Practice question

Dynein mechanochemistry is powered by Mg-ATP, with intracellular concentration typically one to three millimolar far above Km around twenty to fifty micromolar, supporting near maximal velocity in vivo. Cycle involves ATP binding to AAA1 causing microtubule release, hydrolysis triggering primer stroke of linker into bent conformation, microtubule reattachment forward, then phosphate and ADP release driving power stroke returning linker to straight conformation. Without ATP supply motor enters rigor bound to microtubule, transport stalls. Metabolic poisons depleting ATP, such as sodium azide plus deoxyglucose, arrest retrograde organelle movement within minutes, reversible upon washout. Phosphorylation of stalk modulates cargo affinity but not basal stepping, myosin II interaction irrelevant because actin and microtubule systems use distinct ATPases, nuclear envelope attachment mediates nucleokinesis only in specialized migrations requiring additional adaptors. Hence requirement for constant ATP reflects dependence of AAA1 catalytic turnover to reload each step, similar to myosin and kinesin ATPases but with larger ring architecture.