Practice question

Lysosomes function as degradative centers requiring low internal pH between 4.5 and 5 to activate more than fifty acid hydrolases such as cathepsins, sulfatases and lipases, while protecting cytosol from unwanted proteolysis. Acidic environment established primarily by vacuolar proton ATPase V-ATPase, large 900 kilodalton complex composed of peripheral V1 octamer A3B3CDE3FG3H hydrolyzing ATP at A-B interfaces and integral Vo domain a, c, c'', d, e, forming proton pore. Mechanism rotary: ATP hydrolysis in V1 drives rotation of central stalk D F and c-ring proteolipid assembling ten copies each with essential glutamate protonating deprotonating at half channels in subunit a, transporting protons from cytosol to lumen consuming roughly four protons per ATP. No phosphoenzyme intermediate forms. Specific inhibitors bafilomycin and concanamycin bind Vo. Na+/K+ ATPase maintains plasma gradients, F-type ATP synthase in mitochondria normally synthesizes ATP using proton gradient, ABC transporters move organic molecules. Thus proton pumping and organelle acidification for low pH depends on V-class proton pump.