Regulation of the Step-Distance in Shortening Muscles

Abstract

It is argued that the force driving muscular shortening (Ψ) differs from the force (φ) responsible for tension generation, Ψ is associated with ATP-induced dissociation of actomyosin, whereas φ is due to an isomerization reaction of actomyosin, following the hydrolysis of ATP. In a shortening muscle, ATP is thus hydrolyzed after movement commences. Both forces are intimately coupled with appreciable changes in the structure of the hydration shell at the interface between the two proteins, which involves the release of stored energy. When an active muscle is allowed to shorten freely, Ψ gives rise to a step- (or sliding-) distance (Δl₁) which should be a variable and its value depends on the environmental conditions. On the other hand, the step distance (Δl₂) observed upon releasing a muscle which had developed rigor tension isometrically is a constant, the value of which is related to the myosin head’s length. The maximal values of the two forces (Ψ₀ and φ₀), as well as of Δl₂ are calculated on the basis of experimental data. The forces and their corresponding step distances are related through the standard free energies of the two chemical reactions responsible for them. It is claimed that the same mechanochemical mechanisms operate also in all microtube-based locomotion and force-generation systems and, furthermore, that practically the same values of Ψo, φ₀, Δl₁, and Δl₂ are shared by the two types of biological energy convertors.