Function of arginine-166 in the active site of Escherichia coli alkaline phosphatase

The function of arginine residue 166 in the active site of Escherichia coli alkaline phosphatase was investigated by site-directed mutagenesis. Two mutant versions of alkaline phosphatase, with either serine or alanine in the place of arginine at position 166, were generated by using a specially constructed M13 phage carrying the wild-type phoA gene. The mutant enzymes with serine and alanine at position 166 have very similar kinetic properties. Under conditions of no external phosphate acceptor, the kcat for the mutant enzymes decreases by approximately 30-fold while the Km increases by less than 2-fold. When kinetic measurements are carried out in the presence of a phosphate acceptor, 1.0 M Tris, the kcat for the mutant enzymes is reduced by less than 3-fold, while the Km increases by more than 50-fold. For both mutant enzymes, in either the absence or the presence of a phosphate acceptor, the catalytic efficiency as measured by the kcat/Km ratio decreases by approximately 50-fold as compared to the wild type. Measurements of the Ki for inorganic phosphate show an increase of approximately 50-fold for both mutants. Phenylglyoxal, which inactivates the wild-type enzyme, does not inactivate the Arg-166----Ala enzyme. This result indicates that Arg-166 is the same arginine residue that when chemically modified causes loss of activity Daemen, F.J.M., & Riordan, J.F. (1974) Biochemistry 13, 2865-2871]. The data reported here suggest that although Arg-166 is important for activity is not essential. The analysis of the kinetic data also suggests that the loss of arginine-166 at the active site of alkaline phosphatase has two different effects on the enzyme. First, the binding of the substrate, and phosphate as a competitive inhibitor, is reduced; second, the rate of hydrolysis of the covalent phosphoenzyme may be diminished.