Biomotor is not a Heat Engine

ATP synthase (FoF1-ATPase) is the model rotary molecular motor with three sites of ATP syntheses/hydrolyses in the “stator” of F1. The coupling between chemical progress and mechanical one is tight, and both progresses are reversible. Thus, the mechanochemical reversibility of FoF1-ATPase may resemble that of a heat engine. Single-molecule experiment has demonstrated that the efficiency of rotary motor is nearly 100%. However, it is impossible for a heat engine to achieve such high efficiency. On the other hand, kinesin is the model linear biomotor with only two sites of ATP hydrolyses located in the two heads, respectively, while its efficiency is just [Formula: see text]. Myosin V is another processive linear motor with nearly [Formula: see text] efficiency as well. However, the chemical progress of processive linear motors with two heads is irreversible. That is, if they walk backward in hand over hand along with a track by an external force, the energy molecule ATP is not synthesized, but consumed yet. This chemical irreversibility excludes the possibility that kinesin/myosin V can be treated energetically as a heat engine. The most intriguing fact is why the efficiency of a processive linear motor with two sites is just [Formula: see text] of that of a processive rotary motor with three sites.