Pressure-Volume Properties of Molten Globule

关于典型和经典的牛α-乳清蛋白熔融球的体积行为，我们已经得到以下结果：虽然α-乳清蛋白在从天然到熔融球的构象变化过程中体积减小（ΔV【近似等于】- 60 cm^3/mol）s，但从熔融球到完全展开状态的体积接近0 cm^3/mol （Kobashigawa et al., Protein Sci.）。科学通报，8,2765-2772（1999）。对于蛋清溶菌酶，由于其未表现出稳定的熔融球，且其展开和再折叠具有双态机制，因此在高达100 Mpa压力下的展开行为不能用双态机制来解释，特别是在较低温度（0 ~ 20℃）下。这表明体积行为有些复杂。蛋清溶菌酶熔融球在压力下稳定。犬乳溶菌酶呈极稳定的熔融球状。用差示扫描量热法对犬乳溶菌酶的热行为进行了研究，结果表明，犬乳溶菌酶从天然形态到熔融球状形态和熔融球状形态到未折叠形态的构象变化明显分开。这是因为与α-乳清蛋白和常规溶菌酶相比，犬乳溶菌酶的溶球状态非常稳定。从突变研究来看，这种稳定性源于His21、Ile56、Ala93和Val109在熔融球状态下形成疏水团簇。由于稳定性的起源是疏水相互作用，因此在熔融球到未折叠的构象变化过程中，体积预计会增加。然而，由于仪器在较高温度下缺乏稳定性，我们尚未通过实验证明。

We had already obtained the following result concerning to the volumetric behavior of the typical and classical molten globule of bovine α-lactalbumin : although the volume of α-lactalbumin decreased during the conformational change ftom native to molten-globule (ΔV【approximately equal】 - 60 cm^3/mol)s, that from molten-globule to fully-unfolded state was nearly 0 cm^3/mol (Kobashigawa et al., Protein Sci., 8, 2765-2772(1999)). In the case of egg-white lysozyme, which does not show stable molten-globule and unfolds and refolds with two-state mechanism, the unfolding behavior under the pressure up to 100 Mpa cannot be explained with two-state mechanism, especially at lower temperature (0 〜 20℃). It suggests that the volumetric behavior is somewhat complicated. The molten globule of hen egg-white lysozyme is stabilized under pressure. Canine milk lysozyme shows extremely stable molten globule. The thermal behavior of canine milk lysozyme was investigated with differential scanning calorimetry and it is shown that the conformational changes from native to molten-globule and molten-globule to unfolded state are clearly separated to each other. This is because that the molten-globule state of canine milk lysozyme is extremely stable as compared to those of α-lactalbumins and conventional lysozymes. From mutational investigation, this stability is originated from the formation of hydrophobic cluster of His21, Ile56, Ala93 and Val109 in molten-globule state. As the origin of the stability is hydrophobic interaction, the volume is anticipated to increase during the conformational change from molten-globule to unfolded state. However, we have not yet proved it experimentally due to the lack of stability of the instrument under higher temperatures.