A molecular mechanism for the production of muscular work

Abstract

Motility in biological systems is widely thought to result from the transduction of chemical free energy. In muscle a difficulty has been encountered in finding a precise mechanism whereby this conversion is accomplished. We suggest that this difficulty resides in the macroscopic character of free energy which, as a thermodynamic quantity, deals only with large assemblages of molecules. However, the fundamental site of active movement has recently been found to be localized in a single molecule (a myosin head) and is therefore not open to thermodynamic treatment. It is suggested instead that the energetic source of work produced at the myosin head is to be found in the heat (in the form of kinetic energy) evolved during an actomyosin ATPase cycle. This heat equivalent kinetic energy is then converted into useful work by means of a vibrational mode of a single water molecule which is attached to the ADP formed in the myosin head during a portion of the actomyosin ATPase cycle. It is the resonance mode of this water molecule which enables the extremely short durations (∼10-15s) of the chemical reactions taking place in one actomyosin ATPase cycle to result in the much longer duration (∼10-2s) of the resulting movement. This mechanism may also be fundamental to other types of motility in living systems.