18O-exchange by hydrolyzing enzymes: extension of the model to Pi molecules with inequivalent oxygen atoms in the bound state

Enzymes causing an exchange of oxygens from P\ with the surrounding water oxygens are very common. A statistical model for the data evaluation for an observation of this oxygen exchange by isotope methods is presented. It is shown how different cases of inequivalence of the four Pi oxygens may be uncovered. The number of reversals of the oxygen exchange step on the enzyme and the apparent second order rate constant for the binding of Pi to the enzyme are obtained as a result of the data fitting procedure. Cobalt phosphatase, zinc phosphatase, and myosin subfragment 1 are treated as examples