Reverse micelles as a model system with which to study leaving group effects on alkaline phosphatase-catalysed hydrolysis

Abstract

Ionisation of 4-nitrophenol in a reverse micellar system prepared by dissolving Aerosol OT (AOT), a surfactant with an anionic polar head, in isooctane, depends on the degree of hydration ([H₂O]/[AOT]) of the system. The model system provides a convenient instrumental tool with which to study the leaving group effect of alkaline phosphatase-catalysed hydrolysis of 4-nitrophenyl phosphate. The Brønsted constants, β_lg, for k_cat and k_cat/K_m were found to be –0.47 and –1.03, respectively. Assuming that phosphorylation of the enzyme is rate limiting, the strong leaving group effect on catalysis indicates that the apparent pK_a-values observed in reverse micelles are true measures of ionisation.