Deracemization and Stereoinversion of α-Amino Acids by L-Amino Acid Deaminase

Enantiomerically pure alpha-amino acids are compounds of primary interest for the fine chemical, pharmaceutical, and agrochemical sectors. Amino acid oxidases are used for resolving D, L-amino acids in biocatalysis. We recently demonstrated that L-amino acid deaminase from Proteus myxofaciens (PmaLAAD) shows peculiar features for biotechnological applications, such as a high production level as soluble protein in Escherichia coli and a stable binding with the flavin cofactor. Since L-amino acid deaminases are membrane-bound enzymes, previous applications were mainly based on the use of cell-based methods. Now, taking advantage of the broad substrate specificity of PmaLAAD, a number of natural and synthetic L-amino acids were fully converted by the purified enzyme into the corresponding aketo acids: the fastest conversion was obtained for 4-nitrophenylalanine. Analogously, starting from racemic solutions, the full resolution (ee > 99%) was also achieved. Notably, D, L-1-naphthylalanine was resolved either into the D- or the L-enantiomer by using PmaLAAD or the D-amino acid oxidase variant having a glycine at position 213, respectively, and was fully deracemized when the two enzymes were used jointly. Moreover, the complete stereoinversion of L-4-nitrophenylalanine was achieved using PmaLAAD and a small molar excess of borane tert-butylamine complex. Taken together, recombinant PmaLAAD represents an L-specific amino acid deaminase suitable for producing the pure enantiomers of several natural and synthetic amino acids or the corresponding keto acids, compounds of biotechnological or pharmaceutical relevance.