L-乳酸脱氢酶热变性的热力学研究

用差示扫描量热法对L-乳酸脱氢酶的热变性进行了研究(温度扫描范围为290—390K,酶蛋白溶液浓度为0.28—0.72mg蛋白/mg溶液)。实验观察到当酶溶液浓度在0.62—0.72mg蛋白/mg溶液范围内有一个吸热转变,酶溶液浓度小于0.62mg蛋白/mg溶液时有两个未完全分开的吸热转变。 这个酶的量热焓与范德霍夫焓的比远大于1,而接近于2,这表明乳酸脱氢酶的变性过程不是一个简单的两态转变,从热力学和吸热峰的形状、大小分析,可以推断乳酸脱氢酶分子是由两个以弱相互作用相连结的合作结构区组成,而每一个结构区是由两个相互作用很强的亚基组成。也就是说乳酸脱氨酶的变性过程包括两个半独立的合作结构区的转变,每一个结构区的转变都近似一个两态转变,ΔHeal与ΔHvh的比值是随着两个半独立部分相互作用的增强,即蛋白浓度的增加而减小。随着蛋白浓度的减小,蛋白质周围水分子增多,酶分子中两个半独立部分的相对独立性增强,这可由热谱图上一个吸热转变变成两个半独立的转变得到证实。

Thermodynamic Study of Thermal Denaturation of L-Lactic Dehydogenase

Thermal denaturation of L-lactic dehydrogenase, (rabbit muscle) has been studied thermodynamically in solution by Differential Scanning Calprinetry at various protein concentrations (in CB) . At the CB in 0.62-0.72 mg protein/mg solution, only one transition was observed. At the CB<0.62mg protein/mg solution, two quasi-independent transitions were observed. The ratio △Hcal/△Hvh is far above 1.0 and approaches 2.0 for this protein. These facts show that, thermal denaturation of L-lactic dehydrogenase is not a simple two-state transition and that the L-lactic dehydrogenase molecule consists of two slightly interacting , co-operative units;i.e., each of them underwent a quasi-independent transition. Thus denaturation of the L-lactic dehydrogenase includes two two-state transitions. Analysis of the shape of the thermogram shows that the sizes of the two co-operative structural blocks are-different, so this enzyme molecule is not very symmetric.