  We experimentally showed that biological molecular motor F$_1$-ATPase (F$_1$)
implements an optimal rectification mechanism. F$_1$ hardly suppresses
adenosine triphosphate (ATP) synthesis, which is the F$_1$'s physiological role
while inhibiting unfavorable hydrolysis of ATP. This optimal rectification is a
high contrast to a simple ratchet model, where the inhibition of the backward
current is inevitably accompanied by the suppression of the forward current.
The detailed analysis of single-molecule trajectories demonstrated a novel but
simple rectification mechanism of F$_1$ with parallel landscapes and asymmetric
transition rates.
