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(Na⁺/K⁺)-ATPase

This page shows some interactive JSmol views of (Na⁺/K⁺)-ATPase, this is a large protein and may take a few seconds to load.
(Na⁺/K⁺)-ATPase is an enzyme which transports sodium ions out of cells and potassium ions into cells with associated Mg²⁺-catalysed ATP hydrolysis:

3Na⁺_ic + 2K⁺_ec + ATP^4- + H₂O ⇄ 3Na⁺_ec + 2K⁺_ic + ADP^3- + H₃PO₄^2- + H⁺

where ic = intracellular (inside cell) and ex = extracellular (outside cell).

The first figure shows an overview of the Na⁺-bound form of the protein from pig kidney. It shows a dimeric structure with 3 protein chains per monomer. The distinctive α-helices would normally span the cell membrane. Na⁺ ions are shown as yellow spheres, Mg²⁺ as green spheres, and ADP in CPK colours - these are easier to see in the "skeleton view".

The active binding site is near the centre of the α-helices and contains 3 Na⁺ ions (in this structure there is another Na⁺ ion above the α-helix array. Note the hard oxygen donors coordinated to the Na⁺ ions (the coordination sites are probably a bit incomplete in this figure, due to missing or misplaced water molecules). Note also that the ADP is associated with Mg²⁺ ions.

The second set of images shows the K⁺-bound form of shark (Na⁺/K⁺)-ATPase.

This time the first figure shows a monomer with the K⁺ ions shown as purple spheres and Mg²⁺ in green. There is no ATP or ADP crystallised in this form of the protein.
Note that 2 K⁺ ions are bound in the same region of the protein that occupied by 3 Na⁺ ions in the previous structure.