OctanoylACP

Octanoyl-ACP is an acyl carrier protein (ACP) bearing an octanoyl thioester attached to the 4'-phosphopantetheine prosthetic group. In fatty acid biosynthesis by type II systems, ACP shuttles growing acyl chains between catalytic domains; octanoyl-ACP is a common intermediate when the acyl chain reaches eight carbons in length. In many bacteria and in the plastids of plants, octanoyl-ACP also serves as the donor substrate for lipoic acid biosynthesis.

The lipoyl biosynthesis pathway begins with LipB, an octanoyltransferase that transfers the octanoyl group from octanoyl-ACP onto specific lysine residues within the lipoyl domains of dehydrogenase complexes such as the E2 subunit of the pyruvate dehydrogenase complex. This yields a protein-bound octanoyl moiety. LipA then sulfurates the octanoyl group, converting it into the mature lipoyl moiety (lipoyllysine), essential for the catalytic cycles of several oxidative decarboxylation enzymes.

Thus octanoyl-ACP links fatty acid synthesis to cofactor biosynthesis; its availability influences the lipoylation status of key enzyme complexes, impacting central metabolism. Beyond bacteria, plastids in plants and mitochondria in eukaryotes possess analogous systems using ACP-derived octanoyl groups. The specificity of LipB and LipA for their ACP and lipoyl-domain substrates ensures tight regulation of lipoylation.