Control mechanism for carbon chain length in polyunsaturated fatty acid synthases.

Polyunsaturated fatty acids (PUFAs) such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are essential fatty acids for humans. Microalgae and some marine bacteria synthesize these PUFAs with PUFA synthase from acetyl units. The PUFA synthases are composed of three to four subunits and each create a specific PUFA without undesirable byproducts even though the multiple catalytic domains in each huge subunit are very similar. However, detailed biosynthetic mechanisms for controlling final product profiles have been obscure. Here, we carefully dissected bacterial DHA and EPA synthases by in vivo and in vitro experiments. In vivo analysis showed that β-ketoacyl synthase (KS)/chain length factor-like domains in "C"-subunits of EPA and DHA synthases controlled DHA or EPA production. Furthermore, in vitro analysis with two KS domains in the "A" and "C" subunits (KSA and KSC) and acyl-ACP substrates showed that KSA accepted short to medium chain substrates while KSC accepted medium to long chain substrates. Unexpectedly, condensation from C18 to C20, the last elongation step in EPA biosynthesis, was catalyzed by KSA domains in both EPA and DHA synthases. Conversely, condensation from C20 to C22, the last elongation step for DHA biosynthesis, was catalyzed by the KSC domain in DHA synthase. Based on our results, we converted a practical microalgal DHA synthase into an EPA synthase.