Phosphate release in F1-ATPase catalytic cycle follows ADP release

A magnetic tweezer system that arrests the rotary ATP synthase F1 at the ATP-hydrolysis step provides an order of events to the chemomechanical cycle involving ADP release before phosphate release upon F1 rotation in ATP-hydrolysis mode. F1-ATPase is an ATP-driven rotary motor protein in which the γ-subunit rotates against the catalytic stator ring. Although the reaction scheme of F1 has mostly been revealed, the timing of inorganic phosphate (Pi) release remains controversial. Here we addressed this issue by verifying the reversibility of ATP hydrolysis on arrested F1 with magnetic tweezers. ATP hydrolysis was found to be essentially reversible, implying that Pi is released after the γ rotation and ADP release, although extremely slow Pi release was found at the ATP hydrolysis angle as an uncoupling side reaction. On the basis of this finding, we deduced the chemomechanical coupling scheme of F1. We found that the affinity for Pi was strongly angle dependent, implying a large contribution by Pi release to torque generation. These findings imply that under ATP synthesis conditions, Pi binds to an empty catalytic site, preventing solution ATP (though not ADP) from binding. Thus, this supports the concept of selective ADP binding for efficient ATP synthesis.