Stereoselective Synthesis Using Hydantoinases and Carbamoylases

Optically pure d- and L-a-amino acids are of increasing interest as precursors for semi-synthetic antibiotics, new herbicides, insecticides, and physiologically active peptides, and as chiral building blocks for chemical synthesis. For the production of optically active a-amino acids, enzymatic processes have been developed other than fermentation to cover a wide range of products, not only natural L-a-amino acids but also unnatural and D-a-amino acids (Table 1). 

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. 

This chapter provides a comprehensive review of the process development and recent results of biochemical study, along with technical application of enzymatic hydantoin transformation. 

Process Enzyme Substrate Enantio- mer D L Product example 

Hydantoin hydrolysis Hydantoinase with or DL-5-Monosubstituted hydantoins + + D-p-Hydroxyphenylglycine 

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