The catalytic mechanism of gastric H+/K+-ATPase: simulations of pre-steady-state and steady-state kinetic results

A reaction cycle for the gastric H+/K+-ATPase is proposed. This has been used to simulate the results from four types of pre-steady-state and steady-state kinetic experiments: (1) the K+ dependence of the dephosphorylation of the phosphoenzyme; (2) the rate of phosphorylation of the enzyme by ATP at different concentrations; (3) the effect of ATP concentration on the steady-state rate of ATP hydrolysis; (4) the phosphoenzyme levels in the steady state at various ATP concentrations. A single set of equilibrium and rate constants can be used to reproduce the results from all four sets of experiments quite well. It is suggested that the steady-state rate equation is nonhyperbolic because ATP can react with the enzyme in both the E1 and the E2 state, but with a lower affinity in E2. No single step is by itself limiting the maximum turnover rate.