Rate Limitation of the Na1,K1-ATPase Pump Cycle

ABSTRACT The kinetics of Na1-dependent phosphorylation of the Na1,K1-ATPase by ATP were investigated via the stopped-flow technique using the fluorescent label RH421 (saturating [ATP], [Na1], and [Mg21], pH 7.4, and 24°C). The well-established effect of buffer composition on the E2-E1 equilibrium was used as a tool to investigate the effect of the initial enzyme conformation on the rate of phosphorylation of the enzyme. Preincubation of pig kidney enzyme in 25 mM histidine and 0.1 mM EDTA solution (conditions favoring E2) yielded a 1/t value of 59 s 21. Addition of MgCl2 (5 mM), NaCl (2 mM), or ATP (2 mM) to the preincubation solution resulted in increases in 1/t to values of 129, 167, and 143 s21, respectively. The increases can be attributed to a shift in the enzyme conformational equilibrium before phosphorylation from the E2 state to an E1 or E1-like state. The results thus demonstrate conclusively that the E2 3 E1 transition does in fact limit the rate of subsequent reactions of the pump cycle. Based on the experimental results, the rate constant of the E23 E1 transition under physiological conditions could be estimated to be;65 s21 for pig kidney enzyme and 90 s21 for enzyme from rabbit kidney. Taking into account the rates of other partial reactions, computer simulations show these values to be consistent with the turnover number of the enzyme cycle (;48 s21 and;43 s21 for pig and rabbit, respectively) calculated from steady-state measurements. For enzyme of the a1 isoform the E2 3 E1 conformational change is thus shown to be the major rate-determining step of the entire enzyme cycle