Biosynthesis of Fatty Acids and Eicosanoids MCQs

Fatty Acid Synthesis occurs through two enzyme system Acetyl CoA Carboxylase Fatty Acid Synthase Complex Acetyl CoA Carboxylase Converts Acetyl CoA (2C) to Malonyl CoA in the presence of ATP. Acetyl-CoA carboxylase has a requirement for the B vitamin biotin . It is a multienzyme protein. This is the rate limiting step in the Fatty Acid Synthesis. Acetyl CoA carboxylase is not a part FAS Complex Reference : Harper's illustrated biochemistry 32 nd edition, page 235

Fatty Acid Synthase (FAS ) Multienzyme Complex The complex is a homodimer of two identical polypeptide monomers in which six enzyme activities and the acyl carrier protein (ACP) Six enzyme activities of Fatty Acid Synthase Complex are Ketoacyl Synthase Malonyl-Acetyl Transacylase Dehydratase Enoyl Reductase Ketoacyl Reductase Thioesterase (Deacylase). Acetyl CoA carboxylase is not a part FAS Complex. Reference: Harper's illustrated biochemistry 32 nd edition, page 234

Explanation: After a meal (high-energy state), insulin is elevated, promoting lipogenesis and weight gain. The enzymes responsible for that are: Malonyl-CoA : It is the key intermediate for fatty acid synthesis. (statement 1) Acetyl-CoA carboxylase (ACC): Insulin activates ACC (via dephosphorylation), which catalyzes the synthesis of Malonyl-CoA. (statement 2) Tricarboxylate transporter (citrate) : It exports citrate from mitochondria to the cytosol. Citrate is cleaved to provide acetyl-CoA for fatty acid synthesis. (statement 4) Mitochondrial carnitine transporter (CPT-I) is involved in fatty acid oxidation. It helps transport fatty acids into mitochondira for breakdown. Malonyl-CoA potently inhibits CPT-I, blocking fatty acid entry into mitochondria for β-oxidation. (statement 3)

Ans. is 'c' i.e., Acetyl-CoA carboxylase [Ref Harper 29th/e p. 217, 220] Production of malonyl-CoA is the initial° and rate limiting step in fatty acid synthesis. Acetyl-CoA needs to be converted to activated form, which will serve as the donor of carbon units to growing fatty acid chain. Malonyl-CoA°) a 3- carbon compound is such activated form. It is produced by carboxylation of acetyl-CoA, a reaction catalyzed by acetyl-CoA caroxylase°. Acetyl-CoA carboxylase requires biotin as a cofactor°.

Eicosanoids, physiologically and pharmacologically active compounds known as prostaglandins (PG), Thromboxane (TX), Leukotrienes (LT), and Lipoxins (LX). Physiologically, they are considered to act as local hormones functioning through G-protein-linked receptors to elicit their biochemical effects. Reference: Harper's 32nd edition, page 239