The three-carbon dicarboxylic acid malonic acid bound to *acyl carrier protein
(ACP). Malonyl ACP, with *malonyl CoA
, is central to fatty acid synthesis. It is formed from malonyl CoA and ACP and then reacts with enzyme-bound acetate to form acetoacetyl ACP, with concomitant release of carbon dioxide. Acetoacetyl ACP is then reduced to butyryl ACP before the addition of another molecule of malonyl ACP to form a six-carbon chain. Malonyl ACP thus provides the two-carbon building blocks with which long-chain fatty acids are progressively synthesized.