Fatty Acid Synthesis• Occurs mainly in liver and adipocytes, in mammary glands during lactation• Occurs in cytoplasm• FA synthesis and degradation occur by two completely separate pathways• When glucose is plentiful, large amounts of acetyl CoA are produced by glycolysis and can be used for fatty acid synthesis
Three stages of fatty acid synthesis:A. Transport of acetyl CoA intocytosolB. Carboxylation of acetyl CoAC. Assembly of fatty acid chain
A. Transport of Acetyl CoA to the Cytosol• Acetyl CoA from catabolism of carbohydrates and amino acids is exported from mitochondria via the citrate transport system• Cytosolic NADH also converted to NADPH• Two molecules of ATP are expended for each round of this cyclic pathway
Sources of NADPH for Fatty Acid Synthesis1. One molecule of NADPH is generated for eachmolecule of acetyl CoA that is transferred frommitochondria to the cytosol (malic enzyme).2. NADPH molecules come from the pentosephosphate pathway.
B. Carboxylation of Acetyl CoA Enzyme: acetyl CoA carboxylase Prosthetic group - biotinA carboxybiotin intermediate is formed.ATP is hydrolyzed.The CO2 group in carboxybiotin is transferred toacetyl CoA to form malonyl CoA.Acetyl CoA carboxylase is the regulatory enzyme.
C. The Reactions of Fatty Acid Synthesis• Five separate stages: (1) Loading of precursors via thioester derivatives (2) Condensation of the precursors (3) Reduction (4) Dehydration (5) Reduction
During the fatty acid synthesis all intermediates are linkedto the protein called acyl carrier protein (ACP-SH), whichis the component of fatty acyl synthase complex.The pantothenic acid isa component of ACP.Intermediates in thebiosynthetic pathwayare attached to thesulfhydryl terminus ofphosphopantotheinegroup.
The elongation phase of fatty acid synthesis starts withthe formation of acetyl ACP and malonyl ACP.Acetyl transacylase and malonyl transacylase catalyzethese reactions.Acetyl CoA + ACP ⇔ acetyl ACP + CoA Malonyl CoA + ACP ⇔ malonyl ACP + CoA
Reduction.Acetoacetyl ACP isreduced to D-3-hydroxybutyryl ACP.NADPH is thereducing agentEnzyme: β-ketoacylACP reductase
Dehydration.D-3-hydroxybutyrylACP is dehydratedto form crotonylACP(trans-∆2-enoylACP).Enzyme: 3-hydroxyacylACP dehydratase
Reduction.The final step in the cyclereduces crotonyl ACP tobutyryl ACP.NADPH is reductant.Enzyme - enoyl ACPreductase.This is the end of firstelongation cycle (firstround).
In the second roundbutyryl ACP condenseswith malonyl ACP toform a C6-β-ketoacylACP.Reduction, dehydration,and a second reductionconvert the C6-β-ketoacyl ACP into a C6-acyl ACP, which is readyfor a third round ofelongation.
Final reaction of FA synthesis• Rounds of synthesis continue until a C16 palmitoyl group is formed• Palmitoyl-ACP is hydrolyzed by a thioesterase Overall reaction of palmitate synthesis from acetyl CoA and malonyl CoAAcetyl CoA + 7 Malonyl CoA + 14 NADPH + 14 H+Palmitate + 7 CO2 + 14 NADP+ + 8 HS-CoA + 6 H2O
Organization of Multifunctional Enzyme Complex in EukaryotesThe synthase is dimer with antiparallel subunits.Each subunit has three domains.ACP is located in domain 2.Domain 1 contains transacylases, ketoacyl-ACPsynthase (condensing enzyme)Domain 2 contains acyl carrier protein, β-ketoacylreductase, dehydratase, and enoyl reductase.Domain 3 contains thioesterase activity.
Fatty Acid Elongation and DesaturationThe common product of fatty acid synthesis ispalmitate (16:0).Cells contain longer fatty acids and unsaturatedfatty acids they are synthesized in the endoplasmicreticulum.The reactions of elongation are similar to the onesseen with fatty acid synthase (new carbons areadded in the form of malonyl CoA).For the formation of unsaturated fatty acids thereare various desaturases catalizing the formation ofdouble bonds.
THE CONTROL OF FATTY ACID METABOLISMAcetyl CoA carboxylase plays an essential rolein regulating fatty acid synthesis anddegradation.The carboxylase is controlled by hormones: glucagon, epinephrine, and insulin.Another regulatory factors: citrate, palmitoyl CoA, and AMP
Global Regulationis carried out by means of reversible phosphorylationAcetyl CoA carboxylase is switched off by phosphorylationand activated by dephosphorylationInsulin stimulates fatty acid synthesis causingdephosphorylation of carboxylase.Glucagon and epinephrine have the reverse effect (keep thecarboxylase in the inactive phosphorylated state).Protein kinase isactivated by AMP andinhibited by ATP.Carboxylase isinactivated when theenergy charge is low.
Local RegulationAcetyl CoA carboxylase is allosterically stimulated bycitrate.The level of citrate is high when both acetyl CoA and ATPare abundant (isocitrate dehydrogenase is inhibited byATP).Palmitoyl CoA inhibits carboxylase.
Response to DietFed state:• Insulin level is increased• Inhibits hydrolysis of stored TGs• Stimulates formation of malonyl CoA, which inhibits carnitine acyltransferase I• FA remain in cytosol (FA oxidation enzymes are in the mitochondria)Starvation:• Epinephrine and glucagon are produced and stimulate adipose cell lipase and the level of free fatty acids rises• Inactivate carboxylase, so decrease formation of malonyl CoA (lead to increased transport of FA into mitochondria and activate the b-oxidation pathway)
LIPID METABOLISM:BIOSYNTHESIS OF TRIACYLGLYCEROLS AND PHOSPHOLIPIDS
Synthesis of Triacylglycerols (TGs) and Glycerophospholipids (GPLs)Glycerol 3-phosphate can be obtained either by thereduction of dihydroxyecetone phosphate (primarily) orby the phosphorylation of glycerol (to a lesser extent).
Formation of phosphatidateTwo separate acyl transferases (AT) catalyze theacylation of glycerol 3-phosphate.The first AT (esterification at C1) has preference forsaturated fatty acids; the second AT (esterification at C2) prefersunsaturated fatty acids.
• Phosphatidic acid (phosphatidate) is an common intermediate in the synthesis of TGs and GPLsPhosphatidate can be converted to two precursors: - diacylglycerol (precursor for TGs and neutralphospholipids) - cytidine diphosphodiacylglycerol (CDP-diacylglycerol) (precursor for acidic phospholipids)
Synthesis of TGs and neutral phospholipids Phospha-Triacyl- tidyl-glycerol etha- nolamine Phosphatidylcholine
Synthesis of TGs Diacylglycerol can be acylated to triacylglycerol (in adipose tissue and liver) Enzyme: acyltransferase
Synthesis of neutral phospholipidsCDP-choline or CDP-ethanolamine are formed fromCTP by the reaction:CTP + choline phosphate → CDP-choline + PPiCTP + ethanolamine phosphate → CDP-ethanolamine + PPiDiacylglycerol react with CDP-choline or CDP-ethanolamine to form phosphatidylcholine orphosphatidylethanolamine