Repurposing the GNAT Fold in the Initiation of Polyketide Biosynthesis

Natural product biosynthetic pathways are replete with enzymes repurposed for new catalytic functions. In some modular polyketide synthase (PKS) pathways, a GCN5-related N-acetyltransferase (GNAT)-like enzyme with an additional decarboxylation function initiates biosynthesis. Here, we probe two PKS GNAT-like domains for the dual activities of S-acyl transfer from coenzyme A (CoA) to an acyl carrier protein (ACP) and decarboxylation. The GphF and CurA GNAT-like domains selectively decarboxylate substrates that yield the anticipated pathway starter units. The GphF enzyme lacks detectable acyl transfer activity, and a crystal structure with an isobutyryl-CoA product analog reveals a partially occluded acyltransfer acceptor site. Further analysis indicates that the CurA GNAT-like domain also catalyzes only decarboxylation, and the initial acyl transfer is catalyzed by an unidentified enzyme. Thus, PKS GNAT-like domains are re-classified as GNAT-like decarboxylases. Two other decarboxylases, malonyl-CoA decarboxylase and EryM, reside on distant nodes of the superfamily, illustrating the adaptability of the GNAT fold. [Display omitted] •GNAT superfamily catalyzes decarboxylation in addition to acyltransfer•A GNAT-like enzyme initiates biosynthesis of some polyketide natural products•Polyketide synthase GNAT-like enzymes catalyze only decarboxylation Most GCN5-related N-acetyltransferase (GNAT) superfamily members catalyze acyltransfer, but some are decarboxylases. Skiba et al. show that “GNATs” involved in the biosynthesis of two polyketide natural products catalyze only decarboxylation. They identify GNAT-like decarboxylases on distant nodes of the superfamily, illustrating the adaptability of the GNAT fold.