Phosphorylation and dephosphorylation reactions of the red beet plasma membrane ATPase studied in the transient state

The reaction mechanism of the solubilized red beet (Beta vulgaris L.) plasma membrane ATPase was studied with a rapid quenching apparatus. Using a dual-labeled substrate (γ-³²P]ATP and 5′,8-³H]ATP), the presteady-state time course of phosphoenzyme formation, phosphate liberation and ADP liberation was examined. The time course for both phosphoenzyme formation and ADP liberation showed a rapid, initial rise while the timecourse for phosphate liberation showed an initial lag. This indicated that ADP was released with formation of the phosphoenzyme while phosphate was released with phosphoenzyme breakdown. Phosphoenzyme formation was Mg²⁺-dependent and preincubation of the enzyme with free ATP followed by the addition of Mg²⁺ increased the rate of phosphoenzyme formation 2.3-fold. This implied that phosphoenzyme formation could result from a slow reaction of ATP binding followed by a more rapid reaction of phosphate group transfer. Phosphoenzyme formation was accelerated as the pH was decreased, and the relationship between pH and the apparent first-order rate constants for phosphoenzyme formation suggested the role of a histidyl residue in this process. Transient kinetics of phosphoenzyme breakdown confirmed the presence of two phosphoenzyme forms, and the discharge of the ADP-sensitive form by ADP correlated with ATP synthesis. Potassium chloride increased the rate of phosphoenzyme turnover and shifted the steady-state distribution of phosphoenzyme forms. From these results, a minimal catalytic mechanism is proposed for the red beet plasma membrane ATPase, and rate constants for several reaction steps are estimated.