Characterization of glutamyl-tRNA-dependent dehydratases using nonreactive substrate mimics

The peptide natural product nisin has been used as a food preservative for 6 decades with minimal development of resistance. Nisin contains the unusual amino acids dehydroalanine and dehydrobutyrine, which are posttranslationally installed by class I lanthipeptide dehydratases (LanBs) on a linear peptide substrate through an unusual glutamyl-tRNA-dependent dehydration of Ser and Thr. To date, little is known about how LanBs catalyze the transfer of glutamate from charged tRNA^Glu to the peptide substrate, or how they carry out the subsequent elimination of the peptide-glutamyl adducts to afford dehydro amino acids. Here, we describe the synthesis of inert analogs that mimic substrate glutamyl-tRNA^Glu and the glutamylated peptide intermediate, and determine the crystal structures of 2 LanBs in complex with each of these compounds. Mutational studies were used to characterize the function of the glutamylation and glutamate elimination active-site residues identified through the structural analysis. These combined studies provide insights into the mechanisms of substrate recognition, glutamylation, and glutamate elimination by LanBs to effect a net dehydration reaction of Ser and Thr.