(A, B) Close-ups of the transmembrane cation-binding site in the H+,K+-ATPase Y799W(K+)E2-MgFx state in stick representation, viewed approximately parallel to the membrane from the TM6 side (A) or approximately perpendicular to the membrane from the cytoplasmic side (B). Dotted lines indicate atoms that are within 3.5 Å of neighboring atoms, presumably forming hydrogen bonds or electrostatic interactions. The purple dot indicates bound K+ with its Stokes radius shown as a transparent sphere. Water molecules (red dots) are also indicated. (C, D) The (2K+)E2-MgFx state of Na+,K+-ATPase (Shinoda et al., 2009) (gray ribbons) is superimposed on the corresponding reaction state of H+,K+-ATPase Y799W(K+)E2-MgFx (colored ribbons), viewed from the membrane (C) or cytoplasmic (D) sides. For clarity, amino acid residues that contribute to the K+ coordination in H+,K+-ATPase are indicated, with their corresponding amino acids in Na+,K+-ATPase in parentheses. Residues Asn331, Ser782, Asn783, Asp811 and Tyr854 in Na+,K+-ATPase (with corresponding amino acid numbers of Tyr340, Lys791, Asn792, Glu820 and Tyr863, respectively, in H+,K+-ATPase) are shown because three of these amino acids are not conserved. The rest of these amino acids are conserved but Asn 792 in H+,K+-ATPase shows different conformation in the structure. Blue spheres with dotted circles indicate positions of K+ binding sites I and II, and pink dots are water molecules in the Na+,K+-ATPase structure.