Metabolism of lipids

Course ContentDigestion and absorption of lipidsTriacylglycerol metabolismPhospholipid metabolismCholesterol metabolismplasma lipoprotein metabolism

SummaryDefinition Classeslipids fats : triacylglycerols, TGcholesterol, Chcholesteryl ester, CEphospholipids, PLglucolipids, GLlipoids Function

nomenclatureFatty acids Saturated fatty acids 1420C palmitic acid 16C stearic acid 18C Unsaturated fatty acids Linolenic acid 18C three unsaturated bonds Linoleate 18C two unsaturated bonds Arachidonic acid 20C four unsaturated bondsEssential fatty acids required for the growth of mammals and they must be obtained from food. Including linoleatelinolenate, arachidonic acid amount unsaturated in plant

Section I Digestion and absorption of lipidsdigestionsmall intestinebilepancreatic lipasecolipasephospholipase A2 cholesteryl esteraseproduct2-monoacylglycerol(MG)FFACholesterollysophospholipidabsorption

Absorption

Monoacylglycerol synthesis Pathway

Section II

Triacylglycerol metabolism

Chemical Structure of Triacylglycerol

Fatty Acid SynthesisPalmitic acid synthesisElongation of FA carbon-chainERER--MitochondrialSynthesis of Unsaturated FARegulation of unsaturated FA

Biosynthesis of palmitic AcidTissuesliver(major site) kidney breastadipose lung brain---CytosolMaterialsAcetyl-CoANADPH+H+ATPHCO3- and Mn2+Pathway ---Synthesis of malonyl-CoA ---Synthesis of fatty acid

Citrate Pyruvate Cycle

malonyl-CoA Synthesis

The overall reaction, which is spontaneous, may be summarized as:HCO3- + ATP + acetyl-CoA ADP + Pi + malonyl-CoA

Enzyme-biotin

HCO3- + ATP

ADP + Pi

Enzyme-biotin-CO2-

O

CH3-C-SCoA

acetyl-CoA

O

-O2C-CH2-C-SCoA

malonyl-CoA

ll

ll

Enzyme-biotin

1

2

Acetyl-CoA Carboxylase, which converts acetyl-CoA to malonyl-CoA, is the committed step of the fatty acid synthesis pathway. The mammalian enzyme is regulated, by phosphorylationallosteric control by local metabolites.Conformational changes associated with regulation:In the active conformation, Acetyl-CoA Carboxylase associates to form multimeric filamentous complexes. Transition to the inactive conformation is associated with dissociation to yield the monomeric form of the enzyme (protomer).

The decreased production of malonyl-CoA prevents energy-utilizing fatty acid synthesis when cellular energy stores are depleted. (AMP is abundant only when ATP has been extensively dephosphorylated.)AMP-Activated Kinase catalyzes phosphorylation of Acetyl-CoA Carboxylase, causing inhibition.

Phosphorylated protomer of

Acetyl-CoA Carboxylase (inactive)

Dephosphorylated Polymer of

Acetyl-CoA Carboxylase (active)

Citrate

Dephosphorylated, e.g., by insulin-activated Protein Phosphatase

Palmitoyl-CoA

Phosphorylated, e.g., via AMP-activated Kinase when cellular stress or exercise depletes ATP.

Regulation of Acetyl-CoA Carboxylase

The antagonistic effect of insulin, produced when blood glucose is high, is attributed to activation of Protein Phosphatase.

Phosphorylated protomer of

Acetyl-CoA Carboxylase (inactive)

Dephosphorylated Polymer of

Acetyl-CoA Carboxylase (active)

Citrate

Dephosphorylated, e.g., by insulin-activated Protein Phosphatase

Palmitoyl-CoA

Phosphorylated, e.g., via AMP-activated Kinase when cellular stress or exercise depletes ATP.

Regulation of Acetyl-CoA Carboxylase

Palmitoyl-CoA (product of Fatty Acid Synthase) promotes the inactive conformation, diminishing production of malonyl-CoA, the precursor of fatty acid synthesis. This is an example of feedback inhibition.Regulation of Acetyl-CoA Carboxylase by local metabolites:

Phosphorylated protomer of

Acetyl-CoA Carboxylase (inactive)

Dephosphorylated Polymer of

Acetyl-CoA Carboxylase (active)

Citrate

Dephosphorylated, e.g., by insulin-activated Protein Phosphatase

Palmitoyl-CoA

Phosphorylated, e.g., via AMP-activated Kinase when cellular stress or exercise depletes ATP.

Regulation of Acetyl-CoA Carboxylase

[Citrate] is high when there is adequate acetyl-CoA entering Krebs Cycle. Excess acetyl-CoA is then converted via malonyl-CoA to fatty acids for storage. Citrate allosterically activates Acetyl-CoA Carboxylase.

Glucose-6-phosphatase

glucose-6-P glucose

Gluconeogenesis Glycolysis

pyruvate

fatty acids

acetyl CoA ketone bodies

cholesterol

oxaloacetate citrate

Krebs Cycle

Fatty acid synthesis from acetyl-CoA & malonyl-CoA occurs by a series of reactions that are:in bacteria catalyzed by 6 different enzymes plus a separate acyl carrier protein (ACP) in mammals catalyzed by individual domains of a very large polypeptide that includes an ACP domain.NADPH serves as electron donor in the two reactions involving substrate reduction. The NADPH is produced mainly by the Pentose Phosphate Pathway.

Mammalian fatty acid synthaseA dimer of two polypeptides of 240 kDa eachEach polypeptide contains eight domains that represent the seven catalytic centres plus an integral acyl carrier protein (ACP) domain

4 phosphopantetheine

The structure of the mammalian Fatty Acid Synthase protein is summarized above

KS = b-Ketoacyl Synthase (Condensing Enzyme)---(Cys)AT =Acyl transferaseMT = Malonyl/Acetyl-CoA Transacylase DH = DehydrataseER = Enoyl Reductase KR = b-Ketoacyl Reductase TE = Thioesterase ACP = Acyl Carrier Protein ---(Pant)

4. Reduction3.Dehydration2.Reduction1.Condensation5. Acyl transfer

Elongation of FA Carbon-chainERMitochondriaSynthesis of unsaturated FAunsaturated FA Oleatelinoleatelinolenatearachidonic acid Essential FA Essential FArequired for the growth of mammals and they must be obtained from food. Including linoleatelinolenate, arachidonic acid

Regulation of FA synthesisDietary factors: carbohydrate promotes synthesisHormone factors insulinstore hormoneincrease FA synthesis Glucagon release hormoninhibit FA synthesis

Important of polyunsaturated fatty acids---prostaglandins (PG) thromboxanes (TX) leukotrienes (LT)

Chemical structure and nomenclature of PG TX LTSynthesis of PGTX and LTPhysiological functions of PGTX and LT

Thromboxane A2Leukotriene A4(LTA4)

Synthesis of PGTX and LT

Physiological functions of PGTX and LT

PGPGE2 triggers inflammationPGE2PGA2 downregulates blood pressurePGF2promotes ovulationdeliveryTXTXA2 and PGE2 promotes coagulation and thrombosisPGI2inhibiting coagulation and thrombosisLTConstriction of bronchial smooth muscle cellsSlow reaction substancesSRS-Aare mixtures of LTC4LTD4and LTE4

Synthesis of Triglycerideslocationliveradipose tissue and small intestinal materialsglucosedietary fatspathwayAcylglycerol pathway diacylglycerol pathway

Dietary (external)Fat synthesis(internal)CMCMVLDLFFAFFA mobilization

Diacylglycerol Pathway

Degradation of TriacylglycerolsLipolysisGlycerol Metabolism-OxidationOther oxydation modes of fatty acidFormation and utilization of Ketone Bodies

LipolysisConceptCommitted enzymehormone-sensitive triglyceride lipase (HSL)Lipolysis hormonesadrenalin glucagonACTH and TSHAnti-lipolysis hormonesInsulinPEGE2 and Nicotinic Acid

PPiHSL(active)ADPHSL(inactive)PPi

GlucagonInsulin(+)(-)adenylyl cyclase

ATPcAMP(+)Protein kinaseHSLTGDGMGFFAglycerolFFACommitted enzymelipolysis

Glycerol metabolism

Experimental evidence for -Oxidation of fatty acid

-Oxidation of fatty acid-acetyl-CoAStepsActivation of FAenter into mitochondria -oxidation TAC(Tricarboxylic acid cycle)

Activation of fatty acid Formation of Acyl-CoALocationcytosolacyl-CoA

Carnitine(Acyl-CoA)(Carnitine)

Mitochondrion

CoASHCoASHCarnitine acyltransferase Carnitine acyltransferase

Acyl CoA enter into mitochondrionCommitted enzyme

-Oxidation of fatty acidlocationmitochondrial matrix-dehydrogenation hydrationdehydrogenation thiolysisCoA(acyl-CoA)CoA(acetyl-CoA)

FADFADHNAD+NADHCoASHH2OH+(dehydrogenation)(hydration)(dehydrogenation)(thiolysis)

(dehydrogenation)(hydration)(dehydrogenation)(thiolysis)

-OxidationcytosolATPAcyl-CoAcarnitineCAT--Oxidationmitochondrion: including dehydrogenation hydration dehydrogenation thiolysis four repeated steps

C167-8CoA7FADH27NADH 12 8 +27+3 7=131ATP 2131-2=129ATPformula12 +5 ( -1) 2

Difference between synthesis and degradation of palmitic Acid

differencesynthesisdegradationlocationcytosolmitochondriaAcyl carrierACPCoATwo carbon- fragmentMalonyl-CoAAcytel-CoAreducing equivalents NADPHFADNAD+HCO3- and citrateneededNot neededEnergy alterationConsume 7ATP14NADPHForm 129ATP

Difference Between Fatty Acid Synthesis And -Oxidation

DiffferenceSynthesis -OxidationLocationCytoplasmMitochondrionThioester linkageACPCoATwo carbon-fragmentMalonyl-CoAAcetyl-CoAElectron carrierNADPHFADHNADHHCO3- and cytratreneededNod neededEnergy alterationConsume 7ATP14NADPHForm 129ATP

Other oxydation modes of fatty acidOxydation of unsaturated FAFA oxydation in peroxisomes Oxydation of propionic acid

Formation and utilization of Ketone BodiesKetone BodiesAcetoacetate -Hydroxybutyrate and AcetoneKetogenesisUtilization of Ketone Bodies Physiology Significance of KetogenesisRegulation of Ketogenesis

ketone bodies(KB)Acetoacetate-hydroxybutyrateAcetone

NADHNAD+CO2CoASHH+CoASHCoASH

Formation of Ketone Bodies

Utilization of Ketone Bodies

UrineAcetoneLungsCitric acid cycleCitric acid cycleLiver BloodExtrahepatic Tissues

Major energy materials provided for tissues

GlucoseFFAKBRed Blood Cell+Brain++Muscle+(exercise)+(rest)+Liver++

Concentration of energy materials of the blood in full of eating or hungry (mmol/L)

Full(of eating)Hungry(5-6 weeks)Glucose5.04.49-Hydroxybutyrate0.026.67Acetoacetate1.17

Physiology Significance of KetogenesisKetone bodies serve as a fuel for extrohepatic tissues.Ketoacidosis results from prolonged ketosis:Higher than normal quantities of ketone bodies present in the blood or urine constitude ketonemia or ketonuria, respectively.The overall condition is called ketosis.

Three Crucial Steps for Ketogenesis RegulationControl of free fatty acid(FFA) mobilization from adipose tissueThe activity of carnitine acyltransferase (CAT-1) in liver,which determines the propotion of the fatty acid flux that is oxidized rather than esterified;Partition of acetyl-CoA between the pathway of ketogenesis and the citric acid cycle

Regulation of Ketogenesis

1(lipolysis)2(HSL)34-(-oxidation)5(essential fatty acid)6(ketone bodies)

1234CO2H2OATP

1 A B C D E (AB)

2 FAD :A CoAB - CoA C D CoA E - (D)

314 -CoAA2 B7-7CoAC6 FADH2 6 NADH + H+D-(AC)

4 A BCH3CO~CoA CRCH2CH2CH2CO~CoA DRCH2CHOHCH2CO~CoA ENAD+NADH (A)

51CoA180O2 A23 B26 C30 D16 E32 (B)

6 A- B C D (ABCD)

7CO2H2O A B C D (AC)

Section II

Phospholipid Metabolism

Classification of Phospholipids phosphoglyceride Phosphatidylcholine (PC) Phosphatidylethanolamine (PE) Phosphatidylserine (PS) Phosphatidylglycerol (PG) Diphosphatidylglycerol (DPG) phosphatidyl inositol(PI)Sphingomyelin

Chemical Structure of Phosphoglyceride

Most phospholipids have a saturated fatty acid on C-1 and an unsaturated fatty acid (Arachidonic Acid )on C-2 of the glycerol backbone.

Structure of Phospholipid

Classification of phosphoglyceride-1X-OH X- name

Classification of phosphoglyceride-2X-OH X- name

Glycerophospholipid synthesisSite: liverkidneyintestineendoplasmic reticulum, ERSourcesFAglycerolphosphatenitrogenous base cholineethanolamineserineinostol,etcATP, CTPCDP- nitrogenous base CDP-diacylglycerol

CDP-cholineCDP- diacylglycerol

Diacylglycerol Pathway

Diacylglycerol PathwayPE, PC

CDP- Diacylglycerol PathwayPIPSDPG

Synthesis of CDP- nitrogenous base

Phosphoglycerol degradation

Metabolism of SphingolipidsSphingomyelins and Glycosphingolipids

SphingomyelinGlycosphingolipidX=Phosphorylcholin or phosphoethanolamineX=Mono-or Poly- saccharin

Synthesis and Degradation of Sphingomyelin

Site: brain---ERSouces:palmitoyl-CoA, Serine,NADPH+H,FADPathway:

Degradation of Sphingomyelin Sphingomyelinase (PLC ) -----Defects in the enzymes result in genetic diseases such as Niemann- Pick disease

CTP

1234

1CoA180O2 A23 B26 C30 D16 E32 (B)

1 ACoA BCoA C DNADH E

(B)

2CoA A B C DCoA E (B)

3 A B C D (B)

4 ATDP- BADP- CUDP- DGDP- ECDP- (E)

Section IV

Cholesterol(Ch) Metabolism

Cholesterol StructureCholesterol(Ch)Cholesterol Ester(CE)

Roles of CholesterolMembrane component

Steroid synthesis

Bile acid/salt precursor

Vitamin D precursor

Sources of CholesterolDietDe novo synthesisCholesterol synthesized in extrahepatic tissuesLiver cholesterolpoolFree cholesterolIn bileConversion to bile salts/acidsSecretion of HDLand VLDL

Dietary CholesterolAnimal products eggsAbsorb about 50%Increase intake = decreased absorptionExcrete 1 g/day (bile acids)

Dietary CholesterolAssume 400 mg intake / day200 mg is absorbed1000 mg is excreted800 mg from de novo synthesis

Lowering cholesterol in diet has very little effect on blood cholesterol !!!

Cholesterol Synthesis80 % in liver, ~10% intestine, ~5% skin Occurs in cytosol

Requires 18Acetyl-CoA16NADPH36ATP

Similar to ketogenic pathway Highly regulated

Cholesterol Synthesis-1

Cholesterol Synthesis -2

Cholesterol SynthesisSummary

Regulation of Cholesterol Synthesisrate-limiting enzymeHMGCoA reductaseRegulation factorsFamine and saturation: famine (-), fasting (-) cholesterol inactivate HMGCoA reductaseHormons: insulin/thyroxin induce activity of HMGCoA reductase; glucagon/cortisol/Epi inactivate HMGCoA reductase

HMG CoA reductase - Phosphorylation AMP-ActivatedProtein Kinase (high activity)AMP-ActivatedProtein Kinase(low activity)phosphataseInsulin(+)kinaseGlucagon/epi(+) increase cAMPAMP(+)HMG CoA reductase OH (active)HMG CoA reductase P (inactive)

Conversion of Cholesterol Bile acid: liver (2/5)Steroids: adrenal cortex, testicle,ovaryVitamin D: skin(7-dehydrocholeterol and Vitamin D3)

Section IV Metabolism of Plasma Lipoproteins

Plasma lipidsPlasma lipoproteinsApolipoproteins Metabolism of Plasma Lipoproteins Medical implications

Plasma Lipids ----Lipids in plasmaTG100mg/dlPL 200mg /dl lecithins 70% nerve sphingomyelin 20% cephalin 10%Ch and CE200mg /dl Ch:55mg /dl; CE:145mg /dlFFA15mg /dlOrigin of plasma lipids Exogenous: dietary lipids Endogenous: synthetized by liver, adipose tissue and other tissues

Plasma Lipoproteins ClasseselectrophoresisCM (Chylomicron) pro-ultracentrifugation CMVLDL(very low density lipoprotein)LDLHDL

Compositions of plasma lipoproteins

Structure of plasma lipoprotein

B48Chylomicron Carry cholesterol estersLacks LDL recptbinding domain

B100VLDL,IDL,LDL Binds LDL recpt.

C-IIChyl. VLDL, IDL, HDL Activates LPL

C-IIIChyl. VLDL, IDL, HDL Inhibits LPL

E Chyl. Remnant, VLDL, IDL Binds LDL recptHDL

A-1HDL/ChylomicronLCAT activator(lecithin:cholesterol acyltransferase)A HDL HL(+)HDL A HDL,CM LPL(+)Type Association Function Apolipoproteins (apo) -1

D HDL transports CE J HDL binds and transports lipids CETP HDL transports CE,TG PTP HDL transports PLType Association Function Apolipoprotein (apo) -2

Major Enzymes for Lipoprotein Metabolismlipoprotein lipase,LPLhepatic lipase,HLlecithin: cholesterol acyltransferase, LCAT acyl-CoA: cholesterol acyltransferase, ACAT

lipoprotein lipaseLPL

hepatic lipaseHP

lecithin: cholesterol acyltransferase, LCAT

acyl-CoA: cholesterol acyltransferase,ACAT

CM MetabolismapoCLPL

VLDL MetabolismapoCLPL

CE

LDL Metabolism

HDL Metabolism

Function of plasma lipoproteinsCMTransport dietary from intestine to liver (exogenous) VLDL: Transport lipids from liver to peripheral tissues (endogenousLDLendogenous Cholesterol transport HDLreverse Cholesterol transport

Clinical importance for disease

Hypertriglyceridemia and CHD Risk: Associated AbnormalitiesAccumulation of chylomicron remnantsAccumulation of VLDL remnantsGeneration of small, dense LDLAssociation with low HDLIncreased coagulability - plasminogen activator inhibitor (PAI-1) - factor VIIc - Activation of prothrombin to thrombin

Genetic DiseaseLPL DeficiencyLDL receptor Deficiency

LDL

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