Chemomechanical coupling in F₁-ATPase revealed by simultaneous observation of nucleotide kinetics and rotation

F₁-ATPase is a rotary molecular motor in which unidirectional rotation of the central γ subunit is powered by ATP hydrolysis in three catalytic sites arranged 120° apart around γ. To study how hydrolysis reactions produce mechanical rotation, we observed rotation under an optical microscope to see which of the three sites bound and released a fluorescent ATP analog. Assuming that the analog mimics authentic ATP, the following scheme emerges: (i) in the ATP-waiting state, one site, dictated by the orientation of γ, is empty, whereas the other two bind a nucleotide; (ii) ATP binding to the empty site drives an ∼80° rotation of γ; (iii) this triggers a reaction(s), hydrolysis and/or phosphate release, but not ADP release in the site that bound ATP one step earlier; (iv) completion of this reaction induces further ∼40° rotation.