Competitive measurement of β/α naphthyl phosphate catalytic efficiency by phosphatases utilizing quantitative NMR

The two constitutional isomers of naphthyl phosphate have different steric properties, analogous to those of phosphotyrosine versus phosphoserine/threonine within a peptide or protein. The ratios of their respective rates of hydrolysis, assayed by measuring rates of inorganic phosphate release, have been used to probe the steric requirements around the active sites of many phosphatases in the literature. We report an NMR-based competitive method that is simpler to execute and has other advantages. It directly yields the ratio of catalytic efficiencies (V/K) of the two substrates, a more biologically relevant comparison than the ratio of initial rates (v_o) or maximal rates (V_max). The competitive method ensures that temperature, pH, enzyme and substrate concentrations, and the presence of any potential inhibitors are identical and will not skew the results. The method can be easily applied at any chosen temperature or pH, and to mutants, or under any other condition that might influence protein conformation and, thus, substrate specificity. It provides a facile screening method to select conditions for a detailed phosphopeptide screen to provide deeper insight into substrate preference.