Transmission of E1-E2 Structural Changes in Response to Na+ or K+ Binding in Na,K-ATPase
The extensive E1-E2 conformational changes in response to Na+ or K+ binding in the absence of other ligands must be driven by motion of the side chains contributing to cation coordination, but the differences in structure of Na+ and K+ sites have not been resolved. The recent high resolution structure model of the E2 conformation of Ca-ATPase offers the first opportunity to examine and model the changes accompanying the adjustment of the cation sites from an E1 form with specificity for Na+ to an E2 form with specificity for K+. The model of the E2 form provides a remarkable fit to the data of direct Tl+ or K+ binding after site-directed mutagenesis of residues Asp804 and Asp808 in M6, Glu 779, Gln776, and Ser775 in M5, and Glu327 in M4. Cytoplasmic domain movements during E1 E2 conformational transition can be monitored by proteolytic cleavage. Protection of the chymotrypsin-sensitive bond at Leu266 in L2/3 and rotation of the A domain is more complete in the E2Mg-vanadate-ouabain complex than in the E2[2K] form.
na k-atpase e1-e2 transition na+- or k+-ion binding