Practice question

Ouabain is a cardenolide steroid with rhamnose sugar derived from Strophanthus and Acocanthera, historically used as arrow poison and experimentally as specific probe. Pharmacologically it exhibits extremely high affinity for extracellular side of Na+/K+ ATPase, nanomolar Ki, interacting with residues in transmembrane helices M1-M6 and extracellular loops forming ouabain binding pocket accessible only in E2P conformation when outward gate open. Binding stabilizes E2P state, prevents dephosphorylation and return to E1, blocking cycle. Resultant inhibition halts sodium extrusion, intracellular sodium rises, reducing driving force for forward mode Na+/Ca2+ exchanger NCX1 which normally imports three sodium extruding one calcium. Calcium accumulation in sarcoplasmic reticulum enhances release and contractility. Selectivity is high; at low concentrations ouabain does not inhibit sarcoplasmic calcium ATPase SERCA which sensitive to thapsigargin, nor V-type proton pumps sensitive to bafilomycin, nor ABC transporters requiring different nucleotide binding domains. Hence ouabain serves as selective inhibitor defining P-type sodium potassium pump activity.