New Perspective on the Reversibility of ATP Synthesis and Hydrolysis by F_o×F₁-ATP Synthase (Hydrolase)

F_o×F₁-ATPases of mitochondria, chloroplasts, and microorganisms catalyze transformation of proton motive force (the difference between the electrochemical potentials of hydrogen ion across a coupling membrane) to the free energy of ATP phosphoryl potential. It is often stated that F_o×F₁-ATPases operate as reversible chemo-mechano-electrical molecular machines that provide either ATP synthesis or hydrolysis depending on particular physiological demands of an organism; the microreversibility principle of the enzyme catalysis is usually taken as a dogma. Since 1980, the author has upheld the view that the mechanisms of ATP synthesis and hydrolysis by the F_o×F₁ complex are different (Vinogradov, A. D. (2000) J. Exp. Biol., 203, 41-49). In this paper, the author proposes a new model considering the existence in coupling membranes of two non-equilibrium isoforms of F_o×F₁ unidirectionally catalyzing synthesis and/or hydrolysis of ATP.