The ATP-dependent ion pumps are essential for maintaining cellularcation homeostasis and a variety of specialized cellular functions. The Ca2+-ATPase in sarcoplasmic reticulum (SR) induces muscle relaxation by lowering the Ca2+ activity in the cytoplasmic compartment surrounding the myofilaments. The utilization of one molecule of ATP results in the electrogenic uptake of 2 Ca2+ and ejection of 2 H+ at neutral pH. The amplitude of the electrical signal associated with this transport activity is reduced in the presence of K+, suggesting that K+ binds to negatively-charged ligands on the pump protein during cycling. Rapid mixing, quenched-flow studies have shown that K+, in addition to accelerating the hydrolysis of the low energy phosphoenzyme (E2P), dramatically slows the rate of the final conformational transition, E2 to E These results suggest that Ca2+ uptake is coupled to K+ countertransport associated with the E2 to E1 conformational transition. Slowing of this step results from the slow deocclusion of K+ to the cytoplasm following translocation, similar to behavior seen in the Na+/K+-ATPase. Sodium ion translocation by the Na+/K+ pump is coupled to the E1P to E2P conformational transition in the Na+/K+-ATPase reaction cycle. Stopped-flow and quenched-flow kinetic experiments using BIPM-labeled pig kidney Na+/K+-ATPase revealed that 1) the BIPM probe does not alter the kinetics of the ATPase partial reactions; 2) the BIPM fluorescence signal which monitors the conversion of E1P to E2P coincides with the time course of phosphorylation. The rapid conversion of E1P to E2P implied by this result was confirmed by simulations of phosphoenzyme formation and phosphate liberation which yielded rates more than 3000 s-1 at pH 7.4 and 24oC. At pH 6.2, the rate constant for this reaction was 287 s-1, implying that a H+ is released in conjunction with this conformational transition.
