  A tiny bacterial flagellar motor rotates in both counter-clockwise (CCW) and
clockwise (CW) rotational directions. The most important measurable
characteristic of the flagellar motor is its torque versus angular speed
relationship in CCW or CW modes, which is found to be non-symmetrical with each
other, and still, such a phenomenon is not clearly understood.Here, we explain
this asymmetry through a mechanistic model based on the detailed torque
analysis for the rotation of the motor and the revolutionary as well as
spinning motion of the filament and bead. We find out that the asymmetry
results from the conformational changing of the hook due to rotational
switching, rather than any non-symmetric changes in the potential of mean force
generated by the stator-rotor interactions. In CCW mode, when the hook remains
bend and flexible, the revolution motion predominates and the restoring torque
in this motion, originated due to drag, governs the shape of the torque-speed
curve. However, in CW mode, spinning motion dominates as the hook becomes
straight and rigid, and the linear torque-speed relation arises due to the
restoring torque for the drag corresponding to this motion. Our study indicates
the significant role of the hook's conformational change upon the biological
functions of the motor and paves the way for further experimental exploration
on the structural origin of such asymmetry.
