HEAT‐INDUCED TRANSITIONS IN THE PROTEIN‐PROTEIN INTERACTION OF BOVINE NATURAL ACTOMYOSIN

Protein‐protein interaction of bovine natural actomyosin (NAM) was studied by means of optical density changes resulting from discrete particle formation in the temperature range of4°C to 70°C. From Arrhenius plots, the apparent heat of activation (ΔH_a) at pH 5.5 (17.1 kcallmole) was significantly (P<0.05) lower than activation energies in the pH range of 6.0 to 7.5. The lower Δ H_a resulted in initiation of protein‐protein interaction at a temperature near 16°C at pH 5.5, whereas interaction did not proceed until the temperature approached 37°C at pH 6.0 and above. Derivative curves (dOD/dT) at pH 5.5 and 6.0 showed two distinct NAM thermal transition regions. Tm₁ occurred at 43.0°C at pH 5.5 and 48.5°C at pH 6.0, with the 5.5°C difference possibly arising from effects of proton binding in altering protein conformation. Only a 1.5°C difference in Tm₂ (56.0°C at pH 5.5 versus 57.5°C at pH 6.0) was found. Although the overall heat‐mediated NAM aggregation (in dilute solution) was found to follow first order kinetics by two evaluation methods, the existence of two thermal transitions supports a two‐step reaction mechanism proposed for the formation of protein gels (in higher concentration solutions).