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Asymmetry in the F₁-ATPase and Its Implications for the Rotational Cycle

Sean X. Sun ^*, Hongyun Wang  and George Oster 

^* Department of Mechanical Engineering and Whitaker Institute of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland; Department of Applied Mathematics and Statistics, Jack Baskin School of Engineering, University of California, Santa Cruz, California; and Department of Molecular and Cellular Biology and College of Natural Resources, University of California, Berkeley, California

Correspondence: Address reprint requests to George Oster, University of California, Dept. of Molecular and Cellular Biology and ESPM, 201 Wellman Hall, Berkeley, CA 94720-3112. Tel.: 510-642-5277; Fax: 510-642-7428; E-mail: goster{at}nature.berkeley.edu.

ATP synthase uses a rotary mechanism to carry out its cellular function of manufacturing ATP. The central-shaft rotates inside a hexameric cylinder composed of alternating - and ß-subunits. When operating in the hydrolysis direction under high frictional loads and low ATP concentrations, a coordinated mechanochemical cycle in the three catalytic sites of the ß-subunits rotates the -shaft in three 120° steps. At low frictional loads, the 120° steps alternate with three ATP-independent substeps separated by 30°. We present a quantitative model that accounts for these substeps and show that the observed pauses are due to 1), the asymmetry of the F₁ hexamer that produces a propeller-like motion of the power-stroke and 2), the relatively tight binding of ADP to the catalytic sites.

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