The role of a conserved histidine residue, His324, in Trigonopsis variabilis D-amino acid oxidase

To investigate the functional role of an invariant histidine residue in Tri gonopsis variabilis D-amino acid oxidase (DAAO), a set of mutant enzymes wi th replacement of the histidine residue at position 324 was constructed and their enzymatic properties were examined, Wild-type and mutant enzymes hav e been purified to homogeneity using the His-bound column and the molecular masses were determined to be 39.2 kDa. Western blot analysis revealed that the in vivo synthesized mutant enzymes are immune-identical with that of t he wild-type DAAO. The His324Asn and His324Gln mutants displayed comparable enzymatic activity to that of the wild-type enzyme, while the other mutant DAAOs showed markedly decreased or no detectable activity. The mutants, Hi s324/Asn/Gln/Ala/Tyr/Glu, exhibited 38-181% increase in K-m and a 2-10-fold reduction in k(cat)/K-m. Based on the crystal structure of a homologous pr otein, pig kidney DAAO, it is suggested that His324 might play a structural role for proper catalytic function of T. variabilis DAAO. (C) 1999 Federat ion of European Microbiological Societies. Published by Elsevier Science B. V. All rights reserved.