Figure 11. The different metabolic fates for medium- and long-chain fatty acids in A. finegoldii.

A. finegoldii uses two acyl-ACP synthetases with different substrate specificities to activate a spectrum of exogenous fatty acid chain lengths found in the gut environment. R1 and R2 refer to the 1- and 2-positions of PE, respectively. R1 of SL is the fatty acid condensed with cysteic acid, and R2 is the fatty acid attached to the amine (see Fig. 4C for structure). Medium-chain fatty acids <16 carbons are ligated to ACP by AfAas1. These medium-chain acyl-ACPs are elongated by FASII prior to the incorporation of the elongated acyl-ACP into both the R1 and R2 positions of both PE and SL. Long-chain fatty acids ≥16 carbons are activated by AfAas2. Acyl-ACP 16 carbons or longer cannot enter FASII and are funneled into the R2 position of PE at the PlsC step. AfAas2 can also use medium-chain fatty acids as substrates, and these acyl-ACP are handled in the same manner as medium-chain acyl-ACP derived from AfAas1. The only long-chain acyl-ACP used for SL synthesis is 16:0-ACP, and it is incorporated into both the R1 and R2 positions of SL.