Reaction Mechanism of Gramicidin S Synthetase 1, Phenylalanine Racemase, of Bacillus brevis

Abstract

We have demonstrated that gramicidin S synthetase 1 (GS 1), phenylalanine racemase [EC 5. 1. 1. 11], of Bacillus brevis catalyzes the exchange between a proton in the medium and α-hydrogen of phenylalanine in the course of the racemase reaction by using tritiated water or L-phenyl [2, 3-³H] alanine. GS 1 from some gramicidin S non-producing mutants of B. brevis lacking phenylalanine racemase activity did not catalyze the tritium exchange reaction. The proton exchange between phenylalanine bound as thioester on the GS 1-phenylalanine complex and water in the medium was detected, but 5, 5'-dithiobis(2-nitrobenzoic acid)-modified complex lacked both the proton exchange and phenylalanine racemase activity. It is suggested that a base group, probably a sulfhydryl group, on the enzyme functions as proton donor and acceptor during the phenylalanine racemase reaction.