Journal of Chromatography B 2011-11-01

Reaction pathway of tryptophanase-catalyzed L-tryptophan synthesis from D-serine.

Akihiko Shimada, Haruka Ozaki, Takeshi Saito, Noriko Fujii

Index: J. Chromatogr. B. Analyt. Technol. Biomed. Life Sci. 879(29) , 3289-95, (2011)

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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.Copyright © 2011 Elsevier B.V. All rights reserved.