Practice question

Coupled transporters classified as antiporters exchange substrates in opposite directions driven by gradients. Sodium potassium ATPase exemplifies primary active antiporter using ATP rather than secondary gradient. Alpha subunit has ten transmembrane helices forming pocket with three sodium sites and two potassium sites. It binds three intracellular Na+ in high affinity E1 and exports them while binding two extracellular K+ and importing them, so Na+ and K+ move opposite in same cycle. Exchange is electrogenic due to 3:2 stoichiometry, contributing about minus four millivolts to potential and producing outward current measurable in voltage clamp. Antiport contrasts with symport where solutes travel same direction as Na+-glucose symport and uniport where single solute moves passively. Mechanistically antiporters use alternating-access rocker-switch where binding of one substrate lowers affinity for other enforcing exchange in many secondary transporters, while ATPase cycle allows slippage under nonphysiological conditions. E1 to E2 conformational change alternates binding site exposure, coupling ATP hydrolysis to opposing movements essential for excitability and volume control.