Hon-nami K, Oshima T
Biochemistry. 1979 Dec 11;18(25):5693-7. doi: 10.1021/bi00592a027.
The denaturation of Thermus thermophilus cytochrome c-552 by acid, guanidine hydrochloride, and heat was studied by measuring the changes in absorption and circular dichroism. Cytochrome c-552 was remarkably resistant to acid; the pK of the transition from the low- to the high-spin form was roughly 0.3. The effect of guanidine hydrochloride on the heme iron-methionine bond of Thermus and horse cytochromes c was also investigated; a comparison of the free-energy changes for the displacement of the bond indicated that the coordination in cytochrome c-552 is highly stable. The spectra of guanidine hydrochloride unfolded cytochrome c-552 were dependent on the pH; the titration curve showed the presence of a cooperative single transition of pK = 4.7, with a one-proton dissociation, suggesting the ionization of a histidine residue. In the presence of guanidine hydrochloride, the influence of the heat on the ligand bond in cytochrome c-552 was studied. The van't Hoff plots of the reaction were biphasic. The enthalpy changes in the higher temperature range were independent on the guanidine hydrochloride concentration, while those in the lower range were not.
通过测量吸收和圆二色性的变化，研究了嗜热栖热菌细胞色素c-552在酸、盐酸胍和热作用下的变性情况。细胞色素c-552对酸具有显著的抗性；从低自旋形式转变为高自旋形式的pK约为0.3。还研究了盐酸胍对嗜热栖热菌和马细胞色素c的血红素铁-甲硫氨酸键的影响；对该键位移的自由能变化进行比较表明，细胞色素c-552中的配位非常稳定。盐酸胍展开的细胞色素c-552的光谱取决于pH值；滴定曲线显示存在pK = 4.7的协同单转变，伴有一个质子解离，表明存在一个组氨酸残基的电离。在盐酸胍存在的情况下，研究了热对细胞色素c-552中配体键的影响。该反应的范特霍夫图是双相的。较高温度范围内的焓变与盐酸胍浓度无关，而较低温度范围内的焓变则与盐酸胍浓度有关。
