Biophys J, September 2001, p. 1220-1233, Vol. 81, No. 3

Viscoelastic Dynamics of Actin Filaments Coupled to Rotary F-ATPase: Angular Torque Profile of the Enzyme

Division of Biophysics, University of Osnabrück, D-49069 Osnabrück, Germany

ATP synthase (F_OF₁) operates as two rotary motor/generators coupled by a common shaft. Both portions, F₁ and F_O, are rotary steppers. Their symmetries are mismatched (C₃ versus C_10-14). We used the curvature of fluorescent actin filaments, attached to the rotating c-ring, as a spring balance (flexural rigidity of 8 · 10²⁶ Nm²) to gauge the angular profile of the output torque at F_O during ATP hydrolysis by F₁ (see theoretical companion article (Cherepanov, D. A., and W. Junge, 2001. Biophys. J. 81:1234-1244.)). The large average output torque (50 ± 6 pN · nm) proved the absence of any slip. Variations of the torque were small, and the output free energy of the loaded enzyme decayed almost linearly over the angular reaction coordinate. Considering the threefold stepping and high activation barrier of the driving motor proper, the rather constant output torque implied a soft elastic power transmission between F₁ and F_O. It is considered as essential, not only for the robust operation of this ubiquitous enzyme under symmetry mismatch, but also for a high turnover rate of the two counteracting and stepping motor/generators.

Biophys J, September 2001, p. 1220-1233, Vol. 81, No. 3
© 2001 by the Biophysical Society   0006-3495/01/09/1220/14  $2.00

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