KetoacylACP

Ketoacyl-ACP refers to a beta-ketoacyl group that is covalently attached to the acyl carrier protein (ACP) by a thioester bond. This molecule is a central intermediate in type II fatty acid synthesis (FAS II), the pathway used by bacteria, plant plastids, and some fungi for elongating fatty acids. In FAS II, the growing acyl chain remains bound to ACP throughout the cycle, and chain elongation proceeds by condensation of an acyl-ACP with malonyl-ACP catalyzed by beta-ketoacyl-ACP synthase (FabB or FabF). The condensation product is a beta-ketoacyl-ACP, for example acetoacetyl-ACP when starting from acetyl-ACP.

Subsequent steps modify the beta-ketoacyl-ACP to extend the chain by two carbons: reduction of the keto group to beta-hydroxyacyl-ACP by beta-ketoacyl-ACP reductase (FabG), dehydration by 3-hydroxyacyl-ACP dehydratase (FabA or FabZ) to enoyl-ACP, and reduction by enoyl-ACP reductase (FabI) to a saturated acyl-ACP. The cycle repeats with another condensation, lengthening the fatty acyl chain until the desired length is reached, after which de novo synthesis progresses toward desaturation and termination.

In bacteria, two principal KAS enzymes influence substrate preference and final chain length, while in higher organisms the analogous process is carried out by a single multifunctional fatty acid synthase in type I systems. Ketoacyl-ACP intermediates are not only central to fatty acid biosynthesis but also play roles in polyketide biosynthesis, where similar beta-ketoacyl-ACP units are assembled.