Kinetic analysis of three activated phenylalanyl intermediates generated by the initiation module PheATE of gramicidin S synthetase.

The three-domain initiation module PheATE (GrsA) of Bacillus brevis gramicidin S synthetase catalyzes the activation, thiolation and epimerization of L-phenylalanine (L-Phe), the first amino acid incorporated into the decapeptide antibiotic gramicidin S. There are three activated intermediates in the PheATE catalyzed chemical pathway: L-phenylalanyl-adenosine-5'-monophosphate diester (L-Phe-AMP), L-Phe-S-4'-phosphopantetheine(Ppant)- and D-Phe-S-4'-Ppant-acyl enzyme. In this study, we examined PheATE in single-turnover catalysis using rapid chemical quench techniques. Kinetic modeling of the process of disappearance of the substrate L-Phe, transient appearance and disappearance of L-Phe-AMP and the ad seriatim formation and equilibration of the L- and D-Phe-S-Ppant-acyl enzyme adducts allowed evaluation of the microscopic rate constants for the three chemical reactions in the initiation module PheATE. This study provides the first transient-state kinetic analysis of a nonribosomal peptide synthetase (NRPS) module.