Deracemization of a-Amino Acids via DKR

1 Deracemization of a-Amino Acids via Enzyme-catalyzed DKR Coupled with In Situ Racemization 

Several effects can contribute to lowering the pK of the proton on the a-carbon of an amino acid derivative, as the requisite for racemization. Both the 

The Hydantoinase-Carbamoylase System for o-Amino Acid Synthesis 5- 

Hydantoinase-Carbamoylase System for t-Amino Acid Synthesis Despite a number of reports of strains with L-selechve hydantoin-hydrolyzing enzymes [38] the commercial application of the hydantoinase process is stiU restricted to the production of D-amino acids. Processes for the production of L-amino acids are Umited by low space-time yields and high biocatalyst costs. Recently, a new generation of an L-hydantoinase process was developed based on a tailor-made recombinant whole cell biocatalyst. Further reduction of biocatalyst cost by use of recombinant Escherichia coli cells overexpressing hydantoinase, carbamoylase, and hydantoin racemase from Arthrohacter sp. DSM 9771 were achieved. To improve the hydan-toin-converting pathway, the level of expression of the different genes was balanced on the basis of their specific activities. The system has been appUed to the preparation of L-methionine the space-time yield is however still Umited [39]. Improvements in the deracemization process from rac-5-substituted hydantoins to L-amino acids still requires a more selective L-hydantoinase. 

Altogether the hydrolysis has useful characteristics only for the preparation of phenylalanine [40]. Other oxazoHnones are transformed with a much lower e.e. and longer reaction times. 

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Scheme 13.13 Hydantoinase-Carbamoylase system for the deracemization of a-amino acids via DKR with in situ...

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