What happens if a mutation occurs in the phosphorylation site of Na+/K+ ATPase?
P-type ATPases operate through an ordered Post-Albers cycle involving transient phosphorylation of a conserved aspartate within the DKTGTLT motif of the cytosolic P-domain. ATP phosphorylates this aspartate in E1 state with high Na+ affinity open inward, generating E1-P with occluded Na+, then conversion to E2-P with outward openness and high K+ affinity releases Na+ and binds K+. Dephosphorylation returns enzyme to E2 and E1 states. Phosphorylation triggers long-range movement of nucleotide-binding N domain and actuator A domain, coupling chemical energy to alternating access. A missense or deletion mutation that removes or changes this aspartate to a non-acceptor residue such as alanine prevents phosphoenzyme formation, blocks ATP hydrolysis, and arrests conformational cycling. Consequently neither Na+ extrusion nor K+ uptake proceeds, collapsing Na+ and K+ gradients, depolarizing membrane potential, disrupting cell volume regulation, disabling secondary carriers for glucose and amino acids, and causing broad failure of excitability and transepithelial transport dependent on gradient. This illustrates why single residue mutation abolishes entire pump function.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11, P-type ATPases E1-E2 mechanism.