LIPID BIOSYNTHESIS

Role of lipids They are the principal form of stored energy in most organisms & major constituents of cellular membranes. Specialised lipids serve as pigments ( Retinol, carotene); Cofactors ( vitamin K); Detergents ( bile salts) etc. Anchors for membrane proteins ( Signalling)

Biosynthesis of fatty acids and eicosanoids Biosynthesis requires the participation of a 3 carbon intermediate, i.e., Malonyl CoA.

Formation of malonyl CoA Irreversible process, catalyzed by Acetyl – CoA carboxylase. Enzyme contains biotin prosthetic group covalently bound in amide linkage to the E-Amino group of a lys residue in any one polypeptide unit of the enzyme.

Process Carboxyl grou, derived from bicarbonate, transferred to Biotin in an ATP dep. Reaction. The biotinyl group serves as a temporary carrier of CO2. Transfer of CO2 to acetyl CoA to yield Malonyl CoA.

Fatty acid synthesis It is a repeating reaction sequence.

Fatty acid synthase / Multi enzyme complex All the reactions in synthetic processes are catalyzed by a multienzyme complex, Fatty acid synthase. It has 7 separate polypeptides unit.

ACP – Acyl Carrier Protein Small protein consisting of the pros. Group 4-phosphopantetheine. Serves as a flexible arm, tethering the growing fatty acyl chain to the surface of F.A.S complex

Enzymes And there functions

Beginning of 2nd cycle

Net reaction

Structure of fatty acid synthases

Fatty acid synthesis occurs in the cytosol of many organisms but in the chloroplasts of plant In most higher eukaryotes, fatty acid synthase complex is found in cytosol. Usually, NADPH carries electrons for anabolic reactions, and NAD+ in catabolic reactions. In plants, NADPH is produced in the chloroplast stroma and hence favouring the reduction of oxygen as in mitochondria.

Role of citrate Acetate is shuttled out as citrate In nonphotosynt. Eukaryotes, acetyl CoA is formed in mitochondria. Oxidation of fatty acids is not a significant source of Acetyl CoA for fatty acid biosynthesis in animals, because the 2 pathways are reciprocally regulated

Mitochondrial inner membrane is impermeable to acetyl CoA So, In the intramembrane acetyl coA reacts with oxaloacet. To form Citrate by Citrate synthase. Citrate lyase cleaves the Citrate giving back Acetyl coA OAA is reduced to malate by malate dehydrogenase and goes back.

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