On the mechanism of hydrolysis of hydantoins by d-hydantoinase from Vigna angularis: inhibition studies

Inhibition studies were performed with d-hydantoinase from Vigna angularis. These studies were based on the fact that 5,5-di-substituted hydantoins are not recognizable substrates for the enzyme. Rac-5-methyl-5-phenylhydantoin was shown to be an efficient competitive inhibitor of this d-hydantoinase (Ki=1.28 mM). (R)-5-mono-substituted hydantoins were identified as the proper substrates of the enzyme. It is proposed that this reaction is constituted by a prior fast racemization step that provides the necessary and constant feeding of (R)-5-mono-substituted isomer and a latter (R)-specific enzymatic hydrolysis. The enzyme must present a hydrophobic environment in the pro-R face and a basic residue in the pro-S face. The feedstock configuration, its molecular volume and the presence of hydrogen in position 5 of the hydantoin are the main factors responsible for the substrate specificity showed by this enzyme.