Reaction pathway of tryptophanase-catalyzed l-tryptophan synthesis from d-serine

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.