Identification of the Histidine Ligands to the Binuclear Metal Center of Phosphotriesterase by Site-Directed Mutagenesis

Abstract

In order to identify which of the seven histidines in phosphotriesterase participate at the active site/binuclear metal center of the enzyme, site-directed mutagenesis has been employed to change, individually, each of the seven histidine residues to asparagine. In addition, the gene for the wild-type enzyme has been subcloned without its leader sequence behind a modified ribosomal binding site, leading to a 5-fold increase in protein expression. The seven mutants, H55N, H57N, H123N, H201N, H230N, H254N, and H257N, exhibit varying degrees of activity compared to the wild-type enzyme. The H123N and H257N mutants are as active as the wild-type enzyme, but all of the other mutant enzymes have 10% or less activity. The metal content of the cobalt-purified mutant enzymes has been determined to be less than that of the wild-type enzyme in all cases. Each of the mutant enzymes has been converted to apoenzyme and reconstituted with 2 equiv of zinc(II), cadmium(II), or cobalt(II). The kinetic parameters, Vmax and V/Km, and apparent pKa's have been determined for each of the reconstituted enzyme derivatives. In almost all cases, the apparent pKa's have shifted toward higher values. The pH-rate profiles for some of the reconstituted mutant enzymes are significantly different from those for the wild-type enzyme, indicating that other groups may become involved in the reaction mechanism upon mutation of the histidine residue to asparagine. His-123 is the only histidine residue that appears to have no involvement in the catalytic activity of phosphotriesterase.(ABSTRACT TRUNCATED AT 250 WORDS)