Practice question

Facilitated diffusion denotes protein mediated passive transport operating down electrochemical gradient, characterized by saturation kinetics and competitive inhibition but lacking ATP hydrolysis. Several protein families exemplify it. GLUT uniporters possess twelve transmembrane helices organized into two bundles that alternate between outward open and inward open via rocker switch, transporting glucose along blood tissue gradients. Ion channels such as voltage gated sodium and potassium channels form aqueous pores with selectivity filters containing carbonyl oxygens and charged rings permitting millions of ions per second. Aquaporins are tetramers where each monomer forms single file water pore with NPA motifs and aromatic arginine constriction blocking protons while allowing water at billion per second rates. All operate spontaneously without nucleotide. In contrast Na+/K+ ATPase is primary active transporter of P-type family hydrolyzing ATP at DKTGT aspartate forming phosphoenzyme E1P to E2P, extruding three sodium against gradient and importing two potassium, maintained by ouabain sensitivity. ABC and V-ATPases also hydrolyze ATP. Due to direct ATP dependence and uphill ion movement, Na+/K+ ATPase does not represent facilitated diffusion.