SfuPKS2, a novel type III polyketide synthase from Sargassum fusiforme, shows high substrate specificity

A type III polyketide synthase (SfuPKS2) from the edible seaweed Sargassum fusiforme was cloned and biochemically characterized. SfuPKS2 can only accept octanoyl-CoA (C8) and neither other long-chain fatty acyl-CoAs, nor non-fatty acyl-CoAs synthesized in this study as the substrate. In addition, in incubation with octanoyl-CoA, only a single product peak was observed, giving rise to the triketide α-pyrone derivative. To understand the substrate specificity of SfuPKS2, we constructed a structural model and performed docking analysis to reveal amino acid residues critical for enzyme-substrate binding. Putative key residues were then mutated experimentally and the impact on protein function was assessed by incubating the mutant with octanoyl-CoA. We found that key amino acids governing enzyme activity and specificity included Ser171, His236, Phe254, and Ile377. Mutations on these amino acids generally resulted in altered interactions with the substrate and thus the enzyme activity. In summary, our work provided mechanistic insight on how the selectivity toward the substrate is achieved in SfuPKS2.