Mutant D-amino acid oxidase with higher catalytic efficiency toward D-amino acids with bulky side chains

D-Amino acid oxidase from the yeast Trigonopsis variabilis (TvDAAO) is widely used in fine organic synthesis, including the preparation of unnatural L-amino acids and alpha-keto acids. The analysis of the three-dimensional structure of TvDAAO was carried out with the aim of producing the enzyme specific to D-amino acids with bulky side chains. The analysis revealed the residue Phe54 at the entrance to the active site, which controls the substrate access to this site. The residue Phe54 was replaced by residues Ala, Ser, and Tyr. The cultivation of recombinant E. coli strains expressing TvDAAO mutants showed that the mutein with the Phe54Ala substitution had very low stability. Thus, the inactivation of the enzyme occured within 10 min after the cell disruption. The Phe54Ser TvDAAO and Phe54Tyr TvDAAO mutants were obtained as homogeneous preparations, and their thermal stability and catalytic properties were investigated. The introduction of Phe54Ser and Phe54Tyr substitutions resulted in additional stabilization of the protein macromolecule compared to the wild-type TvDAAO. Thus, the half-inactivation time for the mutant enzymes at 54 degrees C increased by a factor of 1.5 and 2, respectively. As in the case of wild-type TvDAAO, the thermal inactivation of the muteins proceeds via a two-step dissociative mechanism. The introduction of mutations led to a strong change in the substrate specificity profile. The mutants have no activity toward a series of D-amino acids (Phe54Ser TvDAAO toward D-Ala, D-Ser, D-Val, and D-Thr; Phe54Tyr TvDAAO toward D-Ser, D-Tyr, D-Thr, and D-Lys). The catalytic efficiency (the k(cat)/K-M ratio) of the Phe54Ser TvDAAO mutant toward D-amino acids with bulky side chains (D-Lys, D-Asn, D-Phe, D-Tyr, D-Trp, and D-Leu) increased from 2.4 to 7.3 times.