D-amino acid tolerant hosts for D-hydantoinase whole cell biocatalysts

Whole cell biocatalysts which enable the concerted use of D-hydantoinase, D-carbamoylase, and racemase enzymes are valuable for the production of D-amino acids. However, Escherichia coli host strains used for this purpose efficiently degrade D-amino acids. This work demonstrates that D-amino acid degradation occurs largely through the concerted action of D-amino acid dehydrogenase, encoded by the dadA gene, and D-serine dehydratase, encoded by the dsdA gene. Deletion mutants of E. coli which lack these activities were constructed and compared against wild type strains in D-amino acid degradation. An E. coli dadA mutant reduced the degradation of D-methionine by one third, D-phenylalanine by two-thirds, and D-2-amino-butyric acid nearly completely. Even though the dadA mutant had no effect on D-serine degradation, a dadA dsdA double mutant of E. coli additionally reduced degradation of D-serine, as well as D-phenylalanine, almost entirely. These strains are appropriate hosts for whole cell biosynthesis of D-amino acids using general approaches such as the hydantoinase system.