Purification and characterization of (Na+, K+)-ATPase. V. Conformational changes in the enzyme Transitions between the Na-form and the K-form studied with tryptic digestion as a tool.

1. Purified (Na+, K+)-ATPase consisting of membrane fragments was digested with trypsin. The time course of enzyme inactivation was related to the electrophoretic pattern of native and cleaved proteins remaining in the membrane. 2. Differences in both the inactivation kinetics and the cleavage of the large chain (mol. wt 98 000) allow distinction of two patterns of tryptic digestion of (Na+, K+)-ATPase seen with Na+ or K+ in the medium. 3. With K+, the inactivation of (Na+, K+)-ATPase is linear with time in semilogarithmic plots and the activity is lost in parallel with cleavage of the large chain to fragments with molecular weights 58 000 and 48 000. 4. With Na+, the inactivation curves are biphasic. In the initial phase of rapid inactivation, 50% of the activity is lost with minor changes in the composition of the large chain. In the final phase, the large chain is cleaved at a low rate to a fragment with a molecular weight of 78 000. 5. It is concluded that the regions of the large chain exposed in the presence of K+ are distinct from the regions exposed in presence of Na+ and that two conformations of (Na+, K+)-ATPase can be sensed with trypsin, a (t)K-form and a (t)Na-form. 6. Reaction of the (t)K-form with ATP cause transition to the (t)Na-form. Relatively high concentrations of ATP are required and Mg2+ is not necessary. Phosphorylation of (Na+, K+)-ATPase is accompanied by transition from the (t)Na-form to the (t)K-form. Previous kinetic data suggest that these conformational changes are accompanied by shifts in the affinities of the enzyme for Na+ and K+.