D-amino Acid Research: My Retrospect and Perspective

the substrate α-hydrogen. It shows no structural similarity to other enzymes, but the structure of active center is closely similar to the heme binding site of hemoglobin and myoglobin. Hemine stoichiometrically binds to the enzyme active center to inhibit the activity. Several species of B. subtilis, in particular the producers of Japanese and Korean soybean fermented food, Natto and Chungkookjang, respectively produce viscous polyγ -glutamate in abundance, which is a polycation composed of D-glutamate and Lglutamate with the molar ratio of about 80:20. The ratio depends on the species and the growth conditions. We showed the occurrence of two isozymes of glutamate racemase, Glr and YrpC. Glr participates in the poly-γ-glutamate synthesis, and YrpC is produced only on cell division, and involved in the synthesis of petidoglycan of cell wall by a supply of D-glutamate, an essential constituent. Polyγ -glutamate synthetase catalyzes synthesis of the polymer from Dand L-glutamates in the presence of ATP(15). The γ -carboxyl group of a C-terminal glutamate residue of the growing poly-γ -glutamate is first activated to form a phosphate ester with hydrolysis of ATP. An amino group of a substrate glutamate attacks the activated carboxyl group as a nucleophile to elongate the polymer: this is an ATP and glutamate-dependent γ -ligase reaction. We showed that poly-γ-glutamate synthetase consists of three components. Component PgsB, which is found inside the cell membrane, catalyzes the ATP and glutamate-dependent γ -ligase reaction probably in association with PgsC. This component occurs in the hydrophobic region of Fig. 2. Stereochemistry of Hydrogen Transfer by Aminotransferase. AspAT; Aspartate Ainotransferase OAT; Ornithineδ-Aminotransferase AlaAT; Alanine Aminotransferase DAAT; D-Amino Acid Aminotransferase BCAT; Branched-Chain Aminotransferase Viva Origino 34 (2006) 81 85