Biochemistry –Second year, Coll. of Medicine-Baghdad Univ.
2011-2012.
Dr.Basil Oied Mohammed Saleh.
Subject:Lipid
Lecture 1 29-3-2011
Lecture 2
6-10-2011
Lecture 3
13-10-2011
Lecture 4 20-10-2011
Introduction to lipid
Objective: Illstrates the meaning of lipid, types and their
functions.
Lipid are compounds that are soluble in organic solvents, such as ether and relatively
insoluble in water or aqeous media e.g.human body fluid such as blood, seminal and
interstitial fluid(water account for about
93 % of blood).The principal unit of
different forms of lipid is fatty acid with general formula:
RCOOH; R: is aliphatic carbon chain structure: CH3(CH2)nCH2; n is the number of
carbon.
In nature fatty acid may be:
1.Saturated; CH3CH2……., the common one in human body is Palmitic acid(C16)
2.Monounsaturated CH3CH2CH=CH……(one double bond), and 3.Polyunsaturated
CH3CH2CH=CHCH2CH2CH=CH…….(2 or more of double bonds ).The most
important of polyunsaturated fatty acids are the Essential Fatty Acids(EFAs).The
EFAs are those fatty acids that are required in human body but cannot be synthesized
in it, so must be supplied in the diet to support the growth and include:
Linoleic acid C18, 2 double bonds
Linolenic acid C18, 3 double bonds
Arachidonic acid C20, 4 double bonds. The absolute EFAs are the linoleic acid, the
precursor of arachidonic acid that is a substrate for Prostaglandins synthesis and the
Linolenic acid, the precursor for other ω-3 fatty acids formula important for growth
and development.These EFAs are important components of phospholipids of cell
membrane and mitochondrial membrane and their deficiencies result in defect in
growth and development.Even the incidence of EFAs deficiencies is rare, it can lead
to scaly dermatitis, visual and neurologic defects.
Fatty acids also may be:
Short : C2-C4
Medium: C6-C10
Long: C12 and more.
The saturated fatty acids are naturally found in Zigzag form, while the unsaturated
fatty acids in kinked form. The degree of unsaturation(number of double bonds) and
the carbon chain length are important in determining of melting point of fatty acids
and so of biological membrane fluidity composed of them(permeability to lipid
soluble substances).
The important structures of lipid materials are:
1. Triglycerides; Simple lipid
2. Phospholipids; Complex lipid
3. Sphingolipids and Glycolipids; Complex lipid
4. Cholesterol, Cyclolipid
5.
Lipoproteins,
Male
and
Female
Sex
Hormones,
Adrenal
Cortical
Hormones(cortisol and aldosterone), Vitamin D, and other derived lipid: Derived
lipid.
Defintions:
Triglyceride; TG (Triacylglycerol):
Are fatty acid ester of glycerol alcohol; 3 fatty acids+ Glycerol (CH2OH-CHOHCH2OH).
Diglyceride; 2 fatty acids+ Glycerol
Monoglycride: 1 fatty acid+ Glycerol. In nature fatty acids contained in acylglycerol
are different in carbon chain length and degree of saturity.
TG represents(its function) the principal storage form of energy in adipose
tissues that needed physiologically in prolonged fasting and starvation and
pathologically, for example in uncontrolled diabetes mellitus. It is also the preferred
form of nutrient that is used by muscle in producing of chemical energy ATP under
normal conditions. TG forms about 95 % of dietary fat.
The general structure of TG is:
CH2OCOR1
CHOCOR2
CH2OCOR3 : R1,R2,R3 are different fatty acids.TG is considered as nonpolar
structure and so not implied in CM formation.
Phospholipids;PL( Phosphoglycerolipids):
These lipids also composed of Fatty acids(R) and Glycerol as TG, but also
phosphoric acid(PO4) and nitrogen base. These two latter structures(PO4 and
nitrogen base) confer the PL compounds the relative polarity and so their function in
CM and mitochondrial membrane structures. The general formula of PL is:
CH2OCOR1
CH2OCOR1
CHOCOR2
CHOCOR2
CH2OPO3-Nitrogen base is PL, while
CH2OH is Phosphatidic acid
Type of PL is defined according to the type of contained nitrogen base:
Base
PL
Choline
Lecithin
Ethanol amine
Cephalin
Serine
Phosphatidyl serine
Inositol
Phosphatidyl inositol triphosphate
And Cardiolipin phospholipid; Diphosphatidyl glycerol.
PLs are also reffered to as amphipathic compounds because of their formation from
polar(PO4 and nitrogen base) and nonpolar(fatty acids) structures.
Lecithin PL is:
1.the predominant type of PL in CM
2.the source of choline component of the neurotransmitter, the Acetylcholine
3.the principal lipid component of Lung surfactant(90 % lipid and 10 % protein), its
deficiency in preterm infants is associated with inadequate production or secretion of
surfactant causes Respiratory Distress Syndrome(RDS), the significant cause of
death.
Lecithin is made and secreted by pneumocytes to act as surfactant, decrease the
surface tension of fluid lining the alveoli so reducing the pressure needed to reinflate
alveoli, thereby preventing alveolar collapse(atelectasis)
4. involved in emulsification of fat diet in small Intestine along with Bile salt.
Cardiolipin PL is the principal type of PL that involved in inner mitochondrial
membrane structure(important for maintenance of certain respiratory complexes).
The laboratory test,
Anti-cardiolipin ACL is used in investigation of abortion or dead infant delivery,
because this PL cardiolipin is recognized by antibodies that raised against Treponema
Pallidum the bacterium that causes Syphilis. Phosphatidyl inositol triphosphate in
CM act as a second messenger(internal messanger) for protein hormones action.
Platelet activating factor PAF and Plasmalogensare compounds that belong to PL
structure but differ in containing ether linkage ROR instead of ester linkage ROCOR
at C1 of PL. PAF is synthesized and released by a variety of cell types, binds to
surface receptors with triggering potent thrombotic and acute inflammatory
processes. It causes platelets to aggregate and degranulate, and neutrophiles and
macrophages to produce superoxide radicals, the killing substance of infected
bacterium.
Sphingolipids(Phosphoshingolipids: Sphingomyelin):
These are also PL but differ from phosphoglycerolipids(previous types) in their
structure: They are composed of Sphingosine alcohol instead of Glycerol.
Sphingosine is C18 monoalcoholamine:
CH3(CH2)12CH=CHCHOHCHNH2CH2OH
Sphingosine+fatty acid=Ceramide
Ceramide+Nitrogen base= sphingolipid. Of the most significant type of these PL in
humans is sphingomyelin in which the base is choline. It is an important component
of myelin sheath of nerve fibers, insulates and protects neuronal fibers of the central
nervous system(preventing the short circulation of nerve electrical pulse
transmission).
Glycolipids:
These are anthor type of lipid. Their structure are relatively similar to sphingolipid;
Ceramide+ carbohydrate moity(or moities)=Glycolipids.
Of which : the simple
forms are glucosylsphingolipid and galactosylsphingolipid(only one unit of
CHO).The complex forms are Globoside and Gangalioside(2-9 units of CHO).They
are found in outer leaflet of plasma membrane and contribute to cell surface
antigen(anti-genicity), cellular interaction(cell-cell adhesion) and blood group(A B O
system).
Cholesterol :
Is anthor form of lipid called sterols. Cholesterol is the major sterol in humans. It is
cycloaliphatic carbon chain C27. It is present in blood in two forms: Free chol.(1/3)
and Esterified chol.(2/3). Total chol. Represents the two forms: The free form is
relatively polar because of free OH group at C3, while the esterified form is nonpolar
because the free OH is occupied by acyl group(fatty acid RCOO) . Cholesterol is
the precursor for synthesis of many vital substances: Male and Female sex hormones
(Androgen such as testosterone and Estrogen, E2 ), vitamin D, Cortisol and
aldosterone hormones.Cholesterol is also an important constituent of CM structure.
2. Lipoproteins:
Objective: Illustrate the definition and metabolism of different types of
lipoproteins
Lipoproteins LPs are spherical structures composed from lipids and proteins and
function in supporting the transport of lipids in circulation. In these structures the
water insoluble lipids (TG and esterified cholesterol) are oriented to the core of the
spherical LP, while the water soluble lipids(PL, Free chol. and added proteins) are
directed to the surface of LP. However, these structures in their later form still
relatively insoluble in systemic circulation and need for addition of specific proteins,
called apolipoproteins(apoLPs) to confer them sufficient water solubilitiy and so
transporting in blood.There are major five LPs in blood of human body,along with
additional sixth one apo a that is related to LPs structurally but not functionally.The
five Lps are classified in order of ascending densities by ultracentrifugation as:
1.Chylomicron
2.Very Low Density Lipoprotein (VLDL)
3.Intermediate Density Lipoprotein(IDL)
4.Low Density Lipoprotein(LDL)
5.High Density Lipoprotein(HDL)
Chylomicron(Exogeneous LP):
It is synthesized in small intestine from dietary lipid after being digested and
absorbed. The reabsorbed lipid(fatty acids, PLs, free cholesterol) are resynthesized in
enterocyte again into TG, PL, and cholesterol ester. These lipid are oriented as
mentioned above into spherical structure and converted into LPs after addition of
apolipoproteins, principally apo B48 and to a lesser extent apo A I. Chylomicron
composed mainly of TG 90 %, and the remainder are PLs, cholesterol and
apoLPs.Because of its low density(large size), it is secreted firstly into lymphatic
system and gives this system its milky appearance, then
passes into systemic
circulation, where this LP accepts apo C II into the circulation from anthor LP(HDL).
In blood, chylomicron is transported to adipose tissue and skeletal muscle where
there are an important enzyme
Lipoprotein Lipase (LPL)
which anchored by
heparan sulfate to the capillary walls of the most tissues, but predominantly those of
adipose tissue and cardiac and skeletal muscle. This LPL catalyze the hydrolysis of
TG contained in this LP into Free fatty acids(FFAs) and glycerol in the presence of
apo CII as coenzyme. FFAs are taken up by adipose tissues(stored as TG) or muscle
for production of energy ATP. Glycerol is transported to the liver to be used in lipid
synthesis TG, glycolysis or gluconeogenesis, however it caanot be used by adipose
tissue ?. As more than 90 % of TG is removed from chylomicron by LPL and as the
apoCII is returned to HDL, the remainder particle is reffered to chylomicron remnant
which is taken up by hepatic receptors via recognition the apo E on the surface of
this remnant by endocytosis(engulf process). Components of the engulfed remnant;
FFAs, cholesterol, PL and amino acids that are produced and released by lysosomal
enzymes are used by hepatocytes.
Pathological condition: Familial Hyperchylomicronemia(Hyperlipoproteinemia
type I).
Is deficiency of LPL or apo CII with resultant hyperchylomicronemia and
hypertriglyceridemia(TG 2000-10,000 mg/dl Normal value= 150 mg/dl) even in
fasting state. Normally, chylomicron is absent from circulation in fasting state and
only present in nonfasting(postprandial condition). This disorder is expressed in
childhood period, less than 10 year of age and characterized by the symptoms; severe
abdominal pain, acute pancreatitis, eruptive xanthoma and lipemia retinalis.
VLDL (Endogeneous pathway):
VLDL is also composed mainly of TG but with less amount compared with
chylomicron (VLDL contains 55-60 % TG). It contains also apo B100, much amount
of PL , cholesterol and apoproteins, so with higher density and smaller size than
chylomicron.VLDL synthesized endogeneously in the liver from chylomicron
remnant(dietary lipid)?? and from those synthesized in the liver from excess ingested
CHO??. The pathway of VLDL metabolism is started from its secretion into the
systemic circulation in the presence of apo B100. It is transported in the blood to
different tissues, mainly the adipose and muscle where there are LPL which acts in
the presence of apoCII on the TG contained into the VLDL particles as in
chylomicron pathway. After removal of much amount of TG as FFAs and glycerol,
VLDL is converted into VLDL remnant which undergoes again subsequent
hydrolysis of TG by LPL to produce what is known: Intermediate density
lipoprotein IDL. This IDL which is present normally in blood transiently, is
composed from equal molar amount of cholesterol and TG, and mainly apo B100
and apo E. The IDL is either taken up by hepatic receptors which recognize the apo E
on the surface of IDL or is converted into LDL after removal of the remaining
amount of TG contained into IDL. The IDL lipid migrate into β position in
electrophoresis technique.
Pathological conditions: HyperVLDLemia due to deficiency of LPL and / or apo
CII with increase in blood level of VLDL and TG. Normally, the increase in TG
blood level in fasting and postprandial states reflects the abnormalities in
chylomicron and VLDL, respectively ?.
Familial dysbetalipoproteinemia,Broad beta disease(Hyperlipoproteinemia Type
III) is abnormal state in lipid metabolism which characterized by appearance of IDL
in blood which is normally undetectable or transient due to presence of abnormal or
variant of apo E2 on the surface of IDL and prevent its uptake by hepatic
receptor.This disorder is characterized by palmar xanthoma, hypercholesterolemia
and premature atherosclerosis.
Abetalipoproteinemia is a rare disorder of lipid metabolism characterized by hypo-
chylomicronemia and hypo-VLDL due to defect in enzyme TG Transfer Protein
which involved in incorporation of apoB 48 and apo B100 in lipid of these LPs with
consequences of accumulation of TG in small intestine and liver(Fatty liver disorder).
Note: The function of chylomicron and VLDL is mainly to transport of TG(and to
very lesser extent the cholesterol) present in diet or synthesized endogeneously to
peripherial tissues, principally the cardiac and skeletal muscle and adipose tissue.
LDL Low Density Lipoprotein:
This type of lipid or LP is differentiated from other LPs in its principally forming
from cholesterol(free and esterified) and only apo B 100. It formed as mentioned
before from ILD in the circulation and functions in transporting of cholesterol(not
TG) from the liver to the peripheral tissues; cardiac and skeletal muscle, adrenal and
gonadal glands?, skin ?and the others. LDL, which scientifically written as LDLcholesterol(LDL-C) is important for growth and development because of need of the
body for cholesterol in CM structure and synthesis of many vital substances?, but it
is(LDL-C) also the Bad Lipid because of direct correlation between the blood levels
of this lipid and the incidence of atherosclerosis; coronary artery diseases(CADs),
cerebrovascular disease(CVD), and the peripheral atherosclerosis.
The metabolism of LDL is mainly by the liver(2/3 of LDL-C) and the remainder(1/3
of LDL-C) by scavenger uptake.The uptake of LDL by the liver is dependent on
hepatocellular Receptor uptake of LDL. These Receptors (R) are negatively charged
glycoproteins and are distributed in the pits on the CM surface of liver.The hepatic
Rs recognize,bind and internalize by endocytosis the LDL particles depending on
the presence of apo B 100 and also the apo E on the surface of LDL-C. After
endocytosis of R-LDL-C complex(known as endosome), the LDL-C dissociates
from complex because of endosomé s low PH and (the LDL particle) is hydrolyzed
by lysosomal enzymes to release the amino acids of apo- B100 and the
free
cholesterol.The cholesterol that carried by LDL particle may derived from dietary
lipid(chylomicron remnant) or mainly hepatic endogeneous synthesis. This pathway
of LDL-C clearance is very important in removal and regulation of blood cholesterol,
but it is saturable pathway. The latter(saturable ) means that when hepatic Rs uptake
of LDL-C increased for any reason such as increased dietary cholesterol, the Rs are
saturated and the uptake of LDL is stopped with accumulation of cholesterol
contained in LDL in circulation; Hypercholesterolemia.
Normally, after hepatocellular uptake of LDL-C and release of free cholesterol
intracellulary,
1.Free
cholesterol
is
converted
into
esterified
cholesterol(E.cholesterol) by enzyme Acylcholesterolacyltransferase(ACAT) which
is less toxic than free one 2. Free cholesterol is transpoted to different tissues for vital
substances synthesis 3. the endocytosized Free cholesterol inhibits the intracellular
de novo synthesis of cholesterol by negative feed back inhibition of regulatory
enzyme of cholesterol synthesis pathway, the Hydroxy Methyl-Glutaryl-CoA
reductase HMG-CoA reductase enzyme to prevent the toxic accumulation of free
cholesterol but with increased blood level of it 4. the endocytosized cholesterol also
inhibits the transcription(gene expression) of LDL-C R and so decreases the
availability of
these
Rs
on
the surface
of liver
cells
with
resultant
hypercholesterolemia. The above pathways of endocytosized cholesterol are
important to regulate the intracellular cholesterol.
Note: Statin drug of different nomenculture;levostatin,avostatin… is mainly act as
anti-hyperlipidaemic or anti- hypercholesterolemic by its inhibition of HMG-CoA
reductase enzyme.
The second pathway of cholesterol clearance is Scavanger receptor class A(SRA).These SR can bind the chemically modified LDL-C such as the oxidized LDLC(oxidation of its fatty acids or the apo B100), but irrespective of their intracellular
levels. This means that the SR-A pathwary is not down-regulated by the intracellular
endocytosized cholesterol levels. These SRs are predominantly found in
Macrophages (the Monocyte leukocytes) which after engulfing and accumulation of
large amounts of cholesterol are, the macrophages transformed into foam cells, which
participate(the early step) in development and progreesion of atherosclerotic plaque.
Pathological condition:
Familial Hypercholesterolemia FH(Hyperlipoproteinemia type II). It is primary
genetic defect in the hepatic LDL-R gene, The R that is responsible for recognition,
binding and removal of LDL molecule from the circulation. Deficiency or
defective(mutant) of LDL-R causes significant elevation of blood LDL-C and
consequently the cholesterol in blood and premature atherosclerosis.Complete
deficiency of LDL-R(homozygotes) leads to
FH in childhood and death from
myocardial infarction MI before the end of the third decade.In partial deficiency of
LDL-R(heterozygotes) the clinical manifestation of MI appear during the fourth
decade of life. FH is characterized clinically by increased plasma LDLcholesterol
levels with their deposition in skin, tendons and the arteries. The level of plasm
LDL-C is 2-3 in heterozygotes, and 4-6 in homozygotes times that of normal
individuals. Tendon xanthoma(Achillus xanthoma), xanthelesma and premature MI
are characteristics of FH disorder.
3. HDL High Density Lipoprotein:
Objective: Part of it is complemntary to prevous lecture and the remainder part
illustrate the Hyperlipidaemia types and some types of Hypolipidaemia and
illustration of Lipid Profile.
Is the good lipid or LP. It is synthesized in the liver and small intestine as diskshapped containing only PL and apo A II(the predominant apoLP),C and E. This
nascent disk-shapped HDL is transported by blood to peripheral tissues for uptake
of intracellular or CM excess amounts of cholesterol(so act as protective LP, prevents
the arterial subendothelial accumulation of cholesterol) and converted into spherical
shape. HDLpartical is the excellent acceptor of free cholesterol(unesterified
cholesterol)and so the carrier of excess amounts of peripheral cholesterol. The
received cholesterol by HDL is esterified promptly by Lecithin Cholesterol Acyl
Transferase(LCAT) enzyme to prevent the escape of cholesterol again to peripheral
tissues?. LCAT is synthesized in the liver and carried after release on the surface of
HDL particle for esterification of cholesterol, the enzyme need for apoA II for its
function. The mechanism of HDL in transporting of excess(unrequired) cholesterol
from peripheral tissues to the liver as the latter is the only organ deal with removing
of excess cholesterol, either by conversion it into bile acids or by excretion as
unmetabolized cholesterol through the bile duct(Reverse cholesterol transport) are:
1. direct uptake of whole molecule of HDL-C
2. Selective uptake of cholesterol
from the HDL-C 3. Exchange of lipid between HDL-C and chylomicron and VLDL
remnant, the E.cholesterol is transffered from HDL-C to the remnant and the TG is
transffered from the remnant to the HDL-C. Then, the transffered E.cholesterol into
the remnant is uptake by the liver through the apo E found on the surface of these
remnants. There two types of HDL: HDL-3 is the newly release particle which is
E.cholesterol poor particle and the HDL-2 which is the E.cholesterol rich particle that
carried the cholesterol to the liver.The inverse relation between the plasma HDL-C
and the incidence of CAD (and in general atherosclerosis) made it the Good lipid or
LP.
Hyperlipidaemia(Hyperlipoproteinemia):
Hyperlipidaemia
was
defined
by
several
ways;
genetic
as
mentioned
above(FH,Familial hyperchylomiconemia….), ultracentrifugation and the laboratory
analysis. The best one in understanding and interpretation of hyperlipidemia is that
dependent on Laboratory Analysis which classified this disorder into:
1. Primary
2. Secondary
Each type is subdivided into: Hypercholesterolemia, Hypertriglyceridemia and
Combined
Hyperlipidemia
CHL.The
primary
Hypercholesterolemia
and
hypertriglyceridemia are those characterized by significant elevation of only plasma
cholesterol
or
TG,
respectively
due
to
genetic
defect
in
metabolism
pathways(Enzymes or Receptors) of LPs, such as FH.The secondary type is due to
diseases other than the genetic causes, for example; Diabetes mellitus and Heavy
alcohol abuse leads to hyperTG, While Hypothyroidism and Nepherotic syndrome
result in hypercholesterolemia. So, in secondary type the defect is not in Enzymes
or receptors involved in lipid pathway metabolisms. The combined Hyperlipidaemia
implied the increased of plasma levels of both cholesterol and TG in the same
individual, also due to genetic abnormalities(primary type) in lipid metabolism
pathways or non-lipid causes(secondary type). The diagnosis of hyperlipidaemia is
firstly made by excluding of secondary type; DM, Hypothyroidism…, which then
make the primary type is the cause. The correction of hyperlipidaemia is depend on
the type; the secondary is by correction of the underlying diseases, while the primary
is by genetic treatment.
Familial Hypercholesterolemia FH is usually diagnosized from the markedly
elevated plasma cholesterol and the presence of tendon xanthomain the patient or
first-degree relation. Possible FH is defined as a plasma cholesterol level of more
than 7.5 mmol/L(NR 3.5-5.0 mmol/L) in adult(more than 6.7 mmol/L in children
under 16 years). In homozygous FH, the plasma cholesterol concentration can be as
high as 20 mmol/L. The severe hypercholesterolemia is accompanied by relatively
normal plasma TG level. HDL-C may be low.
Primary
hypertriglyceridemia
is
often
observed
with
low
HDL-C
concentration.The precipitating causes of hyperTGemia include obesity, high alcohol
intake, DM and the use of estrogen. There may be an increased risk of CAD because
of concomitant increased of TG and decreased of HDL-C. Actually, the increased
risk of CAD that accompanied the hyperTG emia is due to elevation in plasma level
of dense and small size remnant lipid.
Secondary causes of Hypercholesterolemia:
Primary hypothyroidism
Nephroteic syndrome
Cholestasis
Anorexia nervosa
Drug; ciclosporin
Secondary causes Hypertriglycerdaemia:
Alcohol excess
DM
chronic renal failure
Drugs: estrogens, thiazide diuretics cortisol…
Tangier disease:
This pathogenesis of this disorder is due to absolute absence of HDL-C caused by
defect in apo-A II synthesis, the integrated apoLP component of HDL. This disease is
characterized by:
Increased risk of CAD, CVD and peripheral atherosclerosis.
Abetalipoproteinemia:
Absolute deficiency of LDL-C which may lead to mental and physical retardation
and failure to thrive, mainly in infants.
Lipid Profile:
Investigation of patient for hyperlipidaemia need to be in fasting state, the patient
fasts overnight for aroun 12 hours and is only allowed to drink water. The fasting
state must for plasma TG level assessment, but not for plasma cholesterol
concentration. History of therapy usage is also necessary.
Lipid profile means the measurement of plasma or serum levels of
Test
Desirable level
Total cholesterol
less than 200 mg/dl
TG
less than 150 mg/dl
HDL-C
35-60 mg/dl
LDL-C
less
than
100
mg/dl
and VLDL-C. The lipaemic appearance of blood sample on standing reflects the
increased level of TG due to abnormal elevation of VLDL level?. The creamy layer
that appears on surface of blood sample indicates the presence of high level of
chylomicron?. The abnormal increases of HDL-C and LDL-C level is not
accompanied by either the lipaemic or milky layer of the blood sample??
Statin drug for hypercholesterolemia is indicated when plasma level of LDL-C is
more than 100 mg/dl, however this dependent on the presence of the risk factors of
CAD, DM, age of more than 40 year for male and46- 50 year for female ??, BMI and
cigarette smoking. Fibrate drug for hyperTG aemia is indicated when plasma TG is
more than 450-500 mg/dl, the level of 200-450 mg/dl may be trated by dietary
regieme.
Lecture 4
Objective: Cases about hyperlipidaemia and Illstration of pathways of Fatty
acids synthesis.
Case 1
A 23-year-old female had the following plasma lipid levels:
Plasma Total cholesterol 550 mg/dl
Plasma TG
90 mg/dl
Plasma HDL-C
45 mg/dl
On examination, she had tendon xanthoma on her Achilles tendons. What is the
suspected diagnosis?.
Case 2
A 15-year-old female presented to the surgical unit with acute pancreatitis. Some of
her laboratory results were as follows:
Plasma Total cholesterol 500 mg/dl
Plasma TG
HDL-C
2400 mg/dl
40 mg/dl
On examination, she had reuptive xanthoma on her arms and lipaemia retinalis.
What is the suspected diagnosis?
Fatty acids synthesis
Fatty acid is very important component of lipid because it participates in
structures of all forms of lipid compounds. There are three pathways in humans
that used in fatty acids synthesis, of the most important one is the de novo synthesis
pathway. This de novo pathway means the formation of fatty acid, principally the
palmitic acid C16 from the simple unit the acetyl CoA. This pathway occurred
mainly in the liver, lactating glands and to a lesser extent , in the adipose tissue. The
synthesis of fatty acids in humans is derived mainly from excess ingested amounts of
particularly the CHO, and to a lesser extent proteins.
The de novo pathway of fatty acids synthesis occurred in the cytoplasmic portion of
the cells of the above defined organs and included:
1.
Production of Acetyl-CoA in the cytoplasm. This molecule is
produced mainly from excess amount of glucose, beyond the need for production
of ATP energy. Also, acetyl-CoA is produced from fatty acids oxidation, ketone
bodies, and from catabolism of some amino acids(ketogenic amino acids).
However, acetyl-CoA is exclusively formed from these molecules in the
mitochondria and it cannot pass the membrane of mitochondria to be available
into the cytoplasm, the cite of de novo pathway synthesis. The alternative way to
cytoplasmic availability of this unit is by formation of citrate molecule from
condensation of mitochondrial Acetyl-CoA and oxaloacetate(steps of citric acid
cycle, CAC). The formed citrate is directed through the mitochondrial membrane
into the cytoplasm, this movement is enhanced when amount of produced NADH,
and consequently ATP is adequately increased to inhibits the isocitrate DH
enzyme(CAC). The cytoplasmic citrate is then dissociates again into Acetyl-CoA
and oxaloacetate by enzyme ATP-citrate lyase. The result Acetyl-CoA in the
cytoplasm is available for steps of de novo fatty acids synthesis.
∙Acetyl-CoA + oxaloacetate…citrate synthase……»..
citrate(Mitochondria)
∙Citrate(mitochondria)……….».citrate(cytoplasm)…ATP-citrate.lyase…»AcetylCoA(cytoplasm)+oxaloacetate
2.
The rate limiting step, the regulatory step in this pathway
synthesis is the formation of Malonyl-CoA molecule from carboxylation reaction
of Acetyl-CoA by CO2 in the presence of ATP. This reaction is catalyzed by
Acetyl-CoA carboxylase enzyme, the regulatory enzyme of de novo pathway
synthesis of fatty acids.
Acetyl-CoA carboxylase
Acetyl-CoA ………CO2…………………………… Malonyl-CoA
C2 unit
ATP……ADP+Pi
When the regulatory enzyme
C3 unit
Acetyl-CoA carboxylase is stimulated the overall
pathway of fatty acid synthesis is enhanced, and vice versa. Regulation of this
enzyme included:
Short -term regulation of Acetyl-CoA carboxylase:
The enzyme is found in active form, polymer and in inactive form, the dimer. The
formation of polymer active one is stimulated allosterically by citrate and inhibited
by long chain fatty acids or their acyl-CoA(the formation of dimer inactive form).
Also, this regulatory enzyme is activated by Insulin(normally postprandial) and
inhibited
by
antagonsits
hormones,
glucagon
and
adrenaline
through
dephosphorylation and phosphorylation reactions, respectively.
Insulin (+)
Acetyl-CoA carboxylase-P ……………………………»Acetyl-CoA carboxylase
Inactive form
Glucagon(-)
Long- term regulation of Acetyl-CoA carboxylase
active form
The long term regulation is dependent on the stimulation or inhibition of gene
expression of the enzyme with resultant increase or decrease amount of the enzyme
rather than the activity only. This stimulation is by Insulin in case of prolonged
excess amount of ingested CHO meal or caloric diet, while the inhibition is by low
CHO or caloric diet.
3.
Addtion of two carbon unit C2 to the formed malonyl CoA(step
2) under the action of Fatty acid synthase enzyme. This enzyme is dimer, each
monomer is multicomplex enzyme composed of 7 enzymes and acyl carrier
protein ACP needed for the enzyme activity. The enzyme is active only in its
dimer form, its splitting into 2 monomer losses the activity of it. The carrier of the
added C2 is another molecule of malonyle-CoA with loss of CO2. So,with each
turn of pathway the native Malonyl-CoA gained two carbon unit, the number of
turns depended on the chain length of synthesized fatty acid, in lactating glands
the short and medium chains are required, while in nervous system the long and
very long chains are required C24 and more. All steps of addition of C2 units are
carried by components of Fatty acid synthase enzyme, transporting the substrate
from one component to the next one. The end step is catalyzed by Thioesterase
enzyme which split the formed fatty acid from the Fatty acid synthase enzyme.
CH3CoA+OOCCH3CoA…-CO2...» CH3COCH2COO-. .....+.NADPH….»
Acetyl-CoA +Malonyl-CoA
Attached to –SH groups of enzyme
CH3CHOHCH2COO- …………-H2O……» CH3CH=CHCOO-…+NADPH…».
CH3CH2CH2COO- with gain two carbon unit, the growing chain which still
attached to enzyme is processed in next turn to accept another C2 from added
new malonyl-CoA and so.
Sources of NADPH required in pathway synthesis
The required NADPH molecules for reduction steps in synthetic pathway are
derived from:
1.Hexomonophosphate shunt pathway(Pentose phosphate pathway)
2. conversion of cytoplasmic oxaloacetate into malate and then pyruvate with
formation of NADPH in the cytoplasm and so can be used by fatty acid synthetic
pathway.
NAD….»NADH H
NADP……»NADPH H
Oxaloacetate….……………………» Malate……-CO2………………» Pyruvate
Another pathways of fatty acid synthesis are:
Reverse of β-oxidation pathway
Elongase system; This pathways elongates the saturated synthesized fatty acid in
de novo pathway, mainly Palmitic acid. The pathway occurred in the
endoplasmic reticulum ER(microsomal fraction of cell) and mitochondrial of
nervous system to produces very long chain fatty acids required in the synthesis
of brain lipids.
Unsaturated Fatty Acids:
These important fatty acids are synthesized in ER also from the satursted ones,
particulary the Palmitic acid by insertion or adding double bond,one or more by
enzymes known as Oxidases or Desaturase which need for NADH and O2.
Lecture 5
Oxidation of Fatty Acid