Studies on the kinetics of protein denaturation under high pressure

Abstract

The characteristics in the protein denaturation under pressure obtained by- the author up to date are summarized. Thermodynamic functions in the activation process of denaturation are calculated from the relations of absolute reaction rates using the results of ovalbumin and carbonylhemoglobin to discuss the mechanism of protein denaturation under pressure. Considering the signs of the values of ΔH≠, ΔS≠ and ΔV, the results are to be classified into three regions; I: <30℃, >4000 kg/cm^2 (ΔH≠<0, ΔS≠<0, ΔV<0), II: >40℃. >3000kg/cm^2 (ΔH≠>0, ΔS≠≶0, ΔV≠<0), and III: >60℃, <3000kg/cm^2 (ΔH≠>0, ΔS≠>0, ΔV≠>0). In the region III where thermal denaturation is retarded by pressure, the behaviors are to be understood if it is taken into consideration that a disorderliness of configuration by the thermal vibration is retarded by pressure. In the region I where pressure denaturation only occurs, the following mechanism is proposed, P+nH_2O ⇄ P(H_2O)_n → P_D, (1) (2) where P(H_20)_n is the protein hydrate which always exists in the equilibrium relation of (1) under high pressure and is constructed with the rigid structure; P_D is the denatured protein. The rate determining step is the process of (2). The formation of P(H_2O)_n is exothermic and accompanied with the volume decrease. And it is discussed with what mechanism much more plenty of water is adhered to protein and then the volume decreases. It is supposed that in the region III both denaturations by pressure and heat occur at the same time.