Structure Formation Mechanism of Helix-Type Membrane Protein

对典型的膜蛋白细菌视紫红质进行了三种变性实验，以估计各种因素对膜蛋白结构形成的贡献。实验结果如下：1.实验结果表明：经添加醇而变性的细菌视紫红质。在醇变性过程中，三级结构被破坏，二级结构保持完整。2. 细菌视紫红质在纯己烷中不变性。然而，少量醇的加入破坏了结构，表明羟基负责膜蛋白的醇变性。3. 根据热变性动力学常数的温度依赖性，估计了热变性的活化能。活化能约为40 kcal/mol，相当于膜内约10个氢键。这个数是跨膜螺旋之间可能的氢键数的一半。因此，结构稳定性的机制是由于跨膜螺旋之间的氢键和离子对等极性相互作用。

Three kinds of denaturation experiments were performed for a typical membrane protein, bacteriorhodopsin, in order to estimate the contribution of various factors to the structure formation of membrane proteins. The results were as follow : 1. Bacteriorhodopsin denatured by the addition of alcohols. In the alcohol denaturation, the tertiary structure was destroyed with the secondary structure remained intact. 2. Bacteriorhodopsin did not denature in pure hexane. However, the structure was destroyed by the addition of small amount of alcohol, indicating that the hydroxyl group is responsible for the alcohol denaturation of membrane proteins. 3. The activation energy of the thermal denaturation was estimated from the temperature dependence of the denaturation kinetic constant. The activation energy of about 40 kcal/mol corresponds to about ten hydrogen bonds in membrane. Tihs number is the half of possible hydrogen bonds between transmembrane helices. Consequently, the mechanism of the structural stability is due to the polar interactions such as hydrogen bonds and ion pairs between transmembrane helices.