The α/β hydrolase AzpM catalyzes dipeptide synthesis in alazopeptin biosynthesis using two molecules of carrier protein-tethered amino acid

During the biosynthesis of alazopeptin, a tripeptide composed of two molecules of 6-diazo-5-oxo-L-norleucine (DON) and one of alanine, the α/β hydrolase AzpM synthesizes the DON-DON dipeptide using DON tethered to the carrier protein AzpF (DON-AzpF). However, whether AzpM catalyzes the condensation of DON-AzpF with DON or DON-AzpF remains unclear. Here, to distinguish between these two condensation possibilities, the reaction catalyzed by AzpM was examined in vitro using a DON analog, azaserine (AZS). We found that AzpM catalyzed the condensation between AZS-AzpF and DON-AzpF, but not between AZS-AzpF and DON. Possible reaction intermediates, DON-DON-AzpF and AZS-AZS-AzpF, were also detected during AzpM-catalyzed dipeptide formation from DON-AzpF and AZS-AzpF, respectively. From these results, we concluded that AzpM catalyzed the condensation of the two molecules of DON-AzpF and subsequent hydrolysis to produce DON-DON. Thus, AzpM is an unprecedented α/β hydrolase that catalyzes dipeptide synthesis from two molecules of a carrier protein-tethered amino acid.