title: Reaction pathway of tryptophanase-catalyzed L-tryptophan synthesis from D-serine.
abstract: Tryptophanase, L-tryptophan indole-lyase with extremely absolute stereospecificity, can change the stereospecificity in concentrated diammonium hydrogenphosphate solution. While tryptophanase is not inert to D-serine in the absence of diammonium hydrogenphosphate, it can undergo L-tryptophan synthesis from D-serine along with indole in the presence of it. It has been well known that tryptophanase synthesizes L-tryptophan from L-serine through a β-substitution mechanism of the ping-pong type. However, a metabolic pathway of L-tryptophan synthesis from D-serine has remained unclear. The present study aims to elucidate it. Diammonium hydrogenphosphate plays a role in the emergence of catalytic activity on D-serine. The salt gives tryptophanase a small conformational change, which makes it possible to catalyze D-serine. Tryptophanase-bound D-serine produces L-tryptophan synthesis by β-replacement reaction via the enzyme-bound aminoacrylate intermediate. Our result will be valuable in studying the origin of homochirality.
authors: Sustainable Environmental Studies, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan. ashimada@envr.tsukuba.ac.jp
authors: Akihiko
authors: A
authors: Shimada
authors: Haruka
authors: H
authors: Ozaki
authors: Takeshi
authors: T
authors: Saito
authors: Noriko
authors: N
authors: Fujii