Use of site-directed mutagenesis to elucidate the role of arginine-166 in the catalytic mechanism of alkaline phosphatase

Abstract

The guanidinium group of arginine-166 has been postulated to act as an electrophilic species during phosphorylation of alkaline phosphatase. Its role could be either to stabilize the developing negative charge on the oxygen of the leaving group or the pentacoordinate transition state or to help bind the -PO2-3 group. We have produced via site-directed mutagenesis two Escherichia coli alkaline phosphatase mutants (lysine-166 and glutamine-166) to test whether the guanidinium group plays a critical role in catalysis. Comparative kinetic characterization of the lysine-166 and glutamine-166 mutants indicates that the charge at residue 166 is not required for the hydrolysis of phosphate monoesters. Small decreases in kcat are observed for both the lysine and glutamine mutants, relative to the wild-type enzyme, but the value for the uncharged glutamine mutant is only one-third that of lysine. Thus, the stabilizing effect of the positively charged guanidinium group does not appear to play a major role in the rate-limiting step for substrate hydrolysis. A significant effect on the Km value is seen only for the glutamine mutant.