The Osmotic Properties and Free Energy of Formation of the Actomyosin Rigor Complexes from Rabbit Muscle

We have studied the osmotic properties of the calcium-regulated actomyosin complexes from skeletal muscle at the protein osmotic pressure of 18 kPa and a different actin-to-myosin molar ratios. Essentially, protein solutions were equilibrated against a solution of poly(ethylene glycol) 40,000 of known macromolecular osmotic pressure. At the end of the equilibration the water and the protein masses of the protein solutions were determined gravimetrically and the protein molar concentration was calculated. In this reconstructed system we have found following, at the actin-to-molar ratio of 2.6 (the most likely stoichiometry of these two proteins in the dense region of the A band) the average distance between the myosin filaments is 34.2 nm, this equals the interfilament distance in the intact fibre of muscle in rigor, at the sarcomere length of 3.38 micrograms. The formation of the F-actin-myosin and of the tropomyosin-F-actin-myosin rigor complexes involves the largest free energy changes, -5.38 kJ/mol myosin and -5.67 kJ/mol myosin, respectively. The formation of the troponin-tropomyosin-F-actin-myosin(Ca) rigor complex from myosin and troponin-tropomyosin-F-actin(Ca) occurs with the free energy change of -3.43 kJ/mol myosin. Of these -3.43 kJ, -1.81 kJ are provided by the endergonic conversion of troponin-tropomyosin-F-actin(EGTA) into troponin-tropomyosin-F-actin (Ca). The transition of myosin and of troponin-tropomyosin-F-actin(EGTA) into the -F-actin-myosin(Ca) rigor complex is accompanied by a 5.8% increase of volume. The increase of volume is due to a large influx of water, which is essentially protein-hydration water.