Dissecting the free energy of formation of the 1:1 actomyosin complex

The behaviour of solutions of pure myosin, of pure F-actin and of the equimolar mixture of myosin and of F-actin is studied. It is found that the chemical potential of the two proteins, in separate solutions, increases monotonically with the increase of protein osmotic pressure. A method is presented to determine the chemical potential of the 1:1 actin-myosin complex formed from equimolar solutions of myosin and of F-actin (as monomer). This is the first evaluation of the chemical potential of actomyosin under conditions similar to those of skeletal muscle. It is found that the filament suspensions of myosin and of the 1:1 actin-myosin complex display a high non-ideal behavior as well as distinctly different energy profiles as a function of protein osmotic pressure. This supports the hypothesis that, in muscle: (a) detached cross-bridge change significantly their free energy when sarcomere is shifting from the relaxed to the active or to the rigor state; and (b) the cross-bridge attachment-detachment process is accompanied by changes of muscle protein osmotic pressure.