"ATP"

Figure 7. Proposed model for the formation of ATP during rotation of the g unit within the F₁ complex of ATP Synthase. The following description refers to the reactions occurring in the lower left b subunit (blue), one of three sites in the complex

a. The active site on the b unit (lower left) is opened by rotation of the central (green) g protein. In this state it readily accepts a molecule of ADP and a phosphate into the active site. The g protein is rotated under the influence of protons moving through the "c" protein rotor.
b. As a result of this rotation and the consequent change in the shape of the b unit , the apposition of ADP and phosphate is effected resulting in
c. the formation of the "high energy" phosphate bond and a molecule of ATP results.
d. Continued rotation of the g protein results in a return to the initial position, opening the active site, and releasing the newly-formed ATP.

The cycle now begins again. Since there are three identical b subunits in each F₁ complex, three ATP are generated by each complete rotation of the g protein. [Modified from P. Boyer, 1989, FASEB J. 3:2164, and Y. Zhou et al., 1997, Proc. Nat'l. Acad. Sci. USA 94:10583.]

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