Practice question

ATP-driven pumps belong to several mechanistically distinct families distinguished by architecture, subunit composition and mechanism of coupling hydrolysis to transport. P-type family named because first discovered intermediate was phosphorylated enzyme, now understood to include Na+/K+ ATPase maintaining sodium potassium gradients, sarcoplasmic reticulum Ca2+ ATPase SERCA sequestering calcium, plasma membrane Ca2+ ATPase PMCA extruding calcium, gastric H+/K+ ATPase acidifying stomach and heavy metal copper and manganese pumps ATP7A ATP7B. Core mechanism involves conserved aspartate within DKTGT motif in cytoplasmic P domain that transiently accepts gamma phosphate from ATP forming covalent acyl-phosphate intermediate with high energy, creating E1P state occluding ions. Phosphorylation triggers large conformational rearrangement of transmembrane helices to E2P outward facing, releasing ions due to lowered affinity from micromolar to millimolar range, followed by dephosphorylation by TGES motif in actuator domain returning to E1 ready. This phosphorylation cycle detectable by incorporation of 32P, inhibition by vanadate mimicking phosphate transition state analog, and acid stability of phosphoenzyme. In contrast V, F and ABC families use noncovalent ATP binding without phosphoprotein formation. Hence hallmark characteristic defining P-class pumps is transient phosphorylation of pump protein during transport cycle enabling alternating access.