Chapter 8Metabolism of lipidCai danzhao

Lipids are substances that are insoluble in water but soluble in organic solvents.Including: Fats (triglyceridestriacylglycerolsTAG) Function: store and supply energy phospholipids Lipoids cholesterol and cholesterol ester glycolipids Function: as membrane compounds

Triglyceride(TAG) Phospholipid(PL) Cholesterol ester (CE)

CarboxylSaturated FAunsaturated FA (one double bonds)Hydrocarbon chain

Nomenclature: the chain length number of double bonds Palmitic acid 16:0Oleic acid 18:1(9)

Section 8.1Digestion and Absorption of Lipids

Digestion of LipidsLocation: duodenum, small intestineCondition: 1. bile salts (emulsification) 2. lipolytic enzymes Pancreatic lipase Phospholipase A2 Cholesterol esterase

Process bile saltsfood lipids small particles pancreatic lipasetriglyceride 2-monoacylglycerol + 2FFA phospholipase A2phospholipid lysophosphatide + FFA cholesterol esterasecholesterol ester cholesterol + FFA

Pancreatic colipasePancreatic colipse is the necessary cofactor of pancreatic lipase. Excreted by pancreatic acinar cells as a proenzyme, which activated in the small intestine.Can anchor the lipase to the surface of lipid micelles.Assist lipase in two ways: 1.enhances the lipase activity 2.against the inhibitory effects of bile salts and surface denaturation.

Absorption of lipidsMedium and short chain fatty acid (10Cs or less) TAG emulsification absorption intestine mucosal cells degradation FFA and glycerol transport portal vein

blood circulation

Long chain fatty acids (12-26C) + monoacylglycerol absorbed synthesis epithelial cells TAG + lipoproteinslysophosphatide + FFA CM absorbed synthesis (chylomicron) epithelial cells PL cholesterol + FFA lymphatic system absorbed synthesis epithelial cells CE blood circulation

TAG synthesis in epithelial cells: monoacylglycerol pathway1,2-diacylglycerolTAGmonoacylglycerol

Section 8.2Metabolism of Triacylglycerols

Degradation of TAGFAglycerol

Lipolysis: also named fat mobilization, is a process breaking down the fat (TAG) stored in adipose tissue and liberating the glycerol and FFAs from which into the blood circulation.Key enzyme: TAG lipase (hormone sensitive lipase, HSL)

Lipolytic hormones: stimulate TAG hydrolysis Glucagons, Adrenocorticotropic hormone (ATCH) epinephrine norepinephrine

Anti-lipolytic hormones: stimulate TAG formation insulin prostaglandin E2 nicotinic acid

H2O FA H2O FA H2O FA

TAG DAG MAG Glycerol

TAG lipase a DAG lipase MAG lipaseTAG lipase a ADP + Pi TAG lipase b ATP

cAMP dependent protein kinaseATP cAMP 5-AMP Adenylate cyclase Phosphodiesteraselipolytic hormones (Epinephrine )anti-lipolytic hormones (insulin)

FFA +plasma albumins fatty acid- transport albumin complexes all the body

glycerokinaseGlycerol glycerol-3-phosphate dihydroxyacetone phosphate

glucose metabolism Notation: adipose cells and skeletal muscles lack glycerokinase, can not use glycerol well

-Oxidation of Fatty acidsFAs are the major energy source of human the biologically available energy in TAGs: ~ 95 % in their 3 long-chain FAs ~ 5% in their glycerolOxidation location: in the cytoplasm and mitochondria of most body cells (except those of the brain and intestine)

The process of FA degradationActivationTransport into mitochondria-oxidationAcetyl CoA utilization into citric acid cycle change to ketone bodies into other metabolic pathway

1.Activation of FAActivation of FA takes place on the outer mitochondrial membrane (in cytoplasm)Acyl CoA synthaseFatty acidAcyl-CoAATP AMP+PPi

2.Transport of Acyl CoA into mitochondriatranslocasekey enzyme: carnitine acyltransferase I

3. -oxidation of Acyl CoAIn mitochondrial matrix, successive 2-C units are removed from the carboxyl end of the fatty acyl chain in the form of acetyl-CoA by a repeated sequence of 4 reactions, and the oxidation process take place at the -carbon.

To Palmitoyl CoA (C16):1.Dehydrogenation (FAD)

2.Hydration

3.Dehydrogenation (NAD+)

4.Thiolysis

Result of a round of -oxidation : 1NADH, 1FADH2, 1Acytyl CoA, 1Acyl CoA (Cn-2)

C16 Acetyl -CoAAcyl CoA (Cn-2) can now go through another set of -oxidation reactions1234567

1 molecule of palmitoyl-CoA will pass through the sequence 7 times, eventually be oxidized to: 8 Acetyl CoAs 7 NADHs 7 FADH2s

ATP produced during oxidation of palmitate2.67 Glucose (C16): 85.3 ATP

Alternative Oxidation Pathway of Fatty Acids1.Unsaturated FA

oleic acid (18:1, 9) :Can produce a cis-3 C12 acyl CoA, but -oxidation acts only on trans double bonds.Enoyl-CoA isomerase: make a trans- 2 C12 acyl CoA, then -oxidation can continue

2,4-dienoyl-CoA reductaseCan convert cis- 4 double bondto trans- 3,

2.Peroxisomal Fatty Acid Oxidationfor very long chain FA digestion.(C20C22)No ATP produced.FA reduced in length by this pathway will be transferred to mitochondria for further oxidation.Acyl-CoA oxidaseFAD very long chain FA(C20C22) peroxisomal chain shorted FA mitochondria Oxidation

3.Propionyl CoA-oxidation of the odd-chain fatty acids, which are relatively rare in nature, produce a propionyl CoA in the final round.CH3CH2CO~CoA Succinyl CoAcarboxylaseCitric acid cycle

Ketone Bodies Formation and UtilizationKetone Bodies are acetoacetate (30%) -hydroxybutyrate (70%) acetoneGenerated in liver cells (mitochondria), used by extrahepatic tissues (mitochondria also).Precursor: Acetyl CoA

CoASH CoASH NAD+ NADH+H+ HMGCoA synthase acetoacetate-hydroxybutyrate3-hydroxy-3-methylglutaryl CoAAcetoacetyl CoAacetoneKetogenesis

Utilization of ketone bodies(cardiac, kidney, brain, skeletal muscles) NAD+ NADH+H+ Succinyl CoA succinate CoASH+ATP PPi+AMP CoASH -hydroxybutyrate acetoacetateAcetoacetyl CoA

Physiological significance of ketogenesis A way by which liver transfer fuel to extrahepatic tissues (prolonged starvation), ketone bodies can replace glucose as the major source of energy, especially for brain.The normal concentration of ketone bodies in blood is very low. 0.5mmol/L

Under starveling condition, ketogenesis is accelerated.Under some pathological condition (such as diabetes), the synthesis is faster than utilization, so the concentration of ketone bodies in the blood is high, (up to 20mmol/L), which is called ketonemia , if the concentration is too high to be excreted in the urine, that is ketonuria.

Ketone bodies are acidic compounds, the accumulate of which in the blood will decrease the pH of blood , cause ketoacidosis.

Regulation of ketogenesis1.Feeding status: hungry state: FA oxidation ketogenesislipolysislipolytic hormones(glucagon)FFAFA oxidation ketogenesisFeeding state insulinlipolysisFFA

2.Metabolism of glycogen in the hepatic cellsSufficient glucose supply:FFA triacylglycerolsGlucose deficiency: FFA- Oxidation ketogenesis

3.Malonyl CoA concentration Malonyl CoA can inhibit carnitine acyltransferase . malonyl CoAtransportion of fatty acids into mitochondria- Oxidation and ketogenesis

8.2.2 FA BiosynthesisFA synthesis is not the reverse of degradation: different pathways, enzymes, location of cellsLocation of FA synthesis: cytoplasm of liver (major), adipose and other tissue cells.First step: synthesis of palmitic acid

Palmitic Acid BiosynthesisMaterial:acetyl CoA (come mostly from glucose)NADPH (pentose phosphate pathway or produced by malate enzyme.)ATPHCO3-

Acetyl CoA must be transport to cytosol.citratecitrateoxaloacetatemalatepyruvatepyruvateoxaloacetateAcetyl CoAATPAcetyl CoAMITOCHONDRIACYTOSOLCitrate synthaseATP-citrate lyaseInner membrane citrate pyruvate cycle

Formation of malonyl CoA ATP + Acetyl CoA + HCO2- Malonyl CoA + ADP + Pi acetyl CoA carboxylase

biotin act as CO2 carrier

Acetyl CoA carboxylase is the key enzyme of the FA synthesis.Allosteric regulation: up-regulate: citrate and isocitrate down-regulate: palmitoyl CoAPhosphorylation regulation up-regulate: dephosphorylation (insulin) down-regulate: phosphorylation (glucogen)

Repetivity steps catalyzed by Fatty Acid SynthaseThe chain of FA grows 2-carbons per cycle.The reactions are similar to the reversal of FA -oxidation. CH3COSCoA +7 HOOCH2COSCoA + 14NADPH+H+CH3(CH2)14COOH +7 CO2 + 6H2O + 8HSCoA + 14NADP+

Fatty Acid SynthaseIn mammalian: Fatty acid synthase (Type synthase) is a single multifunctional polypeptide with 7 activities.In E.coli (becteria): Fatty acid synthase system (Type system) contain 7 enzymes organized into a cluster.

The acyl carrier protein (ACP) carries a growing fatty acyl chainfrom one active site to the next.

Process :1.Charging -ketoacyl-ACP synthase (KS) with an acetyl group

2.Charging ACP with a malonyl group

1

A. Addition of an activated acyl group with CH2-carbon of malonyl CoA,3. 4 reactions add 2-cabon B. Reduction of the -keto group to an alcohol1

C. Dehydration to create a double bondD. Reduction of the double bond to create saturated fatty acyl group

4. The next round of 4-reactions butyryl group transfers to (KS)

a new round of reactions begin, till a 16-C saturated palmitate synthesized.

Elongation of FA Carbon ChainIn endoplasmic reticulum elongate the chain by 2-C each time, the reactions is similar to what happened in palmitic synthesis, but no ACP is involved.

Material: Acyl CoA, malonyl CoA, NADPH Product: C18 stearate (main)C24 FA

In mitochondria the process here is the reversal of -oxidation, add 2-C each time.

Material: Acyl CoA, Acetyl CoA, NADPH, NADH Product: C18 stearate (main)C24 or C26 FA

Synthesis of polyunsaturated FAHumans have 4589 desaturase, can produce palmitoleic acid (16:1 9) and oleic acid (18:1 9) from palmitate and stearate.NAD+

e- + [Cyt b5-Fe2+] + NADH + H+

Essential Fatty Acids (EFA) linoleic acid (18:2 9,12), linolenic acid (18:3 9,12,15) arachidonic acid (20:4 5,8,11,14) must be obtained from dietary sources, usually from plant, called essential fatty acids.

reasons: there are no desaturases in human can produce double bonds beyond 9-10 carbon position.

Biosynthesis of triacylglycerolsLocation: all tissues, intestine and live are most active tissues.Material: glycerol 3-phosphate (in intestine, monoacylglycerol also be used.) FFA (acyl CoA)Processes: Monoacylglycerol pathway (intestine) Diacylglycerol pathway (liver, adipose)

The origins of glycerolIn intestine, liver and adipose tissues:

In liver and intestine:

dihydroxyacetone phosphateglycerol 3-phosphateglucoseglycolysisdehydrogenateglycerolglycerol 3-phosphatephosphorylationglycerokinase

Diacylglycerol pathway Acyl CoA transferasephosphatidaseAcyl CoA transferase glycerol 3-phosphatelysophosphatidatephosphatidateDiacylglycerol

triacylglycerol

Regulation of TAG metabolismAcetyl CoA carboxylase (ACC) activation: citrate, dephosphorylation (insulin) inhibition: palmitoyl CoA long chain acyl CoA phosphorylation (glucagon)Carnitine acyltransferase inhibition: malonyl CoA

Section 8.3Derivatives of Arachidonic AcidLearned by yourself

Section 8.4Metabolism of phospholipids

Phospholipids are phosphorous-containing lipids.Including:

Glycerophospholipids (phosphoglycerides)

Sphingolipids

1.Metabolism of GlycerophospholipidsConsist of : glycerol, FA, phosphate group, X group.

phosphatidyl serine phosphatidyl inositol11

(lecithin)Hydrophilic headHydrophobic tail

Biosynthesis of glycerolphospholipidsLocation: endoplasmic reticulum and outermitochondrial membrane of all types of cells, mostly in liver, kidney, intestine.Material: FAs and glycerols (glucose, diet) choline or serine, inositol (diet, synthesis) ATP, CTP, Pi

Serine ethanolaminecholine3(S-adenosyl methionine)

CDP-ethanolamineCDP-cholineCMPCMPPhosphatidyl ethanolaminePhosphatidyl choline1.Diacylglycerol pathwayphosphoethanolaminephosphocholine

1CDP-diacylglycerolPhosphatidyl glycerolinositol1111cardiolipinPhosphatidyl inositol2. CDP-diacylglycerol pathwayGlycerol 3-phophatePhosphotidic acid2 acyl CoA2CoACTPppiserineCMPPhosphatidyl serine

Other methodsPhosphatidyl choline 3(S-adenosyl methionine) Phosphatidyl ethanolamine

Phosphatidyl serine change the head Phosphatidyl ethanolamine

Degradation of glycerophospholipidsPhopholipase

Metabolism of sphingolipids

Hydrophilic headHydrophobic tail

Section 8.5Metabolism of cholesterols

Structure of cholesterolBase of the structure: perhydrocyclopenano-phenanthrene (four rings)

Total: 27 carbonsIt is an alcohol found in animal

In plant(29C)In yeast(28C)

1.Cholesterol BiosynthesisAmount: synthesis: 1g/day diet: 0.3g/dayLocation: cytoplasm and endoplasmic reticulum of liver(80%), intestine(10%) and other tissues.

Material: To one cholesterol: 18 acetyl CoA (glucose, amino acid) 36 ATP 16 NADPH+H+ (pentose-phosphate pathway)

Process:Three stages:Conversion of acetyl CoA to mevalonate (C6)Conversion of mevalonate to squalene (C30)Squalene cyclization and conversion to cholesterol (C27)

CoASH CoASH Acetoacetyl CoA3-hydroxy-3-methylglutaryl CoAMevalonate (MVA)HMGCoA reductase (key enzyme)1. Form MVA

2. Form squalenesqualeneMVA5-pyrophospho-mevalonate13.Form cholesterol

Regulation of cholesterol BiosynthesisHMG CoA reductase: 1. phosphorylated (glucagon): inactivation dephosphorylated (insulin): activation 2. feed back inhibition of cholesterol: inhibit the formation of enzyme.

3. Feeding status: starvation: decrease the activation overeating: increase the activation

4. Feedback regulation of degradation : high sterol synthesis: high degradation low sterol synthesis: low degradation

Ch biotransformation and excretionConversion to Bile Acid (in liver) 1g ch is excreted as bile acid each day.Conversion to cholesterol hormones (in adrenal cortex, testis, ovary) pregnanes, androstanes, estranes, glucocorticoids, mineralocorticoids, Conversion to 7-dehydrocholesterol (skin),which can be change to Vit D3 by UV light.

Section 8.6Metabolism of plasma lipoproteins

Plasma lipidsIncluding: FFAs, TAGs, cholesterols (CH), cholesterol esters (CE), phospholipids (PL),Total lipids: 5 mmol/LTotal TAGs: 0.11 ~ 1.69 mmol/L Total PL: 48.44 ~ 80.73 mmol/L Total CH: 2.59 ~ 6.47 mmol/L FFA : 0.195 ~ 0.805 mmol/L

Plasma lipoproteinsLipoproteins are globular, micelle-like particles consisting of a hydrophobic core of triacylglycerols and cholesterol esters surrounded by an amphipathic coat of protein, phospholipid and cholesterol, acting as the transport form of plasma lipids.

protein

Classification:Agarose gel electrophoresis: Chylomicrons(CM)Pre lipoproteins lipoproteins lipoproteins

Density gradient ultracentrifugation

Chylomicron, (CM)Very low density lipoprotein, ( VLDL) Low density lipoprotein, ( LDL)High density lipoprotein, (HDL)

Apolipoproteins (apoproteins, apo):Apolipoproteins are the proteins involved in the lipoproteins.Classification:

Function: -act as cofactors for lipid metabolism enzymes -maintain the lipoprotein structureapo A: AAA apo B: B100B48 apo C: CCC apo D apo E

CM VLDL LDL HDLelectrophoresis CMPre density 0.950.95~1.0061.006~1.0631.063~1.210Composition:TAG 80~90%apo 1%TAG 50~70%apo 5~10%Ch and CE 40~50%apo 20~25%Lipids 50%apo 50%originintestineliverVLDL/liverIntestine, liverapolipoproteinsapoB48, E A, A AC CCapoB100CC C EapoB100apo A A

Functions Dietary TAGs and cholesterol transportEndogenous TAGs and cholesterol transportEndogenous cholesterol and CE transport (from liver to tissue)Reverse transport ofcholesterol and CE (from tissue to liver)

Abnormal metabolism of lipoproteinHyperlipoproteinemia (hyperlipidemia) refer to abnormally high levels of lipids of lipoproteins in blood. It can be divided into 6 types.Genetic disease: associated with the mutation of key enzyme or apo involved in lipoprotein metabolism, such as: apoCBEACLDL recepter.

Familial hypercholesterolemiainherited disorder of lipids.Reason: lack of functional LDL receptorsCharacters: elevated level of Ch in blood cutaneous xanthoma in childhood coronary artery disease atheroscleroticthrombus