Practice question

Generating acidic interior in endocytic compartments essential for cargo sorting, iron release from transferrin, ligand receptor dissociation, viral entry and lysosomal hydrolysis demands active proton translocation against roughly two pH units. Main machinery responsible is vacuolar proton ATPase V-type, large composite enzyme about 900 kilodaltons composed of peripheral V1 complex A3B3CDE3FG3H where A3B3 hexamer contains catalytic ATP hydrolysis sites at alternating interfaces, and integral membrane Vo complex including a subunit with two half channels, c-ring of nine to ten copies c, c double prime providing proton binding glutamate essential carboxylates, subunits d, e and accessory Ac45. Hydrolysis drives rotation of central rotor DF coupled to c-ring, moving protonated glutamates from cytosolic half channel to luminal half channel releasing protons inside. Mechanism rotary without phosphoenzyme intermediate, inhibited by bafilomycin macrolide binding Vo. ABC transporters export xenobiotics, P-type Na+/K+ ATPase maintains plasma gradients not acidic organelle pH. Thus V-class proton pump, not ABC, creates acidic pH in lysosomes and related compartments supporting degradative pathways.