Structures of chloramphenicol acetyltransferase III and Escherichia coli β-ketoacylsynthase III co-crystallized with partially hydrolysed acetyl-oxa(dethia)CoA.

Acetyl coenzyme A (acetyl-CoA) is a reactive metabolite that nonproductively hydrolyzes in a number of enzyme active sites in the crystallization time frame. In order to elucidate the enzyme-acetyl-CoA interactions leading to catalysis, acetyl-CoA substrate analogs are needed. One possible analog for use in structural studies is acetyl-oxa(dethia)CoA (AcOCoA), in which the thioester S atom of CoA is replaced by an O atom. Here, structures of chloramphenicol acetyltransferase III (CATIII) and Escherichia coli ketoacylsynthase III (FabH) from crystals grown in the presence of partially hydrolyzed AcOCoA and the respective nucleophile are presented. Based on the structures, the behavior of AcOCoA differs between the enzymes, with FabH reacting with AcOCoA and CATIII being unreactive. The structure of CATIII reveals insight into the catalytic mechanism, with one active site of the trimer having relatively clear electron density for AcOCoA and chloramphenicol and the other active sites having weaker density for AcOCoA. One FabH structure contains a hydrolyzed AcOCoA product oxa(dethia)CoA (OCoA), while the other FabH structure contains an acyl-enzyme intermediate with OCoA. Together, these structures provide preliminary insight into the use of AcOCoA for enzyme structure-function studies with different nucleophiles.