Practice question

Distinguishing pump families centers on mechanism of coupling ATP hydrolysis to ion translocation. P-class pumps such as Na+/K+ ATPase, SERCA, PMCA and H+/K+ ATPase share formation of covalent phosphoenzyme intermediate on conserved aspartate within DKTGT motif, inhibited by vanadate mimicking phosphate. They typically transport monovalent or divalent cations like sodium, potassium, calcium, heavy metals with fixed stoichiometry. V-class proton pumps acidifying vacuoles, lysosomes, endosomes and Golgi represent rotary ATPases composed of cytosolic V1 domain peripheral stalk and membrane Vo proteolipid ring. They hydrolyze ATP in V1 to drive rotation of central stalk and c-ring translocating protons without ever forming phosphoprotein, mechanism analogous to F-type synthase but operating exclusively as proton ATPase. Importantly substrate specificity limited to protons, not sodium, potassium or calcium. Claim they transport Na+ and K+ misassigns function; Na+/K+ exchange belongs to P-class. Therefore difference lies in absence of phosphorylated intermediate and proton specific rotary mechanism versus phosphoenzyme mediated cation exchange.