Proposed proton transport mechanism and candidate carboxylates. (A) Cartoon of proposed mechanism indicating protonation/deprotonation of a side chain (red circle) with alternate extracellular and cytoplasmic access in kinetically adjacent pump conformations; gray mass represents protein barrier to proton diffusion in the relevant segment of Na⁺/K⁺ pump TM domain structure. (B; from left to right) Increasingly magnified (factors indicated by blue arrows) views of Xenopus α1/β3 Na⁺/K⁺ pump homology model based on x-ray crystal structure of the K⁺-bound E2 · MgF₄²⁻ Na⁺/K⁺-ATPase (Protein Data Bank accession no. 2ZXE; Shinoda et al., 2009), showing the α subunit (gray, with 5 of the 10 TM helices colored), β subunit (green surface), and γ subunit (red helix). (Middle and right) Side and top (from extracellular surface) views of the α-subunit TM domain (helix numbers in white) identifying six candidate carboxylates, three Glu (red spheres) and three Asp (green spheres), at the level of binding sites I and II that contain K⁺ ions (black balls); the colored TM helices are: pale blue, TM1; magenta, TM2; blue, TM4; purple, TM5; and green, TM6. Gray, TM3 and TM7–TM10; lime spheres, C113 in TM1; yellow spheres (left) or sticks (right) mark T806 at the top of TM6.