Practice question

According to Post-Albers catalytic cycle Na+/K+ ATPase interconverts between E1 inward facing and E2 outward facing with phosphorylated intermediates. In E1 primary sites display high affinity for sodium, three ions bind cytosolic side coordinated by carboxyl side chains and backbone carbonyls accommodating sodium ionic radius 0.95 angstrom. ATP phosphorylates Asp369 generating E1P occluded trapping ions, isomerization to E2P opens outward, sodium affinity drops thousand fold due to rearrangement of M4-M6 helices widening pocket incompatible with high affinity sodium coordination, ions released extracellularly. Now pocket geometry reconfigured with different carbonyl spacing and glutamate residues favoring dehydrated potassium radius 1.33 angstrom, potassium binds from extracellular fluid where concentration about 4 millimolar with millimolar affinity, much higher than sodium affinity in same state. Binding triggers dephosphorylation via TGES motif, forming E2(K2) occluded then transitioning back to E1 releasing potassium inward low affinity environment. Hence E2 conformation characterized by high affinity for potassium and low for sodium opposite to E1.