Malonyl-CoA
Malonyl-CoA is a coenzyme A derivative of malonic acid. Functions. It plays a key role in chain elongation in fatty acid biosynthesis and polyketide biosynthesis. Fatty acid biosynthesis. Malonyl-CoA provides 2-carbon units to fatty acids and commits them to fatty acid chain synthesis. Malonyl-CoA is formed by carboxylating acetyl-CoA using the enzyme acetyl-CoA carboxylase. One molecule of acetyl-CoA joins with a molecule of bicarbonate, requiring energy rendered from ATP. Malonyl-CoA is utilised in fatty acid biosynthesis by the enzyme (MCAT). MCAT serves to transfer malonate from malonyl-CoA to the  terminal thiol of "holo"-acyl carrier protein (ACP). Polyketide biosynthesis. MCAT is also involved in bacterial polyketide biosynthesis. The enzyme MCAT together with an acyl carrier protein (ACP), and a polyketide synthase (PKS) and chain-length factor heterodimer, constitutes the minimal PKS of type II polyketides. Regulation. Malonyl-CoA is a highly regulated molecule in fatty acid synthesis; as such, it inhibits the rate-limiting step in beta-oxidation of fatty acids. Malonyl-CoA inhibits fatty acids from associating with carnitine by regulating the enzyme carnitine acyltransferase, thereby preventing them from entering the mitochondria, where fatty acid oxidation and degradation occur