Effect of Active Site Pocket Structure Modification of D-Stereospecific Amidohydrolase on the Recognition of Stereospecific and Hydrophobic Substrates.

D-Stereospecific amidohydrolase (DAH) from Streptomyces sp. 82F2 has potential utility for the synthesis of D/L configuration dipeptides by an aminolysis reaction. Structural comparison of DAH with substrate-bound D-amino acid amidase revealed that three residues located in the active site pocket of DAH (Thr145, Ala267, and Gly271) might be involved in interactions with D-phenylalanine substrate. We substituted Ala267 and Gly271, which are located at the bottom of the hydrophobic pocket of DAH, with Phe and observed changes in the stereoselectivity and specific activity toward the free and acetylated forms of D/L-Phe-methyl esters. In contrast, the mutation of Thr145, which likely supplies negative charge for recognition of the amino group of the substrate, hardly affected the stereoselectivity of the enzyme. A similar effect was observed in an investigation of hydrolysis and aminolysis reactions using the acetylated forms of D/L-Phe-methyl esters and 1,8-diaminooctane as an acyl-donor and acyl-acceptor, respectively. Substrate binding by DAH was disrupted by the mutation of Ala267 to Val or Trp and kinetic analysis showed that the hydrophobicity of the bottom of the active site pocket (Ala267 and Gly271) is important for both stereoselectivity and recognition of hydrophobic substrates.