CARBOHYDRATE METABOLISM
By
Prof. Dr SOUAD M. ABOAZMA
BIOCHEMISTRY DEP.
DIGESTION OF CARBOHYDRATE
•Salivary amylase partially digests starch and glycogen to
dextrin and few maltoses. It acts on cooked starch.
•Pancreatic amylase completely digests starch, glycogen,
and dextrin with help of 1: 6 splitting enzyme into maltose
and few glucose. It acts on cooked and uncooked starch.
Amylase enzyme is hydrolytic enzyme responsible for
splitting α 1: 4 glycosidic link.
•Maltase, lactase and sucrase are enzymes secreted from
intestinal mucosa, which hydrolyses the corresponding
disaccharides to produce glucose, fructose, and
galactose.
•HCl secreted from the stomach can hydrolyse the
disaccharides and polysaccharides.
ABSORPTION OF MONOSACCHARIDES
•Simple absorption (passive diffusion): The absorption depends upon the
concentration gradient of sugar between intestinal lumen and intestinal
mucosa. This is true for all monosaccharides especially fructose &
pentoses.
•Facilitative diffusion by Na+-independent glucose transporter system
(GLUT5). There are mobile carrier proteins responsible for transport of
fructose, glucose, and galactose with their conc. gradient.
•Active transport by sodium-dependent glucose transporter system
(SGLUT1). In the intestinal cell membrane there is a mobile carrier protein
coupled with Na+- K+ pump. The carrier protein has 2 separate sites one
for Na+ ,the other for glucose. It transports Na+ ions (with conc. Gradient)
and glucose (against its conc. Gradient) to the cytoplasm of the cell. Na+
ions is expelled outside the cell by Na+- K+ pump which needs ATP and
expel 3 Na+ against 2 K+.
Exit all sugars from mucosal cell to the blood occur by facilitative
transport through GLUT2.
It is proved that glucose and galactose are absorbed
very fast; fructose and mannose intermediate rate and
pentoses are absorbed slowly. Galactose is absorbed
more rapidly than glucose.
There are 2 pathways for transport of material absorbed
by intestine:
• The hepatic portal system, which leads directly to the liver
and transporting water-soluble nutrients.
• Lymphatic vessels: which lead to the blood by way of thoracic
duct and transport lipid soluble nutrients.
Carrier protein and transport of glucose
GLUCOSE UPTAKE BY TISSUES
Glucose is transported through cell membrane of different tissues by
different protein carriers or transporters. Extracellular glucose binds to
the transporter, which then alters its conformations, then transport glucose
across the membrane.
•GLUT1: present mainly in red cells, and retina.
•GLUT2: present in liver, kidneys, pancreatic B cells, and lateral border of
small intestine, for rapid uptake and release of glucose.
•GLUT3: present mainly in brain.
•GLUT4: present in heart, skeletal muscles, and adipose tissues. It is for
insulin-stimulated uptake of glucose.
•GLUT5: present in small intestine and testes for glucose and fructose
transport.
•SGLUT1: present in small intestine and kidneys, sodium-dependent, for
active transport of glucose and galactose from lumen of small intestine and
reabsorption of glucose from glomerular filtrate in proximal renal tubules.
Role of insulin in transport of glucose in adipose tissue,
skeletal muscles and heart through GLUT4:
1.Insulin produces transfer of GLUT-4 from their
intracellular pool to the outer membrane surface of these
tissues. So, increase GLUT-4 in the cell surface of these
tissues leads to increase glucose transport and uptake by
these tissues.
2-Transport through the previous tissues is insulinindependent.
lGucose
GGG G
Insulin
hormone
G
Insulin
receptor
(nG)
Intracellular signal
for insulin
Intracellular location of GLUT-4
FATE OF ABSORBED SUGARS
The absorbed monosaccharides are either hexoses or
pentoses.
1.The absorbed pentoses are excreted in urine because
the body does not deal with them.
2.The absorbed hexoses are glucose, fructose, or
galactose. Fructose and galactose are converted into
glucose in the liver.
FATE OF ABSORBED GLUCOSE
Blood glucose comes from 3 main sources:
1- Absorbed glucose from diet.
2- Glcogenolysis of liver glycogen.
3- Synthesis of glucose from other substances by gluconeogenesis
The absorbed glucose has the following pathways:
1- Oxidation:
a- For provision of energy: glycolysis, and Kreb’s cycle.
b- Not for energy production:
- HMP for synthesis of phospho-pentoses & NADPH + + H+.
- Uronic acid pathway for synthesis of glucuronic acid.
2- Synthesis of other CHO substances as:
A- Mannose, fucose, neuraminic acid for glycoprotein
formation.
B- Galactose and lactose in mammary gland.
C- Fructose in seminal vesicles.
E- Amino-sugar (glucosamine) for mucopolysaccharides, and
glycoprotein formation.
3- Synthesis of non essential amino acids.
4- Excess glucose is stored as glycogen in liver and muscles
(glycogenesis).
5- More excess glucose is stored as lipid in adipose tissue
(lipogenesis).
IMPORTANT ENZYMES IN CHO METABOLISM
1- Kinase
These are activating enzymes, which convert various metabolites into phosphorylated
form in presence of ATP, Mg++. They are irreversible enzymes with few exception.
1.Hexokinase (present in all tissues except liver) acts on any hexoses (glucose,
fructose, galactose) giving 6-phophorlyated hexose (glucose 6-P, fructose 6-P,
galactose 6-P).
2.Glucokinase (present only in liver) acts only on glucose converting it into glucose
6-P.
3.Fructokinase (present only in liver) acts on fructose to form fructose 1-P.
4.Galactokinase (present only in liver) acts only on galactose to give galactose 1-P.
2- Dehydrogenases
These are oxidizing enzymes that act by removal of H2 from a substrate → the
removed H2 will be carried by special coenzymes, which are hydrogen carriers as NAD,
FAD .
The name of the dehydrogenase is derived from the name of substrate upon, which it
acts as: Lactate dehydrogenase enzyme removes H2 from lactic acids.
The dehydrogenases are reversible enzymes.
.
3- Isomerases
1.These are enzymes that interconvert aldo-keto isomers. They are
reversible enzymes
Fructose 6-P
isomerase
Glucose 6-P
4- Mutases
These are enzymes that transfer a group from carbon to another carbon in
the same molecule. They are reversible enzymes.
e.g.
Mutase
Glucose 6-P
5- Epimerases
Glucose 1-P
•
They are enzymes that transfer a group from a side to the opposite
side of one carbon atom in the molecule. They are reversible enzymes
e.g.
Epimerase
UDP-glactose
UDP-glucose
6- Phosphatases
These are hydrolytic enzymes that remove a phosphate group from a
phosphorylated compound by addition of H2O. They are irreversible
enzymes.
Glucose 6-P
H2O
Glucose + Phosphate
GLYCOGEN METABOLISM
Glycogen is the main storage form of carbohydrates in
animals. It is present mainly in liver and in muscles.
Glycogen is highly branched polymer of α, D-glucose. The
glucose residues are united by α 1: 4 glucosidic linkages
within the branches. At the branching point, the linkages
are α 1: 6. The branches contain about 8-12 glucose
residues.
Glycogen metabolism includes glycogen synthesis
(glycogenesis) and glycogen breakdown (glycogenolysis).
GLYCOGENESIS
Def: it is the formation of glycogen from glucose in
muscles and from CHO and non CHO substances in
liver.
Site of location: In the cytoplasm of every cells
mainly liver and muscles.
Steps :- as the following :-
1- Glucose
Glucokinase,hexokinase
Mg++
ATP
G-6-P
Phosphoglucomutase
ADP
DUP-glucose pyrophosphorylase
2- G-1-P
UDP-glucose
UTP
PPi
H2O
pyrophosphatase
2Pi
G-1-P
N.B.: G-6-P is converted to glucose-1-phosphate by
phosphoglucomutase, glucose-1, 6 diphosphate is an obligatory
intermediate in this reaction.
-Glycogen synthase enzyme in presence of pre-existing
glycogen primer or glycogenin (glycogenin is a small
protein that forms glycogen primer after glycosylation by
UDP-glucose) adds glucose molecule from UDP-glucose
through creation of α 1: 4 glucosidic link.
-When the chain has been lengthened, the branching
enzyme transfers a part of the chain forming α 1: 6
glucosidic link. Thus establishing the branching points in
the molecule. The branches grow by further addition of 1: 4
glucosyl units.
-The key regulatory enzyme of glycogenesis is glycogen
synthase, which present in 2 forms:
1.Active form, which is dephosphorylated enzyme (GSa).
2.Inactive form, which is phosphorylated enzyme.(GSb).
GLYCOGENOLYSIS
Def.: It is the breakdown of glycogen into glucose in
liver and lactic acid in muscles.
Site of location: cytoplasm of many tissues mainly
liver, kidney, and muscles.
Steps:
•Phosphorylase is the first acting enzyme which
is the rate-limiting and key enzyme in
glycogenolysis. With proper activation and in
presence of inorganic phosphate (Pi), the enzyme
breaks the glucosyl α-1:4 linkage and removes by
phosphorolytic cleavage the 1:4 glucosyl residues
from outermost chains of the glycogen molecule
until approximately four (4) glucose residues remain
on either side of α-1 :6 branch (“limit dextrin”).
By phosphorlyase activity glucose is liberated as glucose-1-P
and NOT as free glucose.
•When four glucose residues are left around the
branch point, another enzyme, α-1:4 Glucan
transferase transfers a “trisaccharide” unit from one side
to other thus exposing α-1: 6 branching point.
•The hydrolytic splitting of α-1:6 glucosidic linkage
requires the action of a specific debranching enzyme.
As α-1:6 linkage is hydrolytically split, one molecule of
free glucose is produced.
•Fate of glucose-1-P:
The combined action of
phosphorlyase and other enzymes convert glycogen
mostly
to
glucose-1-P.
By
the
action
of
phosphoglucomutase enzyme glucose-1-P is easily
converted to glucose-6-P as the reaction is reversible.
•In liver and kidney, a specific enzyme glucose-6-phosphatase is
present that removes PO4 from glucose-6-P enabling “free glucose” to
form and diffuse from the cells to extracellular spaces including blood.
This is the final step in hepatic glycogenolysis, which is reflected
by a rise in blood glucose.
•In muscles, enzyme glucose-6-phosphatase is absent. Hence
glucose-6-P enters into glycolytic cycle and forms lactate. Muscle
glycogenolysis does not contribute to blood glucose directly. But
indirectly, lactic acid can go to glucose formation in liver.
The key regulatory enzyme of glycogenolysis is glycogen
phosphorolase enzyme which is present in 2 forms:
1.Active form (phosphorylated form) = phosphorylase a .
2.Inactive form (dephosphorylated form) = phosphorylase b.
Steps in glycogenolysis
1-Phosphorylase enzyme is a phosphorolysis enzyme which
responsible for breaking α 1: 4 glucosidic link of glycogen in
presence of inorganic phosphorus giving G-1-P.
2-Debranching enzyme is hydrolytic enzyme acts on α1: 6
glucosidic link giving free glucose.
NB :- The main function of muscles glycogen is to supply
glucose within muscles during contraction. Liver
glycogen is concerned with the maintenance of blood
glucose especially between meals. After 12-18 hours
fasting, liver glycogen is depleted, whereas muscle
glycogen is only depleted after prolonged exercise
CA2+ SYNCHRONIZES THE ACTIVATION OF PHOSPHORYLASE
WITH MUSCLE CONTRACTION
Glycogenolysis in muscle increases several 100-folds at the
onset of contraction; the same signal (increased cytosolic Ca2+
ion concentration) is responsible for initiation of both contraction
and glycogenolysis. Muscle phosphorylase kinase, which
activates glycogen phsophorylase, is a tetramer of four different
subunits, α, β, γ, and δ. The δ subunit is identical to the Ca2+ binding protein calmodulin and binds 4 Ca2+. The binding of Ca2+
activates the catalytic site of the δ subunit allowing activation of
glycogen phosphorylase and stimulation of glycogenolysis
REGULATION OF GLYCOGEN METABOLISM
1. Regulation of glycogen metabolism is achieved by a balance in
activities
between
glycogen
synthase
and
glycogen
phosphorylase.
2. Not only "phosphorylase" enzyme is activated by a rise in
concentration of phosphorylase kinase via  c-AMP, but "Glycogen
synthase" enzyme is at the sametime converted to "inactive" form,
both effects are mediated via "cyclic -AMP-dependant protein-kinase".
3. Thus glycogenolysis is stimulated while glycogenesis is inhibited. Both
processes cannot occur simultaneously together .
4. Dephosphorylation of "phosphorylase 'a', phsophorylase kinase 'a' and
glycogen synthase 'b' is accomplished by a single enzyme of wide specificity
"protein phsophatase-1", which in turn is inhibited by c-AMP dependant
protein kinase via the protein "Inhibitor-1". Thus, glycogenolysis can be
inhibited and glycogenesis can be stimulated synchronously, or vice versa,
because both processes are geared to the activity of c-AMP dependant
protein-kinase.
HORMONAL CONTROL OF GLYCOGEN METABOLISM
•Epinephrine stimulates α1 adrenergic receptors in liver → activation of
phospholipase-C which hydrolyses phosphatidyl inositol into 1,2 diacylglycerol
and inositol triphosphate → release Ca++ from its intracellular stores into the
cytoplasm raising the intracytoplasmic concentration of Ca++ which reacts with
calmodulin to give Ca++ - calmodulin complex → activation of Ca++ calmodulin
dependent protein kinase → conversion of glycogen synthase a (active) into
glycogen synthase b (inactive) and conversion of phosphorylase kinase b into
phosphorylase kinase a which converts phosphorylase b (inactive) into
phsophorylase a (active) → stimulation of glycogenolysis and inhibition of
glycogenesis →so stimulation of glycogenolysis in liver can be cAMP
independent.
•Epinephrine stimulate β adrenergic receptors in liver and in muscles &
glucagon stimulate its receptors in liver but not in muscles→ stimulation of
adenylate cyclase enzyme → stimulation of cyclic AMP formation → stimulation
of protein kinase A → conversion of glycogen synthase a (active) into glycogen
synthase b (inactive) and conversion of phosphorylase kinase b into
phosphorylase kinase a which converts phosphorylase b (inactive) into
phsophorylase a (active) → stimulation of glycogenolysis and inhibition of
glycogenesis.
•Insulin stimulates phosphatase enzyme so converts
inactive glycogen synthase into active one and converts
active phosphonylase enzyme into inactive one →
stimulation
of
glycogenesis
and
inhibition
of
glycogenolysis. Also it stimulates phosphodiesterase
enzyme → destruction of cyclic AMP.
Control of glycogen metabolism
Epinephrine
(liver, muscle)
Glucagon
(liver)
PHOSPHODIESTERASE
cAMP
Inhibitor-1
phosphate
5’AMP Inhibitor-1
+
+
GLYCOGEN
SYNTHASE b
PROTEIN
PHOSPHATASE-1
PHOSPHORYLASE
KINASE b
cAMP DEPENDANT
PROTEIN KINASE
PROTEIN
PHOSPHATASE-1
-
GLYCOGEN
SYNTHASE a
PHOSPHORYLASE
KINASE a
Glycogen
PHOSPHORYLASE
UDPGIc
PHOSPHORYLASE
b
a
Glucose 1phosphate
Glucose(liver) Glucose Lactate(muscle)
PROTEIN
PHOSPHATASE-1
-
-
Differences between muscle and liver glycogen
Liver glycogen
Muscle glycogen
Amount
Liver has more conc. Muscle has more amounts.
Sources
Blood glucose and
other radicals
Blood glucose only
Hydrolysis
Give blood glucose
Due to absence of phosphatase enzyme not give free
glucose but give lactic acid
Starvation
Changes to blood
glucose
Not affected
Muscular ex. Depleted
Hormones
Depleted
Insulin → ↑
Insulin → ↑
Adrenaline → ↓
Adrenaline → ↓
Thyroxin → ↓
Thyroxin → ↓
Glucagon → ↓
Glucagon → no effect due to absence of its receptors
GLYCOGEN STORAGE DISEASES
A group of diseases results from genetic defects of certain enzymes. The
absence of glucose-6-phosphatase enzyme results in the classical hepatorenal
glycogen storage disease Von Gierke (type I), this is characterized by :
1- It occurs in only 1 person per 200,000 and is transmitted as an
autosomal recessive trait.
2- Symptoms include :
Fasting hypoglycemia, because the liver cannot release enough glucose by
means of glycogenolysis; only the free glucose from debranching enzyme activity
is available.
3- Lactic academia, because the liver cannot form glucose from lactate .The
increased blood lactate reduces blood pH and the alkali reserve.
4- Hyperlipidemia, because the lack of hepatic gluconeogenesis (results in
increased mobilization of fat as a metabolic fuel).
5- Hyperuricemia (with gouty arthritis), due to hyperactivity of the hexose
monophosphate shunt
Other types of glycogenoses
A number of other genetic glycogen storage
defects (glycogenoses) have been described.
Pompe’s (lysosmal glucosidase deficiency),
Forb’s (Debranching enzyme deficiency),
Andersen’s (Branching enzyme system deficiency),
Macardle’s (Muscle phosphorylase deficiency),
Here’s (Liver phosphorylase deficiency) and Taui’s
(Phosphofuctokinase deficiency).
OXIDATION OF GLUCOSE
The pathways for oxidation of glucose are classified into two main
groups:
a- The major pathways for complete oxidation of glucose
into CO2, H2O and energy are:
1- Glycolysis → convert one molecule of glucose into 2 mol of
pyruvic acid + 2 NADH.H+.
2- Oxidative decarboxylation of pyruvic to acetyl CoA +
NADH.H++CO2
3- Complete oxidation of acetyl CoA in Kerb’s cycle into CO2, H2O
and energy .
b- The minor pathways for oxidation, which are not for energy
production.
1- Hexose monophosphate pathway (HMP).
2- Uronic acid pathway.
GLYCOLYSIS
EMBDEN-MEYERHOF PATHWAY
Def.: oxidation of glucose to give pyruvic acid in presence of
O2 and lactic acid in absence of mitochondria (RBCs) and in
absence of O2 .
Site: Cytoplasm of all cells especially muscles and RBCs.
Steps:
H–C=O
H–C=O
H C – OH
OH – C – H
H – C – OH
Hexokinase, glucokinase
H – C – OH
H – C – OH
CH2OH
D-Glucose
ATP
OH – C – H
Mg
H – C – OH
ADP
H – C – OH
CH2O-P
G-6-P
Mechanism of oxidation of glyceraldehydes 3-phosphate. Enz:
glyceraldehydes 3-P dehydrogenase which is inhibited by the –SH poison
iodoacetate, thus able to inhibit glycolysis.
ENERGY PRODUCTION FROM GLYCOLYSIS:
A. glycolysis in presence of O2 (Aerobic glycolysis):
Reaction catalyzed by
ATP production
Stage I
1. Hexokinase/Glucokinase reaction (for
phosphorylation)
2. Phosphofrutokinase-1 (for
phosphorylation)
-1 ATP
-1 ATP
Stage III
3. Glyceraldehyde-3-P dehydrogenase
(oxidation of 2 NADH in electron
transport chain)
4. Phosphoglycerate kinase (substrate level
+ 6 or +4 ATP
+2 ATP
phosphorylation)
Stage IV
5. Pyruvate kinase (substrate level
+2 ATP
phosphorlyation)
Net gain = 10 or 8 - 2
= 8 or 6ATP
B. Glycolysis in Absence of O2 (Anaerobic glycolysis):
•In absence of O2 re-oxidation of NADH at glyceraldehyde-3-Pdehydrogenase stage cannot take place in electron-transport
chain.
But the cells have limited coenzyme. Hence to continue the
glycolytic pathway NADH must be oxidized to NAD+. This is
achieved by reoxidation of NADH by conversion of pyruvate
to lactate (without producing ATP) by the enzyme lactate
dehydrogenase. Occurs in cells with no mitochondria as RBCs
(mature) ,or under low O2 supply as intensive muscular exercise.
In anaerobic glycolysis per molecule of glucose oxidation 4 - 2 = 2 ATP
will be produced.
REGULATION OF GLYCOLYSIS
A- Enzymatic regulation of glycolysis:•
There are 3 types of mechanism can be identified as
responsible for regulation of the enzyme activity of enzymes
concerned with CHO metabolism which are:
1- Changed in rate of enzyme synthesis:
* Induction →↑ rate of enzyme synthesis at gene expression
→↑ mRNA synthesis.
* Repression →↓ rate of enzyme synthesis at gene expression
→↓ mRNA synthesis.
2- Covalent modification by reversible phosphorylation
dephosphorylation.
3- Allosteric effect.
There are 4 regulatory enzymes which responsible for 3 irreversible reaction in
glycolysis.
Hexokinase
1.It is found in most tissues to give G-6-P when blood glucose level is low.
2.Acts on glucose and other hexoses to give hexose-6-P.
3.It has low km and Vmax→ acts maximally at fasting bl. glucose level.
4.It is inhibited by its products, which is G-6-P → feedback inhibition.
Glucokinase
1.It is found in liver and acts maximally after meal.
2.Acts only on glucose.
3.It has a high km and high Vmax → so it is active when bl. glucose level is high (after meal).
4.It is induced (↑its rate of synthesis) by insulin.
5.It is not inhibited by G-6-P.
Phosphofructokinase
1.It is the major regulatory enzyme in most tissues.
2.It is allosterically activated by F-6-P, AMP and inhibited by ATP, citrate, and H+.
Pyruvate kinase
1.It is allosterically inhibited by ATP, fatty acids, alanine, and acetyl CoA. And activated by
F-1-6 diphosphate.
2. It is phosphorylated by cAMP dependent protein kinase, which becomes inactive
and dephosphorylated by phosphatase enzyme, which becomes active.
B- Hormonal regulation:•
Insulin/glucagons ratio is the main hormonal regulation of glucose
utilization; it increases during glucose feeding and decreases during
fasting.
A.Glucagons: it is secreted in case of carbohydrates deficiency or in
response to low blood glucose level (hypoglycemia). It affects liver
cells mainly as follows:
1.It acts as repressor of glycolytic key enzymes.
2.Through cAMP-dependent protein kinase A, it produces
phosphorylation of specific protein enzymes that lead to
inactivation of glycolytic key enzymes.
B.Insulin: it is secreted after feeding of carbohydrate or in response to
high blood glucose level (hyperglycemia). It stimulates all pathways of
glucose utilization. Insulin binds to a specific cell membrane receptors
and produces certain signal cascade, which results in the following:
1.It acts as inducer for glycolytic key enzymes.
2.It activats phosphodiesterase enzyme(decreases cAMP that
leads to inhibition of protein kinase A).
3.It activats protein phosphatase-1 that produces
dephosphorylation of glycolytic key enzymes and their activation.
INHIBITORS OF GLYCOLYSIS:
1- Aresnate: which used in oxidative step
insted of Pi→ so glycolysis proceeds in
presence of arsenate but ATP, which formed
from 1-3 diphosphoglycerate is lost.
2- Iodoacetate produces inhibition of
glyceraldehydes-3-P dehydrogenase (inhibitor
of SH group).
3- Flouride inhibits enolase →↓↓ glycolysis in
bacteria →no production of lactic acid produced
by bacteria, which cause dental caries. It used
as anticoagulant in blood sample used for
estimation of blood glucose →↓↓ glycolysis in
RBCs .
FORMATION OF 2,3 DIPHOSPHOGLYCERATE IN RBCS:
2:3 diphosphoglycerate has an effect on O2 binding power of
haemoglobin→ It lowers O2 affinity by haemoglobin →↑ dissociation of
O2 to the peripheral tissues as in cases of high altitude.
CLINICAL SIGNIFICANCE OF 2,3 DIPHSOPHOGLYCERATE:
1- Persons who live at high altitude undergo state of low
O2 affinity for HB due to simultaneous increase of 2,3
diphosphoglycerate. This increase can be reversed on
returning to sea level.
2- Fetal HB has less 2,3 diphosphoglycerate than adult
HB, so fetal HB has high O2 affinity.
3- During storage of blood in blood banks, there is
decrease in 2,3 diphosphoglycerate so, stored blood has
high O2 affinity, which is not suitable for blood transfusion
especially to ill patients. If 2,3 diphosphoglycerate is
added to stored blood, it can’t penetrate RBCs wall. So, it
is advisable to add insoine, which is a substance that can
penetrate RBCs wall and change it into 2,3
diphosphoglycerate through HMP shunt.
DIFFERENCES BETWEEN AEROBIC AND ANAEROBIC GLYCOLYSIS
- Site
- End products
- Energy production
- Lactate dehdyrogenase
Aerobic glycolysis
Anaerobic glycolysis
Cytoplasm of all
tissues
RBCs and skeletal muscle
during muscular ex.
Pyruvic acid +
NADH.H+
Lactic acid + NAD+
6 OR, 8 ATP
2 ATP
Not needed
Needed
DISEASES ASSOCIATED WITH IMPAIRED GLYCOLYSIS
1- Hexokinase deficiency :
•In patients with inherited defects of hexokinase activity, the red blood cells contain low
concentrations of the glycolytic intermediates including the precursor of 2,3-DPG.
•In consequence, the hemoglobin of these patients has an abnormally high oxygen affinity.
•The oxygen saturation curves of red blood cells from a patient with hexokinase deficiency
are shifted to the left, which indicates that oxygen is less available for the tissues.
2- Pyruvate kinase deficiency (hemolytic anemia):
•All red blood cells are completely dependent upon glycolytic activity for ATP production.
•Failure of the pyruvate kinase reaction, the production of ATP will decrease leading to
hemolysis of red cells.
•Inadequate production of ATP reduces the activity of the Na+ - and K+ -stimulated ATPase
ion pump.
3- Lactic acidosis:•Blood levels of lactic acid are normally less than 1.2 mM. In lactic acidosis, the values for
blood lactate may be 5 mM or more.
•The high concentration of lactate results in lowered blood pH and bicarbonate levels.
•High blood lactate levels can result from increased formation or decreased utilization of
lactate.
•Common cause of hyperlacticidemia is anoxia.
•Tissue anoxia may occur in shock and other conditions that impair blood flow, in respiratory
disorders, and in severe anemia.
AEROBIC AND ANAEROBIC EXERCISE USE DIFFERENT FUELS
Aerobic exercise is exemplified by long-distance running, while
anaerobic exercise by sprinting or weight lifting.
During anaerobic exercise there is really very little inter-organ
cooperation. The vessels within the muscles are compressed during peak
contraction, thus their cells are isolated from the rest of the body. Muscle
largely relies on its own stored glycogen and phosphocreatine.
Phosphocreatine serves as a source of high-energy phosphate for ATP
synthesis until glycogenolysis and glycolysis are stimulated. Glycolysis
becomes the primary source of ATP for want of oxygen.
Aerobic exercise is metabolically more interesting. For moderate
exercise, much of thet energy is derived from glycolysis of muscle glycogen.
There is also stimulation of branched-chain amino acid oxidation, ammonium
production, and alanine release from the exercising muscle. However, a wellfed individual doesn't store enough glucose and glycogen to provide the
energy needed for running long distances. The respiratory quotient, the ratio
of carbon dioxide exhaled to oxygen consumed, falls during distance running.
This indicates the progressive switch from glycogen to fatty acid oxidation
during a race. Lipolysis gradually increases as glucose stores are exhausted,
and, as in the fast state, muscles oxidize fatty acids in preference to glucose
as the former become available.
MAJOR FEATURES OF SKELETAL MUSCLE S METABOLISM
1.Skeletal muscle functions under both aerobic (resting) and anaerobic (eg, sprinting)
conditions, so both aerobic and anaerobic glycolysis operate, depending on conditions.
2.Skeletal muscle contains myoglobin as a reservoir of oxygen.
3.Insulin acts on skeletal muscle to increase uptake of glucose.
4.In the fed state, most glucose is used to synthesize glycogen, which acts as a store of glucose for
use in exercise, 'preloading' with glucose is used by some long-distance athletes to build up stores
of glycogen.
5.Epinephrine stimulates glycogenolysis in skeletal muscle, whereas glucagon does not because of
absence of its receptors.
6.Skeletal muscle cannot contribute directly to blood glucose because it does not contain glucose-6phosphatase.
7.Lactate produced by anaerobic metabolism in skeletal muscle passes to liver, which uses it to
synthesize glucose, which can then return to muscle, (the cori cycle).
8.Skeletal muscle contains phosphocreatine, which acts as an energy store for short-term (seconds)
demands.
9.Free fatty acids in plasma are a major source of energy, particularly under marathon conditions
and in prolonged starvation.
10.Skeletal muscle can utilize ketone bodies during starvation.
11.Skeletal muscle is the principle site of metabolism of branched chain amino acids, which are
used as energy source.
12.Proteolysis of muscle during starvation supplies amino acids for gluconeogenesis.
13.Major amino acids emanating from muscle are alanine (destined mainly for gluconeogenesis in
liver and forming part of the glucose-alanine cycle) and glutamine (destined mainly for the gut and
kidneys).
OXIDATIVE DECARBOXYLATION OF PYRUVIC ACID
Def.: It is conversion of pyruvic acid and other α-keto
acids into CoA derivatives.
Site: In mitochondrial matrix of all tissues except
RBCs.
Steps: The conversion of pyruvic acid into acetyl CoA
is catalyzed by pyruvate dehydrogenase complex, which
composed of 3 enzymes act cooperative with each other
in presence of 5 co-enzymes: TPP, lipoic acid, FAD,
NAD+, and CoASH.
Pyruvate +TTP + Lipoic acid + CoA +FAD+ NAD+ --→ CO2 + Acetyl-CoA + NADH + H+
Steps of oxidative decarboxylation of pyruvic acid:
•Pyruvate is decarboxylated to form a hydroxyethyl
derivative bound to the reactive carbon of thiamine
pyrophosphate, the coenzyme of pyruvate decarboxylase.
•The hydroxyethyl intermediate is oxidized by transfer to the
disulfide form of lipoic acid covalently bound to dithydrolipoyl
transactylase.
•The acetyl group, bound as a thioester to the side chain of
lipoic acid, is transferred to CoA.
•The sulfhydryl form of lipoic acid is oxidized by FADdependent dihydrolipoyl dehydrogenase, leading to the
regeneration of oxidized lipoic acid.
•Reduced flavoprotein is reoxidized to FAD by dihydrolipoyl
dehydrogenase and NAD+.
REGULATION OF OXIDATIVE DECARBOXYLATION OF
PYRUVIC ACID :
1- Product inhibition :
The enzyme complex is inhibited by acetyl CoA, which
accumulates when it is produced faster than it can be oxidized by
citric acid cycle. The enzyme is also inhibited by elevated levels of
NADH+.H, which occure when the electron transport chain is
overloaded with substrate and oxygen is limited.
2- Covalent modification:
The pyruvate dahydrogenase complex exists in two forms: an active
nonphosphorylated form and an inactive phosphorylated form.Phosphorylated
and nonphosphorylated pyruvate dehydrogenase can be interconverted by
two separate enzymes, a kinase and a phosphatase. The kinase is activated
by increase in the ratio of acetylCoA/ CoA or NADH/ NAD+. An increase in the
ratio of ADP/ATP, which signals increased demand for energy production ,
inhibits the kinase and allows the phosphatase to produce more of the active
,nonphosphorylated enzyme.
CLINICAL ASPECTS OF PYRUVATE METABOLISM:
Inhibition of pyruvate metabolism leads to lactic
acidosis, which may be due to:
1- Arsenite or mercuric ions complex the –SH
group of lipoic acid.
2--Dietary deficiency of thiamin as in alcoholics.
These two factors lead to inhibition of pyruvate
dehydrogenase.
3- Inherited pyruvate dehydrogenase deficiency,
which may be due to defects in one or more of
the components of the enzyme complex.
CITRIC ACID CYCLE
TRICARBOXYLIC ACID CYCLE (KREB’S CYCLE)
Def.:
It is the series of reactions in mitochondria, which
oxidized acetyl CoA to CO2, H2O & reduced H2 carriers
that oxidized through respiratory chains for ATP
synthesis.
Site:
Mitochondria of all tissue cells except RBCs, which
not contain mitochondria. The enzymes of the cycle
are present in mitochondrial matrix except succinate
dehydrogenase, which is tightly bound to inner
mitochondrial membrane.
Steps:
ENERGY PRODUCTION:
ENERGY PRODUCTION FROM OXIDATION OF ONE MOLECULE OF GLUCOSE:
REGULATION OF KREB’S CYCLE:
1- As the primary function of TCA cycle is to provide energy, respiratory
control via the E.T.C and oxidative phosphorylation exerts the main control.
2- In addition to this overall and coarse control, several enzymes of TCA
cycle are also important in the regulation.
Three key enzymes are:
(a)Citrate synthase.
(b)Mitochondrial isocitrate dehydrogenase.
(c)α-ketoglutarate dehydrogenase.
These enzymes are responsive to the energy status as expressed by the
[ATP]/[ADP] ratio and [NADH]/[NAD+] ratio.
(a)Citrate synthase enzymes is allosterically inhibited by ATP and long-chain
acyl CoA.
(b)NAD+-dependent mitochondrial iso-citrate dehydrogenase (ICD) is activated
allosterically by ADP and is inhibited by ATP and NADH.
(c)α-ketoglutarate dehyrogenase complex which allosterically inhibited by succinyl
CoA, NADH-H+ and ATP.
3- In addition to above succinate dehydrogenase enzyme is inhibited by oxaloacetate
(OAA) and the avability of OAA is controlled by malate dehydrogenase, which
depends on [NADH]/[NAD+] ratio.
FUNCTIONS OF KREB’S CYCLE
1- It is the final pathway for complete oxidation of all foodstuffs CHO, lipids •
and protein, which are converted to acetyl CoA.
2- It is the major source of energy for cells except cells without mitochondria
as RBCs.
3- It is the major source of succinyl CoA, which used for:
1.Perphyrine and HB synthesis.
2.Ketone bodies activation.
3.Converted to OAA → glucose.
4. Detoxication by conjugation
4- Synthetic functions of Kreb’s cycle:•
a- Amphibolic reactions.
Some components of Kreb’s cycle are used in synthesis of other
substances as:
In fasting state, oxaloacetic acid is used for synthesis of glucose
by gluconeogenesis.
In fed state, citric acid is used for synthesis of fatty acids.
Reactions of Kreb’s cycle are used for synthesis of amino acid
(transamination into non essential amino acids) eg:
-OAA + glutamic acid  aspartic acid + α-ketoglutarate.
-Pyruvic acid + glutamic acid  alanine + α-ketoglutarate.
b- Anaplerotic reactions.•
Synthesis of one or more component of Kreb’s cycle from outside
the cycle:
O.A.A. can be synthesized from pyruvic acid by pyruvate
carboxylase, and from aspartic acid by transamination.
Fumarate can be synthesized from phenylalanine and tyrosine.
Succinyl CoA can be synthesized from valine, isoleucine,
methionine, and threonine.
α-ketogluterate can be synthesized from glutamic acid by
transamination.
Inhibitors of Citric Acid Cycle
1-Flouro-acetate reacts with oxalacetate forming
flourocitrate, which inhibits the aconitase enzyme.
2-Arsenite inhibits α-ketogluterate dehydrogenase.
3-Malonate acts as competitive inhibitor for succinate
dehydrogenase.
ROLES OF VITAMINS IN CITRIC ACID CYCLE
Four of the soluble vitamins of B complex have
important roles in cirtic acid cycle. They are:
1-riboflavin, in the form of FAD, a cofactor in αketogluterate dehydrogenase complex and in
succinate dehydrogenase; 2-niacin, in the form of
NAD, the coenzyme for three dehydrogenases in the
cycle, isocitrate dehydrogenase, α-ketogluterate
dehydrogenase and malate dehydrogenase; 3-thiamin
(vitamin B1), as TPP, the coenzyme for decarboxylation
in α-ketogluterate dehyrdogenase reaction; and
4-pantothenic acid, as part of coenzyme A, which
present in the form of acetyl-CoA and succinyl-CoA.
CO2 fixation or carboxylation
It is an addition of CO2 to the molecule in presence of CO2, biotin, Mn++, ATP,
and specific carboxylase
- CO2 is produced by α – ketoglutarate dehydrogenase , isocitrate
dehydrogenase and pyruvate dehydrogenase complex examples for
carboxylation :Pyruvate carboxylase
1-Pyruvic acid
++biotin,
O.A.A
Mn ,CO2
ATP
Propionyl CoA carboxylase
ADP
→ methylmalonyl CoA -D
2- Propionyl CoA
ATP
biotin ,++Mn ,CO2
L-MMCoA
ADP
Acetyl CoA
carboxylase
3-Acetyl CoA
Malonyl CoA
ATP
4-Pyruvic acid
CAC ← Succinyl CoA
biotin ,++Mn ,CO2
Pyruvate carboxylase
2biotin CO2NADPH-H
ADP
Malic acid
5- Synthesis of carbomyl phosphate of urea cycle and pyrimidine.
6- Formation of C number 6 of purine.
7- Synthesis of H2CO3/NaHCO3 buffer system