Off-loading Mechanism of Products in Polyunsaturated Fatty Acid Synthases

Marine microorganisms de novo biosynthesize polyunsaturated fatty acids such as docosahexaenoic acid and eicosapentaenoic acid by polyunsaturated fatty acid synthases (PUFA synthases) composed of three or four polypeptides in a manner similar to fatty acid synthases (FASs). FASs usually possess thioesterase (TE) domains to release free fatty acids from acyl carrier protein (ACP)-tethered intermediates. However, PUFA synthases have no typical TE domains and their off-loading mechanism is still unclear. Here, we investigated the mechanism with microalgal and bacterial PUFA synthases through in vivo and in vitro experiments. The in vitro experiments with acyltransferase (AT)-like domains and acyl-ACP substrates clearly demonstrated that the AT-like domains catalyzed the hydrolysis of acyl-ACPs to yield free fatty acids.