RECONSIDERATION OF THE ESSENTIAL ROLE OF A HISTIDINE RESIDUE OF L-2-HALO ACID DEHALOGENASE

His20 of L-2-halo acid dehalogenase from Pseudomonas cepacia MBA4 was suggested to serve as a catalytic base [Biochem. J. (1993) 292, 69-74] . In this study, we substituted Asn or Leu for His19 of L-2-halo acid dehalogenase from Pseudomonas sp. YL, which corresponds to His20 of th e P. cepacia enzyme. Although the substrate specificity was affected b y the substitution, the susceptibilities of substrate halo acids were not substantially diminished, and the K-m and k(cat) values of the mut ant enzymes for L-2-chloropropionate were not significantly different from those of the wild-type enzyme. In addition, the wild-type and mut ant enzymes showed the same pH optimum. Accordingly, His19 is not esse ntial for catalysis of L-2-halo acid dehalogenase.