The catalytic cycle of gastric (H+ + K+)-ATPase.

Abstract

The reaction sequence of the gastric (H' + K+)-ATPase and the rate of the distinguishable steps was studied in purified gastric membranes and vesicles using a flow-quench rapid reaction apparatus and steady state measurements.The reaction rates were calculated as pseudo-first order rate constants.In the absence of added K', the initial step is the reversible binding of ATP to the protein with a rate constant of 1400 min".This is followed by a M&+-dependent transfer of the terminal phosphate of ATP to the catalytic subunit of the protein with a rate constant of 4400 min".The addition of K+ to this protein-bound acyl phosphate results in a two-step dephosphorylation.The faster initial step is dependent on the concentration of K' in terms of rate and quantity and the maximum rate is 4000 min".The slower step with a rate of 210 min" is not affected by K' concentrations greater than 0.5 m~ and corresponds to the overall rate of hydrolysis.The K' site for dephosphorylation is present in intact vesi-