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Torque Generation by the F_o motor of the Sodium ATPase

Jianhua Xing ^*, Hongyun Wang , Christoph von Ballmoos , Peter Dimroth  and George Oster ^*

^* Departments of Molecular and Cellular Biology and Environmental Science, Policy and Management, University of California, Berkeley, California 94720-3112 USA; Department of Applied Mathematics and Statistics, University of California, Santa Cruz, California 95064 USA; and Institute of Microbiology, Swiss Federal Institute of Technology, Zurich, Switzerland

Correspondence: Address reprint requests to George Oster, Dept. of Environmental Science, Policy and Management, 201 Wellman Hall, University of California, Berkeley, CA 94720-3112. Tel.: 510-642-5277; E-mail: goster{at}nature.berkeley.edu.

Based on recent structural and functional findings, we have constructed a mathematical model for the sodium-driven F_o motor of the F₁F_o-ATPase from the anaerobic bacterium Propionigenium modestum. The model reveals the mechanochemical principles underlying the F_o motor's operation, and explains all of the existing experimental data on wild-type and mutant F_o motors. In particular, the model predicts a nonmonotonic dependence of the ATP hydrolysis activity on the sodium concentration, a prediction confirmed by new experiments. To explain experimental observations, the positively charged stator residue (R227) must assume different positions in the ATP synthesis and hydrolysis directions. This work also illustrates how to extract a motor mechanism from dynamical experimental observations in the absence of complete structural information.

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