Practice question

Lysosomes maintain a highly acidic lumen essential for optimal activity of about sixty acid hydrolases involved in macromolecular digestion and for solute transport. The steep proton gradient, pH 4.5 to 5.0 inside versus cytosolic pH 7.2, representing over two pH units and hundredfold proton concentration difference, is actively generated by vacuolar-type H+ ATPase, V-ATPase, consuming ATP. This massive multi-subunit rotary pump consists of peripheral V1 domain with A3B3 hexamer that hydrolyzes ATP and integral V0 domain forming proton channel with c-ring rotation. ATP hydrolysis drives rotation of central stalk and c-ring, translocating protons into the lumen against electrochemical gradient at cost of one ATP per two to three protons. Counterion movement of chloride via ClC-7 Cl-/H+ exchanger and potassium channel provides electroneutrality and osmotic balance preventing excessive membrane potential. Resulting acidity activates cathepsins B, D, L and others by protonation of catalytic residues, induces conformational changes releasing enzymes from mannose-6-phosphate receptors after delivery, and denatures substrates improving access. Low pH also prevents hydrolase leakage from causing cytosolic damage. Inhibition by bafilomycin A1 or concanamycin neutralizes lysosomes, blocking degradation, autophagy flux, and cholesterol egress from NPC1 pathway.