Evaluation of mechanical parameters of the mechanochemically 'active state' of actin filaments on the basis of purely chemical data.

Oosawa and his collaborators (cf. F. Oosawa, Biophys. Chem. 11 (1980) 443), employing various optical techniques, have shown that the flexibility of actin filaments increases upon interacting with the enzymatically active myosin fragments, particularly heavy meromyosin (HMM). It has been reported (S. Hitchock, L. Carlsson and U. Lindberg, Cell 7 (1976) 53) that HMM can accelerate the DNase 1-induced depolymerization of F-actin, provided MgATP is also present. Since, as we have demonstrated (cf. J. Borejdo myosin, is endowed with mechanochemical capability, we made an attempt to correlate the enhanced rate of depolymerization with the decrease in rigidity of the G-G bonds in F-actin. On the basis of the chemical kinetic data of Hitchcock et al. we could derive the approximate value of the HMM-MgATP-induced change in rigidity which is a mechanical molecular parameter. Since interaction between HMM or HMM subfragment-1 and F-actin in the presence of MgATP leads to the movement of the myosin heads along the actin filaments, it is argued that the enzymic behavior of this system should not be analyzed on the basis of simple, equilibrium, complex formation.