The effect of pH, temperature, and organic solvents on the kinetic parameters of Escherichia coli alkaline phosphatase.
Abstract A study of the effect of pH on kinetic parameters of Escherichia coli alkaline phosphatase, in presence and in absence of organic solvents, has shown that a cationic acid group with a pK of 7.4 at 25.5° is implicated in catalysis and that a neutral acid group with a pK of 9.2 at 25.5° is involved in enzyme-substrate binding. From the change in pK with temperature, values of 6,500 and 11,000 cal per mole, respectively, have been calculated for the heats of ionization of these two groups. These data are consistent with the proposal that the imidazolium group of a histidine residue participates in catalysis, and that a water molecule coordinated to the zinc atom functions in enzyme-substrate binding.