Identification of Asp804 and Asp808 as Na+ and K+ coordinating residues in α‐subunit of renal Na,K‐ATPase

Mutations to Asp804 and Asp808 in the α‐subunit almost abolish Na,K‐ATPase activity, but high‐affinity binding of [3H]ATP or [3H]ouabain at equilibrium and E1–E2 transitions are preserved. Titration of K+‐ion displacement of [3H]ATP or [3H]ouabain shows that the mutations interfere with occlusion of K+ in the E2[2K] conformation. Reduced phosphorylation levels or affinities for Na+ in presence of oligomycin indicate that Asp804 and Asp808 also contribute to coordination of Na+ in the E1P[3Na] form. Demonstration of alternate interactions of Na+ or K+ with Asp804 and Asp808 support the notion of cation binding in a ping‐pong sequence in catalytic models of Na,K‐pumping.