Site-Directed Mutagenesis of a Hexapeptide Segment Involved in Substrate Recognition of Phenylalanine Dehydrogenase from Thermoactinomyces intermedius

Abstract

Phenylalanine dehydrogenase from Thermoactinomyces intermedius and leucine dehy-drogenase from Bacillus stearothermophilus show a 59% sequence similarity in their substrate-binding domains, although their substrate specificities are different. We prepared a phenylalanine dehydrogenase mutant enzyme whose inherent hexapeptide segment (¹²⁴F-V-H-A-A-¹²⁹R) in the substrate-binding domain was replaced by the corresponding part of leucine dehydrogenase (M-D-I-I-Y-Q) in order to investigate the mechanism of substrate recognition by phenylalanine dehydrogenase. The catalytic efficiencies (k_cat/K_m) of the mutant enzyme with aliphatic amino acids and aliphatic keto acids as substrates were 0.5 to 2% of those of the wild-type enzyme. In contrast, the efficiencies for L-phenylalanine and phenylpyruvate decreased to 0.008 and 0.035% of those of the wild-type enzyme, respectively. These results suggest that the hexapeptide segment plays an important role in the substrate recognition by phenylalanine dehydrogenase.