Metabolism of Lipids

Lipid Metabolism II Ankush Goyal

1. LIPID METABOLISM
ANKUSH GOYAL
ASSISTANT PROFESSOR
PHARMACEUTICAL CHEMISTRY
MAHARAJA AGRASEN SCHOOL OF PHARMACY

2. Contents
1.Introduction
2.β-OxidationofSaturatedFattyAcids
3.Formationandutilizationofketonebodies
4.Ketoacidosis
5.Denovosynthesisoffattyacids(Palmiticacid)
6.Biologicalsignificanceofcholesterolandconversionofcholesterolinto
bileacids,steroidhormoneandvitaminD
7.Disordersoflipidmetabolism:Hypercholesterolemia,atherosclerosis,
fatty liver and obesity.

3. 1.Introduction
1.Lipidsconstituteabout15-20%ofthebodyweightinhumans.
2.Triacylglycerols(formerlytriglycerides)arethemostabundant
lipidscomprising85-90%ofbodylipids.Mostofthetriacylglycerols
(TG;alsocalledneutralfatordepotfat)arestoredintheadipose
tissueandserveasenergyreserveofthebody.
3.Fatalsoactsasaninsulatingmaterialformaintainingthebody
temperatureofanimals.

4. Whyshouldfatbethefuelreserveofthebody?
Therearetwomainreasonsforfatbeingthefuelreserveofthebody
1
•Triacylglycerols(TG)arehighlyconcentratedformofenergy,
yielding9Cal/g,incontrasttocarbohydratesandproteinsthat
produceonly4Cal/g.ThisisbecausefattyacidfoundinTGarein
thereducedform.
2
•The triacylglycerols are non-polar and hydrophobic in nature,
hence stored in pure form without any association with water
(anhydrous form). On the other hand, glycogen and proteins are
polar. One gram of glycogen combines with 2 g of water for storage.
For the two reasons stated above, one gram of fat stored in the body
yields nearly six times as much energy as one gram of (hydrated)
glycogen.

5. Long chain fatty acids (of fat)
containing 13 to 21 carbon atoms, are
the ideal storage fuel reserves of the
body.
Fats can support the body’s energy
needs for long periods of food
deprivation.
In extreme cases,
humans can fast and
survive for 60–90 days,
without food.
In extreme cases, the
obese persons can
survive even longer (6
months to one year!)
without food.

6. Transport of lipids
The insoluble lipids are solubilizedin association with proteins to form lipoproteinsin
which form lipids are transported in the blood stream. Free lipids are undetectable in blood.
Chylomicrons
[2% Protein+98% Lipid]
Very Low Density Lipoproteins (VLDL)
[10%Protein+90% Lipid]
Low Density Lipoproteins (LDL)
[20%Protein+80% Lipid]
High Density Lipoproteins (HDL) [40%
Protein+60% Lipid]
Albumin-Free Fatty Acids
Following are the different lipoprotein complexes that transport lipids in the blood stream.

7. Transport of lipids
•They are synthesized in the intestine and transport
exogenous (dietary) triacylglycerolto various tissues. The
chylomicronsare the least in density and the largest in size.
Chylomicrons
•They are produced in liver and intestine and are responsible
for the transport of endogenously synthesized
triacylglycerols.
VLDL
•They are formed from VLDL in the blood circulation. They
transport cholesterol from liver to other tissues.
LDL
•HDL particles transport cholesterol from peripheral tissues to liver
(reverse cholesterol transport). They are mostly synthesized in liver.
HDL
•Free fatty acids in the circulation are in a bound form to albumin.
Each molecule of albumin can hold about 20-30 molecules of free
fatty acids.
Albumin-Free Fatty Acids

8. Triacylglycerol/fatty acid cycle
Duringstarvation,TGstoredinadipose
tissueishydrolysedtofreefattyacids(for
oxidation)toprovideenergytoskeletaland
cardiacmuscle.
However,about65%oftheseFFAare
convertedtoTG,andsentbacktoadipose
tissuefordeposition.Thisprocessof
lipolysisofTGandresterificationofFFA
toTGistermedastriacylglycerol/fattyacid
cycle.

9. 2. β-Oxidation of Saturated Fatty Acids
The β -oxidation of fatty acids involves three stages
I. Activation of fatty acids occurring in the cytosol
II. Transport of fatty acids into mitochondria
III. β -Oxidation proper in the mitochondrial matrix.
The fatty acids in the body are mostly oxidized by β -oxidation.
β -Oxidation may be defined as the oxidation of fatty acids on the β-carbon atom. This
results in the sequential removal of a two carbon fragment, acetyl CoA.

10. I. Activation of fatty acids occurring in the cytosol
Fatty acids are activated to acylCoA by thiokinasesor acylCoA synthetasesrequires ATP,
Coenzyme A and Mg2+.
The reaction occurs in two steps
Step1:FattyacidreactswithATPtoformAcyladenylate
Step2:AcyladenylatethencombineswithCoenzymeAtoproduce
acylCoA
In the activation, two high energy phosphates are utilized, since ATP is converted to pyrophosphate
(PPi).
The enzyme inorganic pyrophosphatasehydrolyses PPito phosphate (Pi). The immediate elimination
of PPimakes this reaction totally irreversible.

11. I. Activation of fatty acids occurring in the cytosol

12. II. Transport of acylCoA into mitochondria
The inner mitochondrial membrane is impermeable to fatty acids.
A specialized carnitinecarrier system (carnitineshuttle) operates to transport activated fatty
acids from cytosol to the mitochondria
This occurs in four steps :
1.Acylgroup of acylCoA is transferred to carnitine(β-hydroxyγ-trimethylaminobutyrate),
catalysedby carnitineacyltransferaseI (present on the outer surface of inner mitochondrial
membrane).
2. The acyl-carnitineis transported across the membrane to mitochondrial matrix by a specific
carrier protein.
3. CarnitineacyltransferaseII (found on the inner surface of inner mitochondrial membrane)
converts acyl-carnitineto acylCoA.
4. The carnitinereleased returns to cytosol for reuse.

13. II. Transport of acylCoA into mitochondria

14. III. β -Oxidation proper in the mitochondrial matrix.
Each cycle of β-oxidation, liberating a two carbon unit-acetyl CoA,
This stages occurs in a sequence of four reactions :
1. Oxidation : AcylCoA undergoes
dehydrogenation by an FAD-dependent
flavoenzyme, acylCoA dehydrogenase.
A double bond is formed between αand
βcarbons (i.e., 2 and 3 carbons).
2. Hydration : EnoylCoA hydratase
brings about the hydration of the double
bond to form β -hydroxyacylCoA.
3. Oxidation :β -HydroxyacylCoA
dehydrogenasecatalyses the second
oxidation and generates NADH. The
product formed is β –ketoacylCoA.
4. Cleavage : The final reaction in β
-oxidation is the liberation of a 2 carbon
fragment, acetyl CoA from acylCoA.
This occurs by a thiolyticcleavage
catalysedby β -ketoacylCoA thiolase
(or simply thiolase).

15. β-Oxidation of Saturated
Fatty Acids
COMPLETE PATHWAY

16. 3. Formation and utilization of Ketone Bodies
The compounds namely acetone, acetoacetateand β-hydroxybutyrate(or 3-
hydroxybutyrate) are known as ketone bodies.
Only the first two are true ketoneswhile β -Hydroxybutyratedoes not possess a keto
group.
Ketone bodies are water-soluble and Energy yielding.

17. Ketogenesis
(Formation or Synthesis of Ketone Bodies)
1.The synthesis of ketone bodies occurs in the liver.
2.The enzymes for ketone body synthesis are located in the mitochondrial
matrix.
3.Acetyl CoA, formed by oxidation of fatty acids, pyruvate or some amino acids, is
the precursor for ketone bodies.

18. Ketogenesis occurs through the following reactions :
a)Condensation of two moles of acetyl CoA to form acetoacetylCoA
b)Addition of one more acetyl CoA to form HMG CoA
c)Cleavage of HMG CoA to form acetoacetate
d)Decarboxylationof acetoacetateto form acetone
e)Reduction of acetoacetateto form β-hydroxybutyrate

19. Utilization of Ketone Bodies
The ketone bodies, being water-soluble, are easily transported from the liver to various
tissues.
The two ketone bodies—acetoacetate and β-hydroxybutyrateserve as important sources
of energy for the peripheral tissues such as skeletal muscle, cardiac muscle, renal
cortex etc.
The tissues which lack mitochondria (e.g. Erythrocytes) however, cannot utilize ketone
bodies.
The production of ketone bodies and their utilization become more significant when
glucose is in short supply to the tissues, as observed in starvation, and diabetes
mellitus.
During prolonged starvation, ketone bodies are the major fuel source for the brain and
other parts of central nervous system.
The ketone bodies can meet 50-70% of the brain’s energy needs.

20. Utilization of Ketone Bodies

21. Ketoacidosis
Both acetoacetateand β-hydroxybutyrateare strong acids. Increase in their
concentration in blood would cause acidosis. The carboxyl group has a pK
a
around
4. Therefore, the ketone bodies in the blood dissociate and release H+ ions which
lower the pH.Diabetic ketoacidosisis dangerous—may result in coma, and even
death, if not treated. Ketosis due to starvation is not usually accompanied by
ketoacidosis.

22. 4. De novo synthesis of fatty acids (Palmiticacid)
The dietary carbohydrates and amino acids, when consumed in excess, can
be converted to fatty acids and stored as triacylglycerols.
De novo (new) synthesis of fatty acids occurs predominantly in liver, kidney,
adipose tissue and lactating mammary glands.
The enzyme machinery for fatty acid production is located in the cytosomalfraction
of the cell.
Acetyl CoA is the source of carbon atoms while NADPHprovides the reducing
equivalents and ATPsupplies energy for fatty acid formation.
Conti..

23. The fatty acid synthesis occurs in three stages :
I. Production of acetyl CoA and NADPH
II. Conversion of acetyl CoA to malonylCoA
III. Reactions of fatty acid synthasecomplex.

24. I. Production of acetyl CoA and NADPH
Requirements: Acetyl CoA and NADPH are the prerequisites for fatty acid synthesis.
Production & Permeability of Acetyl CoA:Acetyl CoAis produced in the mitochondria by the
following pathways:
Oxidation of pyruvate and fatty acids
Degradation of carbon skeleton of certain amino acids
Degradation of ketone bodies
Mitochondria, however, are not permeableto acetyl CoA. An alternate or a bypass
arrangement is made for the transfer of acetyl CoA to cytosol.
Acetyl CoA condenses with oxaloacetate in mitochondria to form citrate. Citrate is freely
transported to cytosol where it is cleaved by citrate lyase to liberate acetyl CoA and
oxaloacetate. Oxaloacetate in the cytosol is converted to malate.
Malicenzyme converts malate to pyruvate. NADPH and CO
2are generated in this reaction.
Both of them are utilized for fatty acid synthesis.
conti….

25. I. Production of acetyl CoA and NADPH

26. II. Conversion of acetyl CoA to malonylCoA
Acetyl CoA is carboxylated to
malonylCoA by the enzyme acetyl
CoA carboxylase.
This is an ATP-dependent reaction
and requires biotin for CO
2
fixation.

27. III. Reactions of fatty acid synthasecomplex
Theremainingreactionsoffattyacidsynthesisarecatalysedbyamultifunctional
enzymeknownasfattyacidsynthase(FAS)complex.
Ineukaryoticcells,includingman,thefattyacidsynthaseexistsasadimerwith
twoidenticalunits.Eachmonomerpossessestheactivitiesofsevendifferent
enzymesandanacylcarrierprotein(ACP)boundto4’-phosphopantetheine.
Fattyacidsynthasefunctionsasasingleunitcatalysingallthesevenreactions.

28. The sequence of reactions of the extra-mitochondrial synthesis of fatty acids (palmitate) is
as following :

29. Steroids
Steroids are the compounds containing a cyclic steroid nucleus (or ring).
Steroid nucleus is cyclopentanoperhydrophenanthrene(CPPP).
Itconsists of a phenanthrene nucleus (rings A, B and C) to which a cyclopentanering (D)
is attached.
There are several steroids in the biological system. These include
Cholesterol
Bile acids
Vitamin D
Sex hormones
Adrenocortical hormones
Sitosterols
Cardiac glycosides and alkaloids
If the steroid contains one or more hydroxyl
groups it is commonly known as Sterol.

30. Cholesterol
Cholesterol,exclusivelyfoundinanimals,isthe
mostabundantanimalsterol.Cholesterolis
amphipathicinnature,sinceitpossessesboth
hydrophilicandhydrophobicregionsinthe
structure.
Structure of Cholesterol:
It has one hydroxyl group at C-3 and a double bond
between C-5 and C-6. An 8 carbon aliphatic side chain
is attached to C-17. Cholesterol contains a total of 5
methyl groups.
Cholesterol consist of two words i.e.
Greekword Chole = Bile
Sterol = Solid alcohol
Cholesterol was first isolated from bile.
Cholesterol literally means ‘solid alcohol from bile.’

31. Biological Significance of Cholesterol
1. It is a structural component of cell membrane.
2. Cholesterol is the precursor for the synthesis of all other steroids in the
body. These include steroid hormones, vitamin D and bile acids.
3. It is an essential ingredient in the structure of lipoproteins in which
form the lipids in the body are transported.
4. Fatty acids are transported to liver as cholesterylesters for oxidation.

32. Conversion of cholesterol into Bile acids,
Steroid hormone and Vitamin D
BileAcids:Bileacidsserveasemulsifyingagentsintheintestineand
activelyparticipateinthedigestionandabsorptionoflipids.
Primarybileacids:Thesynthesisofprimarybileacidstakesplaceinthe
liver.Cholicacidandchenodeoxycholicacidaretheprimarybileacids.On
conjugationwithglycineortaurine,conjugatedbileacids(glycocholicacid,
taurocholicacidetc.)areformed.
Secondarybileacids:Intheintestine,aportionofprimarybileacids
undergoesdeconjugationanddehydroxylationtoformsecondarybileacids
(deoxycholicacidandlithocholicacid).Thesereactionsarecatalysedby
bacterialenzymesintheintestine.

33. Steroid hormone
Cholesterol is the precursor for the synthesis
of all the five classes of steroid hormones
(A) Glucocorticoids(e.g. Cortisol)
(B) Mineralocorticoids(e.g. Aldosterone)
(C) Progestins(e.g. Progesterone)
(D) Androgens (e.g. Testosterone)
(E) Estrogens (e.g. Estradiol)

34. Conversion of Cholesterol to Vitamin D
Vitamin D is a fat soluble vitamin. It
resembles sterols in structure and
functions like a hormone.
7-dehydrocholesterol, an intermediate in
the synthesis of cholesterol, is converted
to cholecalciferol(Vitamin D3) by
ultraviolet rays in the Skin.

35. Disorders of lipid metabolism
Hypercholesterolemia :
Increase in plasma cholesterol (> 200 mg/dl) concentration is known as hypercholesterolemia.
Hypercholesterolemia is associated with atherosclerosis and coronary heart diseases.
Control of hypertcholestrolemia:
1.Fiberpresentinvegetablesdecreasesthecholesterolabsorptionfromtheintestine.
2.Consumptionofpolyunsaturatedfattyacidsandfiberdecreasescholesterolincirculation.
3.Dietsrichincarbohydrates(e.g.sucrose)shouldbeavoidedtocontrolhypercholesterolemia.
4.Elevationinplasmacholesterolisobsevedinpeoplewithsmoking,abdominalobesity,lackofexercise,stress,high
bloodpressure,consumptionofsoftwateretc.Therefore,adequatechangesinthelifestyleswillbringdownplasma
cholesterol.
5.DrugssuchaslovastatinwhichinhibitHMGCoAreductaseanddecreasecholesterolsynthesisareused.Statinscurrently
inuseincludeatorvastatin,simvastatin,fluvastatinandpravastatin.

36. Atherosclerosis :
It is a complex disease characterized by thickening or hardening of arteries due to the
accumulation of lipids (particularly cholesterol, free, and esterified) collagen, fibrous tissue,
proteoglycans, calcium deposits etc. in the inner arterial wall.
Atherosclerosis is a progressive disorder that narrows and ultimately blocks the arteries.
Infarctionis the term used to indicate the stoppage of blood flow resulting in the death of
affected tissue. Coronary arteries (the arteries supplying blood to heart) are the most
commonly affected leading to myocardial infarction or heart attacks.
The probable causes of atherosclerosis include hyperlipoproteinemias, diabetes mellitus,
obesity, high consumption of saturated fat, lack of exercise and stress.

37. Fatty liver :
The normal concentration of lipid (mostly phospholipid) in liver is around 5%. Liver is not a
storage organ for fat, unlike adipose tissue. However, in certain conditions, lipids (especially the
triacylglycerols) accumulate excessively in liver, resulting in fatty liver.
In the normal liver,kupffercells contain lipids in the form of droplets.
In fatty liver,droplets of triacylglycerols are found in the entire cytoplasm of hepatic cells.
This causes impairment in metabolic functions of liver. Fatty liver is associated with fibrotic
changes and Cirrhosis, fatty liver may occur due to two main causes.
1. Increased synthesis of triacylglycerols
2. Impairment in lipoprotein synthesis.
Certain hormones like ACTH, insulin, thyroid hormones, adrenocorticoidspromote deposition
of fat in liver.

38. Obesity
Obesityisanabnormalincreaseinbodyweightduetoexcessivefatdeposition
(>25%).
Obesityisduetoanincreaseinboththenumberandsizeofadipocytes(ofadipose
tissue).
Therearetwotypesofadiposetissues:
1.Whiteadiposetissue:Thefatismostlystoredandthistissueismetabolicallylessactive.
2.Brownadiposetissue:Thestoredfatislessbutthetissueismetabolicallyveryactive.
Brownadiposetissuepossesseshighproportionofmitochondriaandcytochromesbutlow
activityofATPsynthase.Thisisanactivecentrefortheoxidationoffatandglucoseandis
responsibleforthediet-inducedthermogenesis.Aspecificprotein(thermogenin)hasbeen
isolatedintheinnermembraneofthesemitochondria.Thermogeninfunctionslikean
uncoupleranddissipatestheenergyintheformofheat,andthusblockstheformationofATP.

39. It is significant to note that brown adipose tissue is almost absent in obese
persons. Some individuals are fortunate to have active brown adipose tissue.
They eat and liberate it as heat, and therefore do not become obese.
Pharmacological treatment of obesity : Inrecent years, synthetic lipids such as
olestra and orlistatare used to treat obesity. They taste like natural lipids but cannot be
digested, and excreted unchanged.
1. Pancreatic lipase degrades dietary triacylglycerolto fatty acids and glycerol which are
absorbed. Orlistatis a non-hydrolysable analog of triacylglycerol, and is a powerful
inhibitor of pancreatic lipase, hence prevents fat digestion, and absorption.
2. Olestrais a synthetic lipid, produced by esterificationof natural fatty acids with sucrose
(instead of glycerol). Olestra tastes like a natural lipid. However, it cannot be hydrolysedand
therefore, gets excreted.