Steady-state and pre-steady-state kinetics of the mitochondrial F1Fo ATPase: Is ATP synthase a reversible molecular machine?

H+-ATP synthase (F1Fo ATPase) catalyzes the synthesis and/or hydrolysis of ATP, and the reactions are strongly affected by all the substrates (product s) in a way clearly distinct from that expected of a simple reversibly oper ating enzyme. Recent studies have revealed the structure of F-1, which is i deally suited for the alternating binding change mechanism, with a rotating gamma-subunit as the energy-driven coupling device. According to this mech anism ATP, ADP, inorganic phosphate (P-i) and Mg2+ participate in the forwa rd and reverse overall reactions exclusively as the substrates and products . However, both F-1 and F1Fo demonstrate non-trivial steady-state and pre-s teady-state kinetics as a function of variable substrate (product) concentr ations. Several effecters cause unidirectional inhibition or activation of the enzyme. When considered separately, the unidirectional effects of ADP, P-i, Mg2+ and energy supply on ATP synthesis or hydrolysis may possibly be explained by very complex kinetic schemes; taken together, the results sugg est that different conformational states of the enzyme operate in the ATP h ydrolase and ATP synthase reactions. A possible mechanism for an energy-dep endent switch between the two states of F1Fo ATPase is proposed.