Practice question

Lactose permease functions through classical alternating-access transport mechanism: outward-open conformation binds extracellular lactose plus proton on specific side chains, undergoes conformational change to occluded state, then rearranges to inward-open releasing substrates into cytoplasm. Proton coupling utilizes electrochemical gradient across inner membrane to drive accumulation to concentrations hundredfold higher than environment even when external lactose remains micromolar. Influx supplies beta-galactosidase substrate producing glucose, galactose, and allolactose inducer driving further expression. Transport activity therefore underlies both nutritional acquisition and regulatory induction, not hydrolytic cleavage, repression, or direct activation, distinguishing it sharply from enzymatic components inside operon.