Tautomerization hydantoinase

Another approach for the synthesis of enantiopure amino acids or amino alcohols is the enantioselective enzyme-catalyzed hydrolysis of hydantoins. As discussed above, hydantoins are very easily racemized in weak alkaline solutions via keto enol tautomerism. Sugai et al. have reported the DKR of the hydantoin prepared from DL-phenylalanine. DKR took place smoothly by the use of D-hydantoinase at a pH of 9 employing a borate buffer (Figure 4.17) [42]. 

For example, n-p-hydroxyphenylglydne, a key intermediate in the synthesis of semisynthetic cephalosporins and penicillins, is currently manufactured on a multi-thousand ton scale. The hydantoinase-catalysed reaction is also suitable for the production of unnatural D-amino acids, although the in situ racemization of the remaining substrate via keto-enol tautomerization is generally slow. To facilitate the stereoinversion, base or hydantoin racemase of Pseudomonas and Arthrobacter strains is often used. 

Hydantoinase process, outlined in Fig. 1, includes two hydrolases—hydantoin-hydrolyzing enzyme (hydantoinase) and AT-carbamoyl amino acid-hydrolyzing enzyme (carbamoylase)—and is one of the most efficient and versatile methods for the production of optically active a-amino acids. DL-5-Monosubstituted hydantoins, which are used as common precursors for the chemical synthesis of DL-a-amino acids [1], are the starting material of this enzymatic process. Keto-enol tautomerism is a typical feature of the hydantoin structure. Under neutral conditions, the keto form is dominant in alkaline solution, enolization between the 4 and 5 positions can occur, as has been concluded from the fact that optically pure hydantoins readily racemize. This feature is of practical relevance for the complete conversion of racemic hydantoin derivatives to optically pure L- or D-a-amino acids without any chemical racemization step. A variety of hydantoinase and carbamoylase with different stereospecificity were found. They are D-specific hydantoinase (D-hydantoinase), L-specific hydantoinase (L-hydantoinase), none-specific hydantoinase (DL-hydantoinase), D-specific carbamoylase (D-carbamoylase), and L-specific carbamoylase (L-carbamoylase). With the combination of these enzymes, optically pure amino acids are obtained from DL-5-monosubstituted hydantoins (Fig. 2). The wide substrate range of hydantoinases and carbamoylases also gives generality to the hydantoinase process. 

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