  F$_\mathrm{o}$F$_1$-ATP synthase is a factory for synthesizing ATP in
virtually all cells. Its core machinery is the subcomplex F$_1$-motor
(F$_1$-ATPase) and performs the reversible mechanochemical coupling. Isolated
F$_1$-motor hydrolyzes ATP, which is accompanied by unidirectional rotation of
its central $\gamma$-shaft. When a strong opposing torque is imposed, the
$\gamma$-shaft rotates in the opposite direction and drives the F$_1$-motor to
synthesize ATP. This mechanical-to-chemical free-energy transduction is the
final and central step of the multistep cellular ATP-synthetic pathway. Here,
we determined the amount of mechanical work exploited by the F$_1$-motor to
synthesize an ATP molecule during forced rotations using methodology combining
a nonequilibrium theory and single molecule measurements of responses to
external torque. We found that the internal dissipation of the motor is
negligible even during rotations far from a quasistatic process.
