Distribution of Glutamate Racemase in Lactic Acid Bacteria and Further Characterization of the Enzyme from Pediococcus pentosaceus

Abstract

We investigated the distribution of glutamate racemase (EC 5.1.1.3) in bacteria, and found that the enzyme occurs exclusively in lactic acid bacteria. Pediococcus pentosaceus IFO 3182 produces the enzyme most abundantly. The enzyme purified from extracts of the Escherichia coli clone cells carrying the plasmid pICR221, which contains the enzyme gene of P. pentosaceus, does not require cofactors for its catalytic activity [N. Nakajima, K. Tanizawa, H. Tanaka and K. Soda, Agric. Biol. Chem., 50, 2823 (1986)]. This cofactor dispensability was re-confirmed with the enzyme purified from cell extracts of P. pentosaceus. On the basis of kinetic parameters obtained by measurements of the initial velocities, the glutamate racemase reaction was categorized into a typical racemization with a calculated equilibrium constant of 1.05. d- and l-Glutamate were the specific substrates for the enzyme, although l-homocysteinesulfinate, a sulfur analog of l-glutamate, slightly served as a substrate. l-α-Aminoadipate behaved as a competitive inhibitor in the racemization of glutamate. The enzyme was inactivated significantly by treatment with various thiol-blocking reagents.