The LnmK Bifunctional Acyltransferase/Decarboxylase Specifying (2R)-Methylmalonyl-CoA and Employing Substrate-Assisted Catalysis for Polyketide Biosynthesis.

We previously showed that the bifunctional LnmK acyltransferase/decarboxylase (AT/DC) catalyzed the formation of a propionyl-S-acyl carrier protein (ACP) from methylmalonyl-CoA, but its substrate specificity to (2S)-, (2R)-, or (2RS)-methylmalonyl CoA was not known. We subsequently revealed that LnmK AT and DC activities share the same active site, employing a Tyr as the catalytic residue for AT, but failed to identify a general base within the vicinity of the active site for LnmK catalysis. We now show that (i) LnmK specifies (2R)-methylmalonyl-CoA and (ii) the AT and DC activities are coupled, featuring substrate-assisted catalysis via the enolate to account for the missing general base within the LnmK active site. LnmK and its homologues are the only bifunctional AT/DC enzymes known to date and are widespread. These findings, therefore, enrich PKS chemistry and enzymology. Since only the (2S)-methylmalonyl-CoA enantiomer has been established previously as a substrate for polyketide biosynthesis by PKSs, we now establish a role for both (2R)- and (2S)-methylmalonyl-CoA in polyketide biosynthesis, and (2R)-methylmalonyl-CoA should be considered as a substrate in future efforts for engineered production of polyketides by combinatorial biosynthesis or synthetic biology strategies in model hosts.