrenal medulla.
They are the most potent stimulant of adrenergic α and β receptors
(sympathomimetic). This leads to
Increase of heart rate and force of contraction.
Vasoconstriction.
Relaxation of smooth muscles of bronchi and intestine.
Stimulation of glycogenolysis and lipolysis (through cAMP).
Epinephrine is used in the treatment of
Bronchial asthma
Acute allergic diseases.
Open angle glaucoma.
Heart block.
As topical and local vasoconstrictor.
*
Pheochromocytoma
This is a benign tumor affecting the cells of adrenal medulla.
The affecting cells secrete abnormal amounts of epinephrine and nor
epinephrine. This leads to
aHypertension which may be paroxysmal.
bAttachs of palpitation , headache , nausea dyspnea , anaxicety
pallor and profuse sweeting.
Synthesis of epinephrine and nor epinephrine
This occurs in adrenal medulla and neurons
Tyrosinase
DOPA-Decarboxylase
Tyrosine-----------------Dihydroxy phenyl alanine (DOPA)----------------------------Oxidase
Transferase
Dopamine---------------------------Nor-epinephrine---------------------Epinephrine
72
73
2.Melanin pigments
These are dark brown to black compounds, derived from tyrosine.
They are normally present in melanocytes (pigment cells of skin hair and
iris).
They give the characterestic colour of skin , hair and iris.
Synthesis of melanin
This occur in melanocytes
Tyrosinase
DOPA Oxidase
Tyrosine----------------DOPA------------------------DOPA quinone----------Melanine
74
3-Thyroxine
This is a hormone that produced by the thyroid gland.
Synthesis of thyroxine
Synthesis of thyroid hormone begins by iodination of the tyrosine radicals
which are present in thyroglobulin protein. This protein is present in the
thyroid follicles and contains carbohydrate (glycoprotein).
lodinated thyroglobulin is then brokendown releasing T3 and T4. On
stimulation of thyroid gland by TSH , T4 and T3 are released into plasma.
75
Within the plasma 99.95 of T3.and T4 are transported inassociation of 2
proteins (thyroine binding proteins)
aThyroxine binding gIobulin :
It is the major transporter.
bThyroxine binding prealbumin
About O.O57% of T4 and T3 are present in the free unbound state. Free
T3 and T4 are the metabolically active hormones in the plasma.
If tyrosine is not needed for the synthesis of epinephrine nor epinephrine
,melanin and.thyroxine , it is oxidized to homogentisic acid fumaric acid
and acetoacetic acid.
Inborn errors of phenyl alanine and tyrosine metabolism
1Phenylketonuria.
2Tyrosinosis.
3Alkaptonuria.
4Albinism.
1- Phenylketonuria:
This is a condition resulting from congenital deficiency of phenyl-alanine
hydroxylase. Tyrosine will not be formed. Phenylalnine is increased in
the blood and converted to phenyl pyruvic and phenyl lactic which appear
in the urine.
Effect of phenylketonuria
The infants and children affected with phenylketonuria:developmental
retardation for unknown reason.
Diagnosis
The plasma of every newly born infant is routinly tested for elevated
phenyl alanine 4 days after birth.
Prevention :
76
Children are maintained on a diet containing a very low level of phenyl
alanine. This diet can be terminated at 6 years of age , when high
concentration of phenyl alanine can not cause injury to the brain.
2-Tyrosinosis:
Deficiency of parahydroxy phenyl pyruvic acid oxidase. This results in
excretion of large amount of parahydroxy phenyl pyruvic acid in the
urine.
77
3-Alkaptonuria:
Condition resulting from deficiency of homogentisic acid oxidase.
homogentisic acid will accumulate and appear in the urine.
When homogentisic acid is voided with the urine it is oxidized in the air
to a black pigment.
Effect
Generalized pigmentation of connective tissue (ochronosis) and arthritis.
4- Albinism:
Deficiency of tyrosinase enzyme in melanocytes , so the body can not
synthesize melanine pigments. i.e. skin is whitish , hair is also whitish
and iris is colourless.
III. Tryptophan
Tryptophan is an essential amino acid. It is glucogenic and ketogenic
amino acid.
Tryptophan has the following metabolic pathways
1Serotonin formation
Serotonin is a substance present in the brain , platelets , intestine and
mast cells.
Action of serotonin
Vasoconstriction of blood vessels.
Contraction of smooth muscle fibers.
Stimulatory transmitor in the brain.
78
*
Mono amine oxidase(M.A.O.):
An enzyme which oxidize serotonin to 5-hydroxy indole acetic acid
which passes to the urine. Inhibition of this enzyme by some drugs (e.g.
iproniazid) leads to accumulation of serotonin and Psychic stimulation.
Serotonin may be converted to a substance called melatonine which is Nacetyl,5 methoxy serotonin; It is found in the tissue of pineal body in the
brain. It appears to depress the gonadal function. Melatonine is
conjugated with sulphate in the liver then excreted.
2Niacin (nicotinic acid) formation
Niacin is a memberof vit. B complex. Every 60 mg of tryptophan are
converted to 1 mg niacin.
People depending on diet deficient in either tryptophan or niacin (e.g.
maize) will be suffered from a disease called pellagra.
End product of tryptophan metabolism is indole and skatol which are
excreted in the stool giving its characteristic odour. Indole may be
absorbed to the liver, hydro -xylated and conjugated to form indican
which is excreted in the urine.
79
80
IV. GLUTAMIC ACID
81
It is a glycogenic amino acid and it is non essential amino acid.
It is important in transamination.
Glutamic acid enters in the formation of
1-Glutathione
It is a tripeptide formed of 3 amino acids glycine cysteine and glutamic
acid
2-Folic acid
It is a member of vit. B-complex formed from glutamic acid, paraaminobenzoic acid and pteridine ring.
3-Gamma amino butyric acid = GABA
It is formed from glutamic acid by decarboxylation. GABA is an
inhibitory transmitter in the brain and its deficiency leads to convulsions
especially in children
4-Glutamine :
it is formed by combination of glutamic acid with ammonia.
Functions of glutamine
A) Glutamine is stored in the kidney and by the action of glutaminase
enzyme it gives ammonia and glutamic acid again. Ammonia is used in
the regulation of acid-base balance.
B)
Glutamine is the source of N3 N9 of purines.
C)
Glutamine formation in the brain removes excess NH3 formed in
the brain
D)
Glutamine is used in detoxication of phenyl acetic acid.
V. Sulphur Containing Amino Acids
These are Cysteine , cystin and methionine.
Methionine is essential amino acid but cysteine and cystin are formed in
the body.
All are glycogenic amino acids.
Cysteine
It enters in the formation of
1.
Glutathione
82
2.
Taurine
formed in the liver.
Taurine is conjugated to bile acid to form one of the bile salts.
3- Thioethanolamine
Thioethanolamine is a constituent of CoASH fatty acid synthetase,
mevalonate synthetase and other enzymes related- to lipid metabolism.
4- Cystin
Cystin is produced from 2 molecules of cysteine by oxidation.
4. Many hormones as insulin are rich in cysteine. Also , many enzymes
contain in their active center -SH group derived from cysteine.
6- H2S and sulphate prbduction
-Sulphite oxidase will result in presence of H2S and SO2 in expired air
and it causes mental retardation.
Methionine
It is essential amino acid it is important for
a- Entering in structure of protein synthesis.
b- Its conversion to S-adenosyl methionine , the main methyl group
donner in the body.
c.Its conversion to homocysteine and cysteine.
83
Transmethylation
It is a transfer of methyl group from a methyl donner to a methyl
acceptor.
Methyl donner : S-adenosylmethionine
the main methyl group
donner.
Methyl acceptor
e.g. Nor-adrenaline
CH3
Adrenaline.
Uracil
CH3
Thymine.
Guanidoacetic acid
CH3
Creatine.
Ethanolaminehydrate
CH3
Choline.
VII. SERINE
Glycogenic , non essential amino acid. It enters in:
1- Ethanolamine and choline base :
2- Sphingosine
Sphingosine is formed from serine and palmityl CoA
3- Cysteine
It is formed of serine and methionine as follows:
84
4-Phospholipids and phosphoproteins
Serine is an important constituent of phosphoprotein e.g. casein and
phospholipids e.g. cephaline.
DYNAMIC STATE OF BODY PROTEINS
All the body proteins with the exception of collagen, are in a constant
state of degradation and resynthesis. The total amount of protein
synthesized in the body is much greater than that degradated This
provides the body needs of protein for
1Growth
2Replacement of cells after their death and
3For milk protein during lactation.
PROTEIN REQUIREMENT
An adult man needs 1 gm/kg body weight/day and he synthesizes about
.400 gram of protein each day.
The exact daily protein requirement depends on many factors
1-Type of protein
a-Protein of high biological value
It contains all essential amino acids and are easily digested.
b.Good quality protein
It does not contain all essential amino acids.
c.Proteins of plants
Are deficient of most amino acids and are not easily digested.
2-Age , sex and lactation
Children lactating females and women need more protein than adult
males do.
NITROGEN BALANCE
Nitrogen balance means that nitrogen intake is equal to nitrogen loss from
the body
Nitrogen intake
Nitrogen is taken in the form of proteins of diet. Every 100 gm proteins
contain 16 gm of nitrogen.
Nitrogen loss
Nitrogen is lost from the body
85
1-
23-
In the urine
Urea
20 - 40 gm/day.
Uric acid
0.7
gm/day.
Hippuric acid
0.7
gm/day.
Ammonia
0.7
gm/day.
Creatinine 0.7- l.7 gm/day.
Creatine
0-0.2 gm/day.
In the feces One gram/day is excreted in the feces.
In milk and menstrual fluids in female.
POSITIVE AND NEGATI\'E NITROGEN BALANCE
Positive nitrogen balance
It means that nitrogen intake is greater than nitrogen loss. It occurs in
conditions where the formation of tissue protein is increased e.g. growing
children and muscle training.
Negative nitrogen balance:
It means that nitrogen intake is less than nitrogen loss. It occurs in
conditions where the breakdown of tissue protein is increased e.g.
Diabetes mellitus and starvation.
FUNCTIONS OF PROTEIN IN THE BODY
1Protein enters in structure of every body cell.
2- All enzymes are protein in nature.
3- Some hormones are protein in nature e.g. pituitary hormones ,
glucagon and insulin.
4- Some hormones are amino acids derivatives e.g. nor-epinephrine,
epinephrine and thyroxine.
5- Nucleic acids (RNA and DNA) are present in the form of
nucleoproteins?
6- Plasma proteins have many functions including immunity , blood
coagulation and fluid exchange.
HYPOPROTEINOSIS
This is a condition due to dietary deficiency of proteins.
In adult
This leads to hypoproteinaemia with subsequent oederna , weakness of
muscle , anaemia and increased susceptibility to infections.
In infants
a- Kwashiorker
This is a disease resulting from deficiency of dietary protein only.
It leads to growth retardation, anaemia, and anorexia.
b- Marsmus
This is a disease resulting from deficiency of dietary protein together with
86
dietary carbohydrate and fat.
87
Enumerate 5 of the following
1. Essential amino acid
2. Non polar side chain amino acid
3. Conjugated proteins
4. Amino acids involved in urea cycle
5. Biologically active compound derived from glycine
6. Metabollic errors of aromatic neutral amino acid
88
Lipid Metabolism
DIGESTION, ABSORPTION AND TRANSPORT OF
LIPIDS
An Adult ingests 60 ----150 g of lipid/ day more than 90% of
which is triglycerids (TG). The remainder is made up of
cholesterol, cholesterol ester, phospholipids and free fatty acids.
DIGESTION OF DIETARY FATS: Order of events that
happens in digesting fats
A-Limited digestion in mouth and stomach
Milk lipids can be degraded by the gastric lipase (active at
neutral pH), not used in adult but has a role in infant.
In adults dietary lipids are not digested to any extent in mouth or
the stomach.
B. Emulsification of dietary lipids in the small intestine:
Physical (not chemical) breakdown of fats. Since the lipids are
water insoluble, therefore, enzymatic hydrolysis of lipids can
occur only on the surface of the lipids droplet.
Mixed Micelles: Emulsification of lipids in the GI-Tract leads to
mixed micelles.
Mixed Micelles are partially degraded lipids with detergent-like
properties. They contain bile acids, dietary lipids and
phospholipids, fat-soluble vitamins, cholesterol, etc.
Chemical breakdown: Pancreatic Lipolytic enzymes break down
triacylglycerols to individual fatty acids. These enzymes are
hormonally regulated as follows:
-Cholecystokinin (CCK; pancreozymin) is a small peptide
hormone secreted
from lower duodenum in response to
presence of lipids. It causes the gall bladder to contract
releasing the bile and digestive enzymes from pancrease.
-Secretin: small peptide hormone released from intestine in
response to low pH of the chime. It stimulates bicarbonates
89
secretion from pancrease leading to neutral pH required for
enzymatic activity.
-Co-lipase: is a second protein secreted from pancrease helps
to anchor and stabilize the lipase enzyme.
1) Triacylglycerol degradation
Lipase
Degrades triacylglycerols. The proenzyme (zymogen) is
activated by the presence of bile acids, phospholipids, or
regulatory enzyme called co-lipase.
90
2)Cholesterol ester degradation
Cholesteryl Ester Hydrolase Degrades dietary cholesterol,
usually present in the form of cholesteryl esters. The
proenzyme is activated by bile salts.
3)Phospholipids degradation
Phospholipase A2
Degrades phospholipids. The proenzyme (zymogen) is
activated by trypsin and Ca+2
The enzyme can remove fatty acid at carbon 2 of
phospholipids leading to lysophospholipids
91
ABSORPTION: Individual fatty acids (not triacylglycerols)
free cholesterol and 2-monoacylglycerol are absorbed
through the brush border.
Resynthesis of triacylglycerol& Cholesterol esters by
Intestinal Mucosal Cells: Once inside intestinal enterocytes,
triacylglycerols and cholesteryl esters are reformed.
92
Secretion of lipids from intestinal mucosal cells
TG and cholesterol are very hydrophobic, must be packaged in a
soluble form. This form consists of lipids surrounded by thin
layer of proteins, phospholipids and free cholesterol. The newly
formed triacylglycerol droplets are then assembled into a
chylomicron:
CHYLOMICRON ASSEMBLY: Chylomicron is a form of
lipoprotein, consisting of lipid and protein. This assembly
occurs in the intestinal epithelial cells.
Secretion of Chylomicron: it is transport into the lymphatic
system for the most part. Chylomicrons always travel in lymph
to the extrahepatic tissues -- except short chained fats.
Short-chained fats (up to 10 carbons): The majority of these fats
are carried through the portal circulation back to the liver -- not
the lymph! Here is reason that fetuses utilize short-chains fats -they are absorbed and sent directly to the infant's liver, rather
than going through lymphatic system.
Long-chained fats: (greater than 12 carbons): The majority of
them are carried through the lymphatic system and thereby
distributed throughout the body.
Fats travel to liver: Fats make their way back to liver, from
lymphatic or portal circulation. They then utilize lipoproteins
(see below) to get distributed to target tissues.
Utilization at target tissue: At the target tissue, fats are broken
back down again, using lipoprotein lipase.
Lipoprotein Lipase: Triacylglycerol ------> Monoacylglycerol +
2 Fatty Acids.
FATE OF ABSORBED LIPIDS
A-Uptake by tissues
The absorbed chylomicrons are taken up by the tissues after
hydrolysis to glycerol and free fatty acids. This requires the
93
enzyme lipoprotein lipase present in the vascular endothelium of
extrahepatic tissues particulary in adipose tissues. This enzyme
hydrolysis 90% of the TG in chylomicrons leaving remnant
chylomicron.
B- Utilization by tissues
1-Oxidation: Fatty Acids------------acetyl CoA------Krebs cycle--CO2 + water +energy
2-Conversion to glucose : glycerol-------glycerol phosphate----reverse of glycolysis--------glucose
3- Formation of tissues fat: all tissues can synthesize all their
requirement of structural fat
C- Storage
Adipose tissues take up and stores a large part of the absorbed
fat. As a store of energy because fat is better than carbohydrates
and proteins in its caloric value and its associated with less
water in storage
Types of body lipids
1- Tissue lipids: This includes structural and functional
lipids. They includes lipids present in cell membrane,
mitochondria , lysosomal, and plasma membrane. They consist
of phospholipids, glycolipids and cholesterol with little
triglycerides. It is generally rich in unsaturated fatty acids.
Tissues lipid does not changes in composition or amount with
changes in the nutritional state and hence termed constant
element
2- Depot Lipids: They are mainly fat TG that is stored in
adipose tissues. It is principle reservoir of energy. It is vary
according to nutritional state of the individual and hence termed
variable element
Lipogenesis
Lipogenesis is the biosynthesis of triacylglycerols . It
occurs in most tissues specially adipose tissue, liver and
lactating mammary glands. It is concerned with the storage of
excess glucose after a high carbohydrate meal and also provides
a large part of milk fat. The biosynthesis of TG requires:
94
Biosynthesis of glycerol phosphate .
Biosynthesis of fatty acids .
Glycerol + FA's
Triglycerides .
I- Biosynthesis of glycerol (glycerol-3-phosphate) :
Reduction
Glycolysis
Glucose
dihydroxyacetone
glycerol-3-p
NADHH+
II- Biosynthesis of Fatty acids :
Like many other degradative and synthesis processes ( e.g.
glycogenolysis and glycogenesis ), fatty acids synthesis
(lipogenesis) was formerly considered to be merely the reversal
of oxidation within the mitochondria . However, it is now
apparent that a highly active extramitochondrial system is
responsible for the complete synthesis of palmitate from acetylCoA in the cytosol. Another system for fatty acids chain
elongation is also present in the liver endoplasmic reticulum.
There are three enzyme systems that are involved in the
biosynthesis of fatty acids from acetyl-CoA (resulted from
glucose by glycolysis). These include extramitochondrial,
mitochondrial, and microsomal systems.
1- Extramitochondrial ( cytosolic ) or De novo systems :
This is the main pathway for de novo synthesis of fatty
acids in the cytosol. It is present in many tissues including liver,
kidney, lung, mammary glands and adipose tissues. It needs the
following precursors and cofactors.
Acetyl-CoA.
NADPH.
ATP, Mn2+
HCO3- (as a source of CO2)
Biotin (co-carboxylase) .
The end product of de novo system is free palmitate (C16)
Sources of reduced NADP ( NADPH+H) required for de
novo synthesis of FA :
Hexose monophosphate shunt (G-6-p dehydrogenase and 6-PG
dehydrogenase reactions) .
95
Extramitochondrial isocitrate dehydrogenase reaction: This
reaction is the same that occurs in the mitochondria ( in Kreb's
cycle ) but use NADP instead of NAD as co-dehydrogenase .
Acetyl-CoA is the Principal Building Block of Fatty Acids
It is formed from carbohydrate via the oxidation of
pyruvate within the mitochondria. However, acetyl CoA does
not diffuse readily into the extramitochodrial cytosol, the
principal site of fatty acid synthesis.
The activity of the extramitochondrial ATP –citratre lyase, like
the "malic enzyme" increase in activity in the well-fed state,
closely paralleling the activity of the fatty-acid synthesizing
system. It is now believed that utilization of glucose for
lipogenesis is by way of citrate. The pathway involves
glycolysis followed by the oxidative decarboxylation of
pyruvate to acetyl-CoA within the mitochondria and subsequent
condensation with oxaloacetate to form citrate, as part of the
citric acid cycle. This is followed by the translocation of citrate
into the extramitochondrial compartment via the transcarboxylate transporter, where in the presence of CoA and ATP
,it undergoes cleavage to acetyl-CoA and oxaloacetate catalyzed
by ATP-citrate lyase . The acetyl-CoA is then available for
malonyl-CoA formation and synthesis to palmitate. The
resulting oxaloacetate can form malate via NADH-linked malate
dehydrogenase, followed by the generation of NADPH via the
malic enzyme. In turn, the NADPH becomes available for
96
lipogenesis. This pathway is means of transferring reducing
equivalents from extramitochondrial NADH to NADP .
Alternatively, malate can be transported into the mitochondrion,
where it is able to reform oxaloacetate . It is to be noted that the
citrate ( tricarboxylate ) transporter in the mitochondrial
membrane requires malate to exchange with citrate .
Eight molecules of acetyl-CoA are used for the formation of one
mole of palmitic acid (C16). Seven of the 8 mole of acetyl-CoA
are first converted to malonyl-CoA before being utilized for the
enzyme acetyl-CoA carboxylase .
The process occurs in 7 repeated cycles, each requires 2 mol of
NADPH+H . The overall reaction can be summarized in the
following equations :
Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH+H
Palmitic acid + 8 CoA + 14 NADP+ + 7CO2 + 6H2O
Fatty acid synthase multienzyme :
An acyl-carrier protein ( ACP ) and the group of enzymes
responsible for the biosynthesis of palmitate are all found in one
polypeptide chain forming a multienzyme complex (FA
synthase). The aggregation of all the enzymes of a particular
97
pathway into one multienzyme functional unit has many
advantages:
a) It offers great efficiency and freedom from interference by
competing reactions.
b) The synthesis of all enzymes is a dimer. In mammals, each
monomer is identical, consisting of one remarkable polypeptide
chain containing 7 enzyme activities of fatty acid synthase and
ACP with a 4'–phosphopantetheine-SH group. In close
proximity is another thiol of a cysteine residue of 3-ketoacyl
synthase (condensing enzyme) of the other monomer (see
figure)
98
99
Short and long-term mechanisms regulate lipogenesis
Long-chain fatty acid synthesis is controlled in the short
term by allosteric and covalent modification of enzymes and in
the long term by changes ih gene expression governing sites of
synthesis of enzymes.
A- Citrate and Acyl-CoA Regulate Acetyl-CoA Carboxylase
The most important reaction regulating the lipogenic
pathway is at the acetyl-CoA carboxylase step. Acetyl-CoA
carboxylase is an allosteric enzyme which is activated by citrate,
and increases in concentration in the well-fed state and is an
indicator of plentiful supply of acetyl-CoA . However, it is
inhibited by long-chain acyl-CoA molecules, an example of
metabolic negative feedback inhibition by a product of a
reaction sequence. Thus, if acyl-CoA accumulates because it is
not esterified quickly enough, it will automatically reduce the
synthesis of new fatty acid. Likewise, if acyl-CoA accumulates
as a result of increased lipolysis or an influx of free fatty acid
into the tissue, this will also inhibit synthesis of new fatty acid.
Allosteric activation of the enzyme involves aggregation from
monomeric to polymeric configuration of several million in
molecular mass. Acyl-CoA may also inhibit the mitochondrial
tricarboxylate transporter, thus preventing activation of the
enzyme by egress of citrate from the mitochondria into the
cytosol.
B- Insulin Also Regulates Lipogenesis :
Insulin stimulates lipogenesis by several mechanisms :
I] It increases the transport of glucose into the cell ( e.g. in
adipose tissue ) and thereby increase the availability of both for
FA synthesis and glycerol-3- phosphate for esterification of
newly formed fatty acids .
II] Insulin converts the inactive form of pyruvate dehydrogenase
to the active form .
III] Insulin activates acetyl CoA carboxylase .
100
C- The Fatty Acid Synthase Complex and Acetyl-CoA
carboxylase are adaptive Enzymes :
These enzymes adapt to the body's physiological needs by
increasing in total amounts in feed and decreasing in fasting,
feeding of fat, and in diabetes. Insulin is an important hormone
causing gene expression and induction of enzyme biosynthesis,
and glucagon antagonize this effect. Feeding fats, containing
polyunsaturated fatty acids causes inhibition of expression of
key enzymes of glycolysis and lipogenesis. These mechanisms
for longer-term regulation of lipogenesis, take several days to
become fully manifested and augment the direct an immediate
effect of free fatty acids and hormones such as insulin and
glucagons.
2- Mitochondrial system for FA synthesis:
This system can elongate FA of moderate chain length
(C10 or C12) into long chain FA (C16 to C20), but cannot
catalyze de novo synthesis of FA. It works only under aerobic
condition and its physiologic significance is uncertain. The
enzymes used are those of β-oxidation , working in the opposite
direction, except for the conversion of enoyl-CoA to the
corresponding saturated acyl-CoA which is catalyzed by the
enzyme enoyl-CoA reductase using NADPH+H as co-reductase
3- Microsomal system ( elongase ) :
This is the main system for elongation of fatty acids from
C10 to C22 . Its enzymes convert acyl-CoA to the longer by 2
carbons using malonyl-CoA as acetyl donor and NADPH as
hydrogen donor. It cannot catalyze do novo synthesis of FA, and
its enzymes can work separately.
101
Desaturation of fatty acids :
1-Synthesis of monosaturated FA :
An enzyme, Δ9 desaturase, in the endoplasmic reticulum
will catalyze the conversion of palmitoyl-CoA or stearyl-CoA to
palmitoleyl-CoA or oleyl-CoA respectively. Oxygen and either
NADPH or NADH are necessary for the reaction.
2- Synthesis of polyunsaturated FA :
Additional double bonds introduced into the existing
monounsaturated FA are always separated by a methylene
group. In animals, the additional double bonds are all introduced
between the existing double bonds and the carboxyl group (in
plants they may be introduced between the existing double
bonds and ω-carbon). Since animals are unable to synthesize
either linoleic (ω6) or α-linoleinic (ω3 ) acids because the
required desaturase is absent , these acids must be supplied in
102
diet to accomplish the synthesis of other members of ω6 and ω3
families of polyunsaturated fatty acids . Linoleic acids may be
converted to arachidonic acid. The pathway is first by
dehydration of the CoA ester through γ-linolenate , followed by
the addition of two-carbon unit via malonyl-CoA in the
microsomal system for chain elongation to give eicosatrienoate .
The latter forms arachidonic acid by further dehydrogenation.
The nutritional requirements for arachidonic acid may thus be
dispensed with if there is adequate linoleate in the diet .
103
Oxidation of fatty acid
1- β - oxidation of fatty acids
Fatty acids are both oxidized to acetyl-CoA and
synthesized from acetyl-CoA .
Although the starting material of one process is identical to the
product of the other and the chemical stages involved are
comparable, fatty acids oxidation is not the simple reverse of
fatty acid biosynthesis but an entirely different process taking
place in a separate compartment of the cell. The separation of
fatty acid oxidation from biosynthesis allows each process to be
individually controlled and integrated with tissue requirements.
Fatty acid oxidation takes place in mitochondria; each step
involves acyl-CoA derivatives catalyzed by separate enzymes,
utilizes NAD+ and FAD as coenzymes, and generate ATP. In
contrast, fatty acid biosynthesis (lipogenesis) takes place in the
cytosol, involves acyl derivatives continuously attached to a
multienzyme complex, utilizes NADP+ as coenzyme, and
requires both ATP and bicarbonate ion . Fatty acid oxidation is
an aerobic, and requiring the presence of oxygen .
FA oxidation requires preliminary activation of the FA to
acyl-CoA by the enzyme acyl-CoA synthetase ( FA thiokinase )
present in the outer mitochondrial membrane :
Acyl-CoA
Synthetase
Fatty Acid + ATP + CoASH
Acyl-CoA + AMP + PPi
Role of carnitine in FA transport :
Acyl-CoA (synthesized in the outer mitochondrial
membrane) cannot penetrate the inner mitochondrial membrane
to enter the β-oxidation reaction or to be elongated. Therefore,
introduction of the acyl radical into the mitochondrial matrix
requires a carrier known as carnitine (β-hydroxy-γtrimethylammonium butyrate) which is widely distributed and is
particularly abundant in muscle. It is synthesized from lysine
and methionine in liver and kidney. An enzyme, carnitine
palmitoyltranseferase I, present in outer mitochondrial
104
membrane, converts long-chain acyl-CoA to acylcarnitine which
is able to penetrate the inner membrane of mitochondrial and
gain access to β-oxidation system of enzymes. Carnitine
acylcarnitine translocase acts as inner membrane carnitine
exchange transporter.
105
B- Oxidation of Fatty Acids
106
Energy yield from complete oxidation of one mole of
palmitic acid ( C16 ) :
The oxidation of one mole of palmitate ( C16 ) results in
the formation of 8 moles of acetyl-CoA ( C2 ) by passing
through 7 β-oxidation cycles .
Each β-oxidation cycle yield 5 mole of ATP (2 moles via
FADH2 and 3 moles via NADH+H). Thus we get: 7 β-oxidation
cycles x 5 moles of ATP = 35 ATP ( 35 ATP ) .
The 8 moles of acetyl-CoA ( from β-oxidation ) each yield 12
ATP via Kreb's cycle thus : 12 x 8 = 96 ATP .
Thus the total ATP = 35 + 96 = 131 ATP ( 131 ATP ) .
Since 2 ATP were lost in the activation of FA to acyl-CoA ,
therefore the net gain will be : 131 - 2 = 129 ATP .
Biomedical importance
Increased fatty acid oxidation is characteristic of starvation
and of diabetes mellitus, leading to ketone body production by
the liver (ketosis). Ketone bodies are acidic and when produced
in excess over long periods, as in diabetes, cause ketoacidosis,
which is ultimately fatal. Because gluconeogenesis is dependent
upon fatty acid oxidation, any impairment in fatty acid oxidation
leads to hypoglycemia.
This occurs in various states of carnitine palmitoyltransferase ,
or inhibition of fatty acid oxidation by poisons , e.g., hypoglycin
Regulation of β-oxidation :
107
Regulation of long-chain fatty acids oxidation in liver :
It was shown that, livers of starved animals oxidize
considerably more fatty acids to CO2 while esterify less to
acylglycerol. This may be explained by the fact that carnitine
palmitiyltransferase I activity in the outer mitochondrial
membrane regulate the entery of long-chain acyl groups into
mitochondria prior to β-oxidation. Its activity is low in fed state,
when fatty acids oxidation is depressed , and high in starvation,
when fatty acid oxidation increases . Malonyl-CoA , the initial
intermediate in fatty acids biosynthesis , which increases in
concentration in the fed state , inhibits this enzyme , thereby
switching off β-oxidation . Thus , in the fed condition there is
active lipogenesis and high malonyl-CoA , which inhibits
carnitine palmitoyltransferase . Free fatty acids , in the fed state ,
enter the liver cell in low concentrations and are nearly all
esterified to acylglycerols and transported out of the liver in
very low density lipoproteins ( VLDL ). However, as the
concentrations of free fatty acids increases with onset of
starvation, acetyl-CoA carboxylase is inhibited directly by acylCoA , and malonyl-CoA decreases , releasing the inhibition of
carnitine palmitoyltransferase I and allowing more acyl-CoA to
β-oxidized. These events are reinforced in starvation by
decrease in [ insulin ] / [ glucagons ] ratio .
108
Oxidation of odd-carbon fatty acids :
II- Omega oxidation of fatty acids :
FA's of average chain length, and to lesser extent long
chain FA's, may initially undergo ω-oxidation ( oxidation of the
terminal methyl group), forming ω-dicarboxylic acid. The
enzymes required are present in the liver microcosoma. The
dicarboxylic acid may be further shortened by β-oxidation.
III- Alpha oxidation :
α-oxidation results in the removal of one carbon at a time
from the carboxyl end of FA molecule . It occurs in brain tissue.
It does not require CoA and does not generate ATP. It is
important for the oxidation of FA with methyl groups on βcarbon, which block β-oxidation e.g. Phytanic acid present in
animal and milk fats and in certain plants.
109
Biosynthesis of triacylglycerol
The enzyme glycerokinase is present only in the liver and
kidney but not in the adipose tissues. Thus glycerol-3-phosphate
involved in the synthesis of TG can be derived directly from
glycerol in the liver and kidney while in adipose tissues it is
derived from glucose via glycolytic process.
110
Metabolism of adipose tissue :
Triacylglycerols of adipose tissues are continually undergoing :
1] Lipolysis ( hydrolysis ) .
2] Reestrification (lipogenesis)
The resultant of these two processes determines the amount of
FFA released from adipose tissue into the blood .
The glycerol liberated from the process of lipolysis cannot
be utilized by the fat cells due to deficiency of glycerokinase ,
so glycerol liberated diffuse to the blood to be utilized by the
liver .
111
The reesterification of the FA requires activation to acylCoA and a supply of glycerol-3-p the later is obtained from
glucose via glycolysis . High carbohydrate diet stimulates
insulin release which increases the uptake and oxidation of
glucose by adipose tissue that results in increased glycerol-3-P
formation and hence stimulates the reesterification of FA and
decreases their release in blood. During fasting the rate of
lipolysis exceeds the rate of reesterification so TG in adipose
tissue decreases and FFA's in blood increase.
The hydrolysis of TG is controlled by the enzyme
hormone sensitive lipase (HSL). This enzyme may exist in 2
forms, an active phosphorylated form " lipase a " and an inactive
dephosphorylated form " lipase b ". The phosphate is attached to
serine residues in the enzyme protein.
The process of activation and inactivation of HSL is controlled
by many hormones.
Control of adipose tissue lipolysis . ( TSH , thyroid-stimulating hormone ; FFA , free fatty
acids ) Note the cascade sequence of reactions affording amplification at each step . The lipolytic
stimulus is " switched off " by removal of the stimulating hormone ; the action of lipase phosphatase ;
the inhibition of the lipase and adenyl cyclase by high concentrations of FFA : the inhibition of adenyl
cyclase by adenosine ; and the removal of cAMP by the action of phosphodiesterase . ACTH , TSH ,
and glucagons may not activate adenyl cyclase in vivo , since the concentration of each hormone
required in vitro is much higher than is found in the circulation . Positive ion ((+)) and negative ((-))
regulatory effects are represented by broken lines and substrate flow by solid lines .
112
HSL is inhibited by insulin, PGE1 and nicotinic acid. It is
stimulated by epinephrine, norepinephrine, glucagons, and
ACTH. Thyroxin increases the sensitivity of adenylate cyclase
to the effect of epinephrine and norepinephrine probably by
increasing the membrane receptors. Glucocorticoids increase
lipolysis by augmenting the synthesis of HSL enzyme .
Brown adipose tissue :
It is a type of adipose tissue that differs from white adipose
tissue by having a well developed blood supply and a high
content of mitochondria and cytochromes but low activity of
ATP synthase . It is involved in metabolism particularly at times
when heat generation is necessary. Thus, the tissue is extremely
active as in some species of animals exposed to cold and in the
newborn animal. Though not a prominent tissue in human, it is
present in normal individuals, where it appears to be responsible
for " diet-induced thermogenesis" which may account for how
some persons can eat and not get fat. It is noteworthy that brown
adipose tissue is reduced or absent in obese persons.
Phospholipids and Glycolipids Metabolism
These compounds are present in high concentrations in the
brain, egg yolk, liver and kidney. They are not essential dietary
components, every tissue is able to synthesize them provided
that the essential FA's, choline and inistol are available.
1] Biosynthesis of Phospholipids :
113
Degradation and remodeling of phospholipids :
Although phospholipids are actively degraded, each portion of
the molecule can turn over at different rate. This is due to the
presence of enzyme that allow partial degradation followed by
resynthesis. Phospholipase A2 catalyzes the hydrolysis of the
ester bond in position 2 to form lysophospholipids which in
turn may be reacylated by another acyl-CoA in the presence of
114
acyl transferase. Then lysophospholipids is attacked by
lysophospholipase ( phospholipase B ) removing 1-acyl group
Phospholipase A1 , attacks the ester bond in position 1 while
phospholipase A2 attacks the bond at position 2 of
phospholipids . Phospholipase C ( which is the major toxin of
some bacteria ) attacks the ester bond in position 3 .
Phospholipase D is an enzyme, described mainly in plants , that
hydrolyzes the nitrogenous base from phospholipids .
Lysolecithin may be formed by an alternative route involves the
enzyme
lecithin-cholesterolacyltransferase(LCAT).
This
enzyme , found in plasma and synthesized in liver , catalyzes the
transfer of a fatty acids residue from the 2 position of lecithin to
cholesterol to form cholesteryl ester and is considered to be
responsible for much of cholestryl ester in plasma lipoprotein .
Lecithin + Cholesterol
LCAT
Lysolecithin + Cholesteryl ester
115
Biosynthesis of Sphingolipids
116
Ketone bodies metabolism
1- Ketogenesis :
Under certain metabolic conditions associated with a high
rate of fatty acids oxidation , the liver produces considerable
quantities of acetoacetate and 3-hydroxybutyrate. Acetoacetate
continually undergoes spontaneous decarboxylation to yield
acetone. These three substances are collectively known as
ketone bodies (or acetone bodies). Acetoacetate and 3hydroxybutyrate are interconverted by the mitochondrial
enzyme 3-hydroxybutyrate dehydrogenase .
117
Ketogenesis becomes of great physiological importance during
starvation when carbohydrate stores are depleted and the
oxidation of fats becomes a major source of energy to the body.
The brain normally uses glucose as the only fuel. Brain takes
some time to adopt to ketone body utilization as an energy
sources in starvation but cannot utilize fatty acids .
2- Ketolysis :
Ketone bodies formed in liver go via the blood to
extrahepatic tissue where they become available to oxidation to
CO2 and H2O. Although most tissues can oxidize FAs , they can
more easily oxidize ketone bodies. Thus ketogenesis may be
considered as a preparatory step performed by the liver to
facilitate the oxidation of FA by extrahepatic tissue .
118
While liver is equipped with an active enzymatic mechanism for
the production of acetoacetate from acetoacetyl-CoA ,
acetoacetate once formed cannot be reactivated directly in the
liver except in the cytosol , where it is a precursor in cholesterol
synthesis, a much less active pathway. This accounts for the net
production of ketone bodies by the liver.
Ketosis
Normally the level of ketone bodies in the blood do not
exceed 1 mg/dl. However in some cases the rate of ketogenesis
may exceed much the rate of ketolysis leading to increased
ketone bodies in the blood " Ketonemia " then excess ketone
bodies are excreted in urine " Ketonuria " . This pathological
case is called Ketosis , Ketosis occurs in states of decreased
carbohydrate utilization e.g :
1] Starvation .
2] Uncontrolled diabetes mellitus .
3] Low carbohydrates and high fat diet .
4] Pregnancy .
All the above conditions are associated with decreased insulin
relative to the antiinsulin hormone , leading to increased
lipolysis and release of FFA from the adipose tissue as well as
decreased oxidation of glucose by the liver . This increases the
uptake and oxidation of FA by the liver, forming excess acetylCoA. The decreased glucose oxidation decreases the availability
of oxaloacetate in the liver mitochondria which leads to
inhibition of acetyl-CoA oxidation via kreb's cycle , also
decreased the translocation of acetyl-CoA to the cytosol for FA
synthesis. This results in the accumulation of acetyl-CoA and
increased HMG-CoA production leads to increased ketogenesis .
119
LIPID TRANSPORT AND STORAGE
* Plasma Lipoproteins*
They are molecular complex of lipids and specific proteins
called apolipoproteins
General Structure of Lipoproteins: the major lipids carried by
lipoproteins particle are triglycerides and cholesterol obtained
from diet or denovo synthesis. They are composed of
Core of non-polar lipids.
Monolayer of phospholipids surrounding the core.
Integral and peripheral apoproteins dispersed throughout.
Function: They function to keep lipids soluble since they
transport them in plasma and to provide an efficient mechanism
for delivering their lipids to tissues,
1. Categories of Lipoproteins: Lipoproteins are often
categorized according to density, which is dependent upon the
relative amount of protein present. The more protein present, the
higher the density of the lipoprotein.
120
Chylomicrons -- These are the lowest density, therefore contain
the most lipid and lowest percentage of proteins. Largest in size
lipoproteins. Half-life about 30 minutes
VLDL: Very Low Density. These are the initial lipoproteins to
carry triacylglycerols from the liver to target tissues. Half-life
about a few hours
IDL: Intermediate Density . Transient lipoprotein. VLDL's
become IDL's when they lose the triacylglycerol component.
LDL: Low density. Half-life is about a few days. IDL's become
LDL's when they lose the Apo-E protein.
HDL2: High Density lipoprotein. The most dense particle.
Half-life = a few days.
HDL3: High Density . It is another variety of HDL.
Plasma lipoprotein can be separated on the basis of their
electrophoretic mobility.
2-Apoproteins
-They serve as structural component of the lipoprotein particles
-They provide recognition sites for cell-surface receptors
-They also serve as activator for enzymes involved in
lipoprotein metabolism
-They are divided into classes A to H and subclasses apoA-1
and apo C-II
TYPES OF APO-PROTEINS:
Apo-A1: Found in HDL and Chylomicrons. It activates LCAT,
which in turn promotes breakdown of cholesterol-containing
lipoproteins.
Apo-B100: Found in LDL and VLDL and serves as a receptor
for uptake of these particles by target cells. Also plays structural
role.
Apo-B48: Structural protein in chylomicrons.
121
Apo-CII: Serves as a cofactor for lipoprotein lipase -- to aid in
breakdown of lipoproteins. Found in HDL2, VLDL, and
Chylomicrons.
Apo-E: Serves as a receptor-ligand for HDL particles and
Chylomicrons. Also found on ApoE-Rich HDL, a special form
of HDL.
The presence of different alleles of this protein in brain cells has
related to Alzheimer's Disease, in recent research.
Metabolism of Chylomicron
1-Chylomicrons are assembled in the intestinal mucosal cells
and carry dietary TG, cholesterol, cholesterol ester and other
lipids made in these cells. They are released by exocytosis from
the intestinal mucosal cells and travel through lymphatic vessels
to thoracic duct then to the blood. These particles are called
nascent chylomicrons and contain apo-B-48.
2- When nascent chylomicrons reach the plasma they received
apo-E and apo-C II from circulating HDL.
*-Apo-E together with apo-B48 help to recognize the modified
particles by receptors located on the hepatocytes membrane.
*-Apo-C II is necessary for the activation of lipoprotein lipase
3- Degradation of TG contained in chylomicron by lipoprotein
lipase which is found on the capillary wall of most tissues
especially adipose tissues and muscles. This enzyme is activated
122
by apo C II leads to the formation of free fatty acids and
monoacylglycerol. Chylomicron becomes smaller in size and
more dense. The remaining particles are known as chylomicron
remnants.
4- the chylomicron remanants are removed from the circulation
by the liver. Hepatocytes membrane contain lipoprotein
receptors that recognize the combination of apo-B48 and apo-E.
Remnant bind to these receptors and are taken up into
hepatocyte cells by endocytosis fused with the lysosomes and
degraded into their components, free fatty acids, TG,
cholesterol.
Metabolism of very low density lipoprotein (VLDL)
1-VLDL are formed in the liver, composed mainly of TG (
endogenously synthesized ) and their function is to carry TG
from liver to peripheral tissues.
2- VLDL are released from liver containing apo- B100 and apo
A. Once they released into circulation they received apo CII and
apo E from circulating HDL
3- In the circulation TG are removed from VLDL by lipoprotein
lipase ----- modified VLDL which become smaller in size and
more dense. Apo E and apo CII are returned again to HDL
4-Finally, cholesterol; ester are transferred from HDL to VLDL
in an exchange reaction with the reverse transfer of TG and
phospholipids from VLDL to HDL.
5- After this modification VLDL have been converted to LDL.
123
Metabolism of low density lipoprotein "LDL"
- LDL particles retain apo-B-100, but lose their other
lipoproteins to HDL. They contain less TG than VLDL and have
high concentration of cholesterol and cholesterol ester.
-The primary function of LDL is to provide cholesterol to the
peripheral tissues. They provide cholesterol to the peripheral
cells via receptors on the cell surface membrane that recognize
apo-B 100
- LDL receptors are negatively charged glycoproteins that are
clustered in pits on the cell membranes. The intracellular side of
the pit is coated with protein called clathrin which stabilizes the
shape of the pit. LDL are internalized as an intact particles.
- The vesicle containing LDL loses its clathrin coat and fuses
with other similar vesicles called endosomes
-The pH of endosome falls due to proton puming activity of
endosomal ATPase causing separation of LDL from its
receptors
124
125
- The receptor can be recycled whereas LDL are degraded by
lysosomal enzymes into amino acids, cholesterol, fatty acids and
phospholipids.
-The number of cell surface receptors for LDL varies according
to the availability of these LDL particles and need of the cell.
Therefore there is down-regulation of the cell receptor if there
is large amount of LDL in plasma and Up-regulation if the cells
are starved for cholesterol they increase cell surface receptors.
Effect of endocytosed cholesterol on cell cholesterol content:-High cholesterol content inhibits HMG-CoA reductase via
allosteric site.
-High cholesterol inhibit de novo synthesis by inhibiting HMGCoA reductase gene activation
-If cholesterol is not required by the cells immediately, it is
stored as cholesterol ester by the aid of the enzyme lecithin
cholesterol acyltransferase (LCAT)
Metabolism of high density lipoprotein "HDL"
Why are they good?
-It is synthesized in a nasent immature form in the liver and are
released into bloodstream by exocytosis, absorbs cholesterol in
many extra-hepatic tissues. It is like a cholesterol sponge.
-It is secreted in the blood, modified after interaction with
chylomicrons and VLDL as it exchanges lipids and apoproteins
-HDL has the higher amount of proteins and the lowest amount
of TG than other lipoproteins so it is the most dense.
-HDL transfers the apoproteins CII and E to chylomicron and
VLDL.
-HDL also functions in picking up cholesterol from the surface
of the cells and from lipoproteins to return it back to the liver so
it retards the atherosclerotic process.
-Esterification of free cholesterol: once cholesterol is taken up
by HDL, it is immediately esterified by Phosphatidylcholinecholesterolacyltransferase PCAT, plasma enzyme synthesized
by the liver and activated by Apo A-1 of the HDL. The resulting
126
cholesterol ester is so hydrophobic that is effectively trapped in
the HDL and can no longer transfer to cell membrane.
-HDL can remove cholesterol ester to VLDL by cholesteroyl
ester transfer protein
-HDL particle are taken by the liver by receptor mediated
endocytosis and the cholesteryl ester are degraded and converted
into Bile acids, Recycled with lipoprotein or secreted into bile.
ESTROGEN: Estrogen increases the levels of HDL. Hence premenopausal women are at lower rish for CHD than men. Other
things (exercise and loss of body fat) also increase HDL.
Function of HDL
1- transfer apoproteins to other lipoproteins
2- pick up cholesterol from from cell membrane
3- convert cholesterol into cholesterol ester
4- transport cholesterol ester to other lipoproteins or to liver
127
128
Three cellular receptors involved in lipoprotein metabolism:
All of the following receptors recognize various Apo-Proteins
and take up lipoprotein-particles into the cell via receptormediated endocytosis.
LDL-RECEPTOR -- "Apo-BE Receptor" -- It recognizes ApoBE particles -- i.e. Apo-B100 or Apo-E It recognizes and takes
up LDL-Particles; widely distributed throughout different
tissues.
Regulated -- The delivery of the cholesterol to the cell inhibits
this receptor! That is, the product of the endocytosis gives
negative feedback.
Functions: Regulation of LDL-Levels in blood, redistribution
and utilization of cholesterol.
Structure -- The Apo-BE Receptor is a single-pass
transmembrane protein, which sports the following domains... a
defect in can result in Hypercholesterolemia:
Ligand-Binding domain: It will bind both Apo-B100 and ApoE.
O and N-Linked glycosylation regions. EGF-Homologous
domain
REMNANT RECEPTOR -- Apo-E
Found in the liver only. It recognizes Chylomicron Remnants
and Apo-E Rich HDL receptors. (Remnants are the leftovers of
the chylomicron particles, after they have already dumped off
their fatty acids).
Remember -- Chylomicrons remnants still have lots of
cholesterol in them!
Function: Uptake of cholesterol-loaded remnants and delivery of
cholesterol to the liver. NOT REGULATED
SCAVENGER RECEPTOR -- Uptake of oxidized or
chemically modified LDL-Particles by Monocytes in circulation
or Macrophages in tissues.
Functions: Degradation and uptake of modified ("damaged")
lipoproteins, and uptake of bacteria to protect us from endotoxic
shock.
129
Lipoprotein
Chylomicrons
Very low density
lipoproteins
(VLDL)
Low density
lipoproteins (LDL)
Main
apoproteins
Function
B48, A-1, C11,
E
Largest LP, synthesized
in gut after a meal . Not
presentin normal fasting
plasma . Main carrier of
dietary TG.
B100 C-11, E
Generated from VLDL
in the circulation . Main
carrier of cholesterol .
B100
High density
A-1, A-11
Lipoproteins (HDL)
Apolipoproteins
A-I
A-H
B100
B48
C-I
C-II
E
Synthesized in the liver
Main carrier of endogen
-ously produced TG.
Site of synthesis
Intestine , Liver
Intestine , Liver
Liver
Smallest
but
most
abundant.
Protective
function. Takes chole sterol from extra hepatic
tissues to the liver for
excretion .
Functions
Activates LCAT
TG & Cholesterol transport
Bind to LDL receptor .
Intestine
TG Transport
Liver
Activates LCAT
Liver
Activates LPL
Liver , Intestine Binds to LDL receptor and
Macrophage .
probably also to another
specific liver receptor .
130
Cholesterol Metabolism
Cholesterol is a typical animal sterol, the brain and egg
yolk are very rich sources. The liver, Kidney and meat are also
rich sources. Average diet provides about 0.5g/day. Cholesterol
is not a dietary essential, the greater part of body cholesterol
arise by synthesis.
Cholesterol biosynthesis:
All tissue can synthesize and degrade most of their
cholesterol requirement in situ. The liver is the main source of
plasma cholesterol, but the intestine also participates.
131
Estrification of Cholesterol :
a- Within the cell :
Cholesterol-OH + Acyl-CoA
Acyl-cholesterol
acyl transferase
ACAT
Cholesterol ester
b- In Plasma :
Cholesterol-OH
Leuthin-cholesterol
transferase
+ Lecithin acyl(LCAT)
Cholesterol ester + Lysolecithin
Regulation of cholesterol biosynthesis :
HMG-CoA reductase is the key enzyme in the
biosynthesis of cholesterol. This enzyme exists in 2 forms a
"phosphorylated inactive form" and "dephosphorylated active
form" phosphorylation is catalyzed by a specific " reductase
kinase " and dephosphorylation by a " protein phosphatase " .
HMG-CoA reductase is activated by insulin and
inactivated by glucagon, thus the enzyme activity is inhibited
during starvation, directly acetyl-CoA to the formation of ketone
bodies, and increased by feeding of carbohydrates .
132
Factors influence the cholesterol balance in tissues :
At the tissue level, in the following are considered to
govern the cholesterol balance of cells.
The increase in cholesterol influx in tissues is caused by:
1- Uptake of cholesterol-containing lipoproteins by receptors,
e.g., the LDL receptor or the scavenger receptor.
2- Uptake of cholesterol-containing lipoproteins by a nonreceptor-membrane pathway.
3- Uptake of free cholesterol from cholesterol-rich lipoproteins
to the cell membrane.
4- Cholesterol synthesis.
5- Hydrolysis of cholesteryl esters by the enzyme cholesterol
ester hydrolase .
The decrease is caused by :
1- Efflux of cholesterol from the membrane to lipoproteins of
low cholesterol potential, particularly to HDL, promoted by
LCAT.
2- Esterification of cholesterol by ACAT.
3- Utilization of cholesterol for synthesis of other steroids, such
as hormones, or bile acids in liver.
Catabolism and excretion of cholesterol :
About 1 gram of cholesterol undergoes catabolism and
excretion from the body per day, About 50% is excreted in
stools in the form of bile acids, the remainder is also excreted in
stools in the form of neutral sterols.
1- Neutral sterols :
Cholesterol is excreted via the bile into the small intestine,
part of the secreted cholesterol is reabsorbed ( enterohepatic
circulation ), and part is partially reduced by intestinal bacteria
to coprostanol ( coprosterol ) . Coprostanol together with a small
amount of cholesterol are excreted in feces.
2- Bile acids :
Two primary bile acids are synthesized from cholesterol
by liver, namely, cholic acid and chenodeoxycholic acid,
133
principally the former. First cholesterol is converted, by 7 αhydroxylase, to 7 α-hydroxycholesterol, which undergoes
oxidative shortening of the side chain forming cholyl-CoA and
chenodeoxycholyl-CoA. These are then reacted with glycine
(75%) or taurine (25%) to form glycol- or tauracholic ( or
chenodeoxycholic) acids. This conjugated bile acids are secreted
in bile as their K+ or Na+ salty ( bile salts ) .
In small intestine, by the action of intestinal bacteria, part
of bile salts undergoes deconjugation and α -dehydroxylation .
In this way cholic and chenodeoxycholic acids are converted to
deoxycholic and lithocholic acids ( secondary bile acids ) .
The greater portion of bile acids are reabsorbed from the ileum
into the portal circulation, taken up the liver and reexcreted in
the bile (enterohepatic circulation). The bile acids that not
reabsorbed are excreted in feces.
Bile salts are important for the digestion and absorption of
fats because of their ability to lower the surface tension .
134
135
Cholesterol, Atherosclerosis and Cornary heart disease(
CHD ) :
1- There is a positive correlation between raised serum lipids,
specially cholesterol, and cholesteryl ester and the incidence of
CHD and atherosclerosis in humans.
2- Atherosclerosis is characterized by the deposition of
cholesterol, cholesterol ester, of lipoproteins VLDL and LDL in
connective tissue of arterial walls.
3- Disease in which prolonged elevated levels of LDL and
VLDL occur in the blood (such as diabetes mellitus,
hypothyroidism and other conditions of hyperlipidemia) are
accompanied by premature or severe atherosclerosis.
4- People with high level of LDL have an increased risk of CHD
5- People with high level of HDL have a decreased risk. This is
because LDL transports cholesterol to tissue while HDL
transports cholesterol from the tissues and arterial walls to liver
to be excreted.
6- Additional factors which may play a role in atherosclerosis
include hypertension, obesity and lack of exercise.
136
Lipid-lowering drugs
Drug group
HMG CoA reductase
inhibitors
Fibrates
Nicotinic acid and
derivatives
Principal actions
Inhibit cholesterol biosnthesis and
lower total and LDL cholesterol
Activate LPL and lower TG and
LDL cholesterol
Inhibit lipolysis within adipocyte
lower TG, increase HDL
Effect of diet on serum cholesterol level :
The factors play the greatest role in determining individual
blood cholesterol concentrations, but the dietary and environmental factors also play a part .
1- Diet rich in cholesterol raises its concentration in the blood
and increases the formation of fatty deposits in the arteries.
2- Fats rich in saturated FA's e.g eggs and animal fats, raise
blood cholesterol level.
3- Naturally occurring oils that contain a high proportion of
polyunsaturated fatty acids include sunflower, cottonseed, corn
and soybean oil and olive oil that contain a high concentration
of monosaturated lowered plasma cholesterol concentration. The
reason for cholesterol-lowering effect of polyunsaturated FA's is
still not clear. However several hypotheses have been advanced,
including the stimulation of cholesterol excretion into the
intestine and the stimulation of the oxidation of cholesterol to
bile acids. Diets rich in palmitic acid inhibits the conversion of
cholesterol to bile acids.
137
Factor
Cholesterol
Lipoproteins
Blood pressure
Weight
Exercise
Menopause
Coffe
Smoking
Alcohol intake
Stress
Increased Risk
High
Raised LDL
High
Overweight
Non or infrequent
Post-menopausal
High intake
Smokers
High
High
Decreased Risk
Low
Raised HDL
Normal
Correct weight
Moderate or regular
Pre-menopausal
Low intake
Non-smokers
Low
Role of liver in lipid metabolism :
The liver has a central and unique role in lipid metabolism:
1- It facilitates the digestion and absorption of lipids by the
production of bile which contains cholesterol and bile salts
synthesized within the liver de novo or from uptake of
lipoprotein cholesterol.
2- Liver has active enzyme systems for synthesizing, oxidizing
and desaturating fatty acids and for synthesizing TG and
phospholipids.
3- Liver converts FA's to ketone bodies ( Ketogenesis ) .
4- Liver plays an integral part synthesis of cholesterol and in the
synthesis and metabolism of plasma lipoproteins.
Fatty liver
The lipid content of the normal liver is approximately 4%
of which only about 1/4 is TG, for a variety of reasons, lipidsmainly triacylglycerol-can accumulate in the liver.
Excessive accumulation is regarded as a pathogenic condition
called fatty liver. When accumulation of lipid is chronic, fibrotic
changes occur in the cells that progress to cirrhosis and impaired
liver function .
Fartty liver may result from :
138
1- Over-feeding of Fats :
A very high fat diet increases the uptake of fats by the
liver. If this exceeds the capacity of the liver to synthesize
VLDL, fatty liver occurs.
2- Over-feeding of Carbohydrates :
If the excess carbohydrate overload the capacity of the
liver to store glycogen, it is converted to TG. If this exceeds the
capacity of the liver to synthesize VLDL, fatty liver occurs.
3- Overmobilization of Fat from Depot to Liver :
This occurs in conditions characterized by decreased
carbohydrate utilization, e.g. diabetes mellitus, starvation and
low carbohydrate diet, such conditions are associated with the
release of large amount of FFA's from adipose tissue. Increasing
amounts of FFA's are taken up by the liver and esterified to TG,
which exceeds the capacity of the liver to form VLDL leading to
fatty liver which is usually accompanied by hyperlipemia .
4- Inhibition of Fatty acids oxidation :
This directs fatty acids to esterification and formation of
TG, leading to fatty liver. Fatty acids oxidation is inhibited in
the following conditions:
a- Deficiency of Pantothenic acid:
↓ CoA formation
↓ acyl-CoA
b- Deficiency of carnitine : leads to decrease in the FA transfer
into the mitochondria for B-oxidation .
c- Alcoholism : Alcoholic fatty liver is chiefly caused by
decreased oxidation of FA as a result of oxidation of ethanol to
acetaldehyde .
Alcohol dehydrogenase
CH3CH2OH + NAD+
Ethanol
CH3CHO + NADHH+
Acetaldehyde
139
This leads to increased NADH/NAD+ ratio that causes a
shift to the left in the equilibrium "malate ↔ oxaloacetate" ( in
Kreb's cycle ) leads to inhibition of FA oxidation . In addition
acetaldehyde is further oxidized to acetyl-CoA which increase
FA synthesis .
5- Undermobilization of Fat from liver to blood :
Triacylglycerols synthesized in the liver are normaly
mobilized to the blood by forming VLDL. Failure to synthesize
VLDL may be caused by either decreased protein synthesis or
decreased phospholipids synthesis, leading to fatty liver.
a- Decreased protein synthesis :
This occurs in the following conditions
i- Deficiency of the essential amino acids due to mal-nutrition
and intake of diet deficiency in proteins of high biological value
ii- Toxic factors , e.g. carbon tetrachloride , chloroform ,
phosphorus and arsenic , which inhibit hepatic protein synthesis
iii- Abetalipoprotein , a genetic defect caused by failure of the
liver to synthesize the protein apo-100 .
b- Decreased phospholipids synthesis :
This occurs in the following conditions :
I] Deficiency of essential fatty acids ( EFA ) which are required
in the 2-position of phospholipids . Their deficiency may be
produced by a diet deficient in plant oils . It may also be
produced by increased cholesterol intake since cholesterol
competes with phospholipids for estrification with EFA .
Ii] Deficiency of choline which is required for biosynthesis of
phospholipids . It may be resulted from a diet deficient in
choline , in methyl donor ( methionine and betaine ) and in
vitamins involved methyl group synthesis ( folic acid and B12 )
A deficiency of Vitamin E enhances the hepatic necrosis of the
choline deficiency type of fatty liver . Added Vit E or a source
of selenium has a protective effect by combating lipid
peroxidation .
140
Iii] Deficiency of inositol which is required for biosynthesis of
lipositol . Inositol deficiency may be induced by a diet poor in
pyridoxine or rich in biotin .
Lipotropic factors :
These are factors, which help the metabolization of TG
from the liver. Therefore, they include substances essential for
the biosynthesis of phospholipids and of proteins, thus helping
the formation of VLDL which is needed for mobilization of TG
from the liver Lipotropic factors include:
1- EFA .
2- Inositol .
3- Choline .
4- Methionine and betaine .
5- Folic acid , vitB12 , pantothenic acid .
EICOSANOIDS
Prostaglandins and related compounds :
Archidonic acid and some other C20 fatty acids with
methylene-interrupted bonds give rise to eicosanoids ,
physiologically and pharmacologically active compounds
known as :
1- Prostaglandins ( PG ) .
2- Thromboxanes ( TX ) .
3- Leukotrienes ( LT ) .
4- Lipoxins ( LX ) .
Biosynthesis of Eicosanoids :
Archidonate, usually derived from the 2-position of
phospholipids in plasma membrane, as a result of phospholipase
A2 activity, is the precursor for the synthesis of PGs, TXs , LTs
and LXs compounds .
Specific agonists stimulate the cells to release arachidonic
acid. These agonists have specific target cells, e.g. Thrombin
141
stimulates the arachidonic acid platelets and Bradykinin causes
the release of arachidonic acid in renal tubules.
Two pathways are involved in eicosanoids biosynthesis:
1- The cyclooxygenase system : which converts arachidonic
acid to PG and TX . This cyclooxygenase system is inhibited by
NSAID such as aspirin , indometacin , ibuprofen or ketoprofen .
2- The lipooxygenase system which converts arachidonic acid to
LT and LX .
Regulation of arachidonic acid release :
An inhibitory protein ( Lipocortin ), whose synthesis is
induced by glucocorticoids, regulates the release of arachidonic
acid from membrane phospholipids. This inhibitory effect on
arachidonic acid release is the so-called anti-inflammatory
action of steroids since PG's and PG-like substances play an
important role in inflammatory reactions.
Prostaglandins ( PG )
- These are 20-carbons hydroxyl fatty acids containing a 5member ring .
- Therapeutic uses :
1- Prevention of conception.
2- Induction of labor at term.
142
3- Termination of pregnancy.
4- Prevention of allevation of gastric ulcer.
5- Control of inflammation.
6- Control of blood pressure.
7- Relief of asthma.
8- Relief of nasal congestion.
- PG's increase c-AMP in platelets, thyroid and lung but lower cAMP in adipose.
- Prostacyclins ( PGI2 ) are a type of prostaglandins produced
by blood vessel walls and are potent inhibitors of platelet
aggregation and inhibitors of adherence to the endothelial
surface . PGI2 are vasodilator particularly for arteries .
Thromboxanes A2 ( TXA2 ) :
- TXA2 is a 20-carbon hydroxyl fatty acid conyaining 6-member ring .
- It is synthesized in platelets and has opposite effect to PGI2 ,
contracting arteries promote platelet aggregation .
- So TXA2 and PGI2 have antagonist effects .
Fish oil and vascular heart disease :
The low incidence of heart disease and diminished platelet
aggregation in Eskimos may be due to high intake of fish oil
which contain eicosapentaenoic acid ( EPA ). This EPA gives a
rise to a type of PGI which is PGI3 which is more potent as
astiaggregator of platelets than PGI2. Also EPA gives rise to a
type of TX which is TX3 is a weaker aggregator than TXA2 .
The net result is that the balance of activity is shifted toward no
aggregation.
143
Fish oil also lowers plasma cholesterol, TG, LDL and VLDL
while it increases HDL concentration, so protect against
atherosclerosis.
Leukotrienes ( LT )
- Leukotrienes are hydroxyl fatty acid derivatives of arachidonic
acid and do not contain ring structure.
- They are formed in Leukocytes , Platelets and macrophage .
- They are involved in inflammation, chemotaxis (attraction of
leukocytes to site of inflammation) and allergic effect.
- The slow-reacting substance of anaphylaxis (SRS-A) is a
mixture of leukotrienesC4, D4 and E4. This mixture is 100-1000
times more potent than histamine or PG as a constrictor of the
bronchial airway musculature .
- Leukotrienes B4 (LTB4) attract leukocytes ( neutrophils and
eusinophils ) , which are found in large number at site of
inflammation .
Lipoxins ( LX ) :
- Lipoxins are a family of conjugated tetraenes also
arising in leukocytes. They are formed by the combined
action of more than one lipooxygenase.
- Evidence supports a role of LX's in vasoactive and
immunoregulatory function, e.g.. as counter-regulator
compounds ( chalones ) of the immune response .
144