Dependence of the phosphorylation of alkaline phosphatase by phosphate monoesters on the pKa of the leaving group

The hydrolysis and transphosphorylation reactions of a series of phosphate monoesters, ROPO3(2)-(R = 2,4-dinitrophenyl, 4-nitrophenyl, phenyl, glucose-1, glycerol-1, methyl, ethyl, and dodecyl), catalyzed by Escherichia coli alkaline phosphatase and a mutant enzyme, Ser102Cys, have been studied at alkaline pH using the rates of change in the 31P NMR signals of substrate, the hydrolysis product (inorganic phosphate), and the transphosphorylation product (O-Tris phosphate) as the assay. The kcat at pH 8.0 for the wild-type enzyme is approximately 30 s-1 and is independent of the nature of the R group, when the pKa of the leaving group is < 10. Under these conditions the rate of phosphorylation is much faster than dissociation of inorganic phosphate, 15-60 s-1. If the pKa of the leaving group is between 10 and 15, phosphorylation and dissociation of the product phosphate both contribute to the rate limit. If the pKa of the leaving group is > 15, phosphorylation is rate limiting. A Bronsted plot of log kcat vs pKa of the leaving group for those substrates for which phosphorylation is rate limiting yields a beta lg of approximately -0.6. In contrast to the wild-type enzyme, the log kcat values for the S102C mutant enzyme catalyzing the hydrolysis of phosphate esters are linearly dependent on the pKa's of the leaving group throughout the range of pKa from 4 to 16. Phosphorylation of C102 is the rate controlling step, and kcat is independent of the Tris concentration as predicted for rate limiting phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)