Practice question

Primary active transport is defined by direct coupling of ATP hydrolysis within same protein to drive solute against gradient uphill requiring work. Sodium potassium ATPase exemplifies true primary pump prototype with ten transmembrane helices and three cytosolic domains forming P-type family characterized by transient phosphorylation of Asp369 during cycle. One ATP hydrolyzed phosphorylates P-domain inducing rearrangement from E1 to phosphorylated E2 state that translocates three sodium outward and two potassium inward per cycle consuming about quarter of ATP at rest generating electrogenic potential. In contrast GLUT1 and GLUT2 encoded by SLC2A are facilitative uniporters operating by alternating access rocker switch without ATP moving glucose down gradient passively saturable Km one to twenty millimolar stereoselective not energy linked expressed in erythrocytes brain liver beta cells for basal uptake. Aquaporins are tetrameric water channels each monomer forming pore allowing passive water diffusion at billion per second following osmotic gradient without energy gating by pH. Distinguishing categories crucial for pharmacology since ouabain digoxin specifically inhibit Na+/K+ ATPase binding extracellular side of E2-P while cytochalasin B inhibits GLUTs. Therefore primary active example among membrane proteins is sodium potassium ATPase requiring ATP directly.