The molecular basis of chemomechanical coupling in muscle and in other biological engines.

It is argued that the force driving muscular shortening (psi) differs from that (phi) responsible for rigor tension generation. psi is associated with ATP-induced dissociation of actomyosin (a.m.), whereas phi is due to an isomerization reaction of a.m., following the hydrolysis of ATP. Both forces are intimately coupled with appreciable changes in the structure of the hydration shell of a.m., mainly at the interface between the two proteins, which involve the release of stored energy. When an active muscle is allowed to shorten freely, psi gives rise to a sliding distance (s.d.) delta l1 which differs in character and in magnitude from the s.d. (delta l2) observed when a muscle which had developed rigor tension isometrically is released. The maximal values of the two forces (psi 0 and phi 0) as well as delta l2 are calculated on the basis of experimental data. The forces and their corresponding s.d.'s are related through the standard free energies of the chemical reactions which are responsible for them. It is claimed that the same mechanochemical (m.c.) mechanisms operate also in all microtube-based locomotion and force-generation systems and, furthermore, that practically the same values of psi 0, phi 0, delta l1, and delta l2 are shared by the two types of biological m.c. convertors.