Figure 1.

Schematic diagram of fatty acid biosynthesis process. The catalytic process is divided into initiation and elongation stages. In the initiation stage, apo-form of acyl carrier protein (apo-ACP, green) is covalently installed with a 4′-phosphopantetheine moiety (4′-Pan-arm) by holo-ACP synthase (AcpS), forming holo-ACP. Subsequently, a short acetyl (or malonyl) group is covalently attached to the terminal sulfhydryl of the 4′-Pan-arm by malonyl-CoA-/acetyl-CoA-ACP transacylase (MAT domain in FAS-I; FabD in FAS-II). The elongation cycle mainly includes four catalytic steps to process fatty acid extension successively. The first step is decarboxylative condensation to form β-ketoacyl-ACP by β-ketoacyl synthase (KS domain in FAS-I; FabH, FanB and FabF in FAS-II); the second step is NADPH-dependent reduction of β-ketoacyl-ACP to β-hydroxyacyl-ACP by β-ketoacyl reductase (KR domain in FAS-I; FabG in FAS-II); the third step is β-hydroxyacyl-ACP dehydration to β-enoyl-ACP by β-hydroxyacyl-ACP dehydratase (DH domain in FAS-I; FabA and FabZ in FAS-II); the last step is β-enoyl-ACP reduction by NADPH-dependent β-enoyl reductase (ER domain in FAS-I; FabI, FabK and FabL in FAS-II). After four steps, two carbons are added to the fatty acid chain, and the fatty acid chain either re-enter the elongation cycle for further extension or is released from ACP by thioesterase (TE domain in FAS-I; TE in FAS-II) once the length reaches 16-18 carbons.