LIPID METABOLISM BIOSYNTHESIS or DE NOVO SYNTHESIS OF FATTY ACID The majority of the fatty acids required supplied through our diet. Fatty acids are synthesised whenever there is a caloric excess in the our diet. The excess carbohydrate and protein obtained through diet can be converted to fatty acids which are stored as triacylglycerol.  Fatty acid synthesis involves the similar steps involved in  -oxidation of fatty acid but in a reverse way.  Mammals can synthesise major portion of the saturated fatty acid as well as monounsaturated fatty acids.  The system for the fresh synthesis of fatty acid is known as de novo synthesis of fatty acid

 Takes place in liver and lactating mammary glands and to a lesser extent in adipose tissue, kidney.  The enzyme machinery is located in cytoplasm.  Enzyme system is referred to as extra mitochondrial or cytoplasmic fatty acid synthase system.  Palmitic acid is the major fatty acid synthesised  All the 16 carbon atoms are from acetyl CoA. The acetyl CoA used as a primer and which forms carbons 15 and 16 of palmitate. The addition of all the subsequent 2-C units is through malonyl CoA formation

 Acetyl CoA and NADPH are the prerequisites for the fatty acid synthesis.  Acetyl CoA produced in the mitochondria cannot enter into cytoplasm through inner mitochondrial membrane. So 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.

Mitochondria Cytoplasm Glucose Pyruvate Pyruvate HMP Shunt NADPH+H +, CO 2 Malic Enzyme NADP + PDH Malate NAD + Malate Dh FA, amino acids NADH+H + FA Oxaloacetate Acetyl CoA + Oxaloacetate AcetylCoA, ADP+Pi Citrate synthase Citrate lyase CoASH, ATP Citrate Citrate For FA synthesis, 8 acetyl CoA are transported from the mitochondria to cytosol, which is linked with the synthesis of 8 NADPH.

14 NADPH are required to synthesise one molecule of Palmitate. The remaining 6 NADPH supplied from HMP shunt. * Acetyl CoA carboxylase 1)Acetyl CoA malonyl CoA Biotin, ATP, CO 2 ADP+Pi * Regulatory enzyme in FA synthesis.  The remaining reactions of FA synthesis are catalysed by multifunctional enzyme known as fatty acid synthase complex [FAS]  FAS is a dimer with two identical subunits.  Each monomer possesses the activities of seven different enzymes and an acyl carrier protein (ACP ) bound to 4’phosphopantetheine-SH group.  Two subunits lie in antiparallel (head to tail) orientation.

 The-SH group of phosphopantetheine of one subunit is in close proximity to the –SH of cysteine residue of the other subunit.  Each monomer of FAS contains all the enzyme activities of fatty acid synthesis.  Dimer form of enzyme is functionally active. Because the functional unit consists of half of each subunit interacting with the complimentary half of the other. Components of fatty acid synthase complex: 1. Acetyl transferase [AT] 2. Malonyl transferase [MT] 3.  -Keto acyl synthase [KS] 4.  -Keto acyl reductase [KR] 5.  -Hydroxy acyl dehydratase [HD] 6. Enoyl reductase [ER] 7. Thioestarase [TE] Acyl carrier protein [ACP].

Functional division KS AT MT HD ER KR ACP TE Cys 4’ phosphopantetheine SH SH Subunit SH SH division 4’ phosphopantetheine Cys TE ACP KR ER HD MT AT KS

1. Fatty acid synthesis starts with the transfer of an acetyl CoA to cysteinyl SH group of ACP Acetyl CoA + (CE)-SH AT Acetyl S-(CE) + CoA Malonyl CoA-ACP transferase transfers malonate from malonyl CoA to bind to ACP Malonyl CoA+ACP-SH MT Malonyl-S-ACP + CoA—2 3) The acetyl unit attached to cysteine is transferred to malonyl group attached to ACP. Malonyl moiety loses CO 2,which was added by acetyl CoA carboxylase & form β-ketoacyl enzyme.

4)  -Ketoacyl-enzyme is reduced to  -hydroxy butyryl enzyme complex using NADPH+H +. 5)Molecule of H 2 O is removed from  -OH butyryl enzyme to form ,  unsaturated acyl enzyme. 6)The unsaturated bond in ,  unsaturated acyl enzyme is again reduced using NADPH+H + to form butyryl or acyl enzyme.The carbon chain attached to ACP is transferred to cysteine residue and the reactions 2-6 are repeated 6 more times and finally palmitic acid is synthesised. 7) The completely synthesized fatty acid is released from the enzyme system by the action of thioesterase enzyme.

Fatty acid chain elongation and desaturation occurs in the microsomes of endoplasmic reticulum and mitochondria. Of the 16 carbons present in palmitate, only two come from acetyl CoA directly. The remaining 14 are from malonyl CoA (produced from acetyl CoA). During elongation in microsomes palmitate activated to palmitoyl CoA. Malonyl Co serves as the donor of two carbons at a time in series of reactions. Major elongation reaction occurs in the body involves the formation of stearyl CoA [C 18 ] from palmitoyl CoA [C 16 ] Elongation of this stearyl CoA in brain increases during myelination to provide C 22 and C 24 fatty acids of sphingolipids Mitochondrial elongation is less active and uses acetyl CoA as the source of two carbon units 8 AcetylCoA+7ATP+14NADPH+14H +  Palmitate+8CoA+7ADP+7Pi+6H 2 O+14 NADP +

Regulation Acetyl CoA carboxylase enzyme controls a committed step in fatty acid synthesis. This enzyme exists as an inactive monomer or an active polymer. Citrate promotes polymer formation, hence increases FA synthesis. Palmitoyl CoA and malonyl CoA causes depolymerisation of the enzyme and inhibit FA synthesis. Hormonal influence Glucagon, epinephrine & norepinephrine inactivate the enzyme by cAMP dependent phosphorylation and inhibits FA synthesis Insulin dephosphorylates & activates the enzyme and promotes FA synthesis.

Dietary Regulation:  High carbohydrate and fat free diet increases the synthesis of Acetyl CoA carboxylase and FA synthase, which promotes FA synthesis.  Fasting and high fat diet decreases FA production. NADPH influences FA synthesis. Reference: Essentials of Biochemistry Dr. S. NayakMarch 2011