Homology modeling of Na,K-ATPase: a putative third sodium binding site suggests a relay mechanism compatible with the electrogenic profile of Na+ translocation

Identification of the third Na(+) binding site would be crucial in interpretation of the electrophysiological behavior of Na,K-ATPase. To address this question a three-dimensional homology model of Na,K-ATPase was built from the known crystallographic structure of Ca-ATPase (1EUL). Phe760, which is conserved in virtually all Ca-ATPases, is replaced by Ser768 in Na,K-ATPase, resulting in a small cavity between M4, M5, and M6. A partially hydrated Na(+) ion can be bound at this third site on the cytoplasmic side of cation binding sites 1 and 2. This leads to the proposal that the conductance of the "third Na(+)" ion across approximately 70% of the membrane dielectric may be achieved by adding up the passage of one Na(+) ion from the described cytoplasmic cavity to cation site 1 and the further conductance of the previously bound Na(+) ion from cation site 1 to the extracellular phase. This relay mechanism may therefore be compatible with the electrogenic profile of Na(+) translocation.