Saigo S
J Biochem. 1986 Jul;100(1):157-65. doi: 10.1093/oxfordjournals.jbchem.a121688.
To elucidate the relationship between local and gross conformational changes, three types of conformational changes, i.e., alkaline isomerization, ligand replacement, and guanidine hydrochloride (GuHCl) denaturation, of a set of homologous and modified cytochromes c were investigated by spectroscopic methods. Cytochromes c examined include the horse, tuna, Candida, monomeric Saccharomyces, and disulfide-linked dimeric Saccharomyces proteins. Correlations were found between the apparent pK (pKa) for the isomerization and the equilibrium constants for the binding of imidazole and azide to the heme iron: the lower the pKa value, the higher the binding constant. This is explained by the fact that the isomerization and ligand replacement involve similar conformational changes localized around the sixth coordination position of the heme. A good correlation was also observed between the susceptibilities to local and gross conformational changes: the lower the pKa value for the isomerization, the lower the GuHCl concentration for the midpoint of the denaturation. Thermodynamic analysis suggests that this correlation is not due to the involvement of similar conformational changes in the two processes. Cooperative stabilization of the tertiary structure is proposed to interpret this correlation.
为阐明局部构象变化与整体构象变化之间的关系，采用光谱学方法研究了一组同源且经修饰的细胞色素c的三种构象变化，即碱性异构化、配体置换和盐酸胍（GuHCl）变性。所研究的细胞色素c包括马、金枪鱼、念珠菌、单体酿酒酵母和二硫键连接的二聚体酿酒酵母蛋白。发现异构化的表观pK（pKa）与咪唑和叠氮化物与血红素铁结合的平衡常数之间存在相关性：pKa值越低，结合常数越高。这可以通过异构化和配体置换涉及血红素第六配位位置周围类似的局部构象变化这一事实来解释。在对局部和整体构象变化的敏感性之间也观察到了良好的相关性：异构化的pKa值越低，变性中点的GuHCl浓度越低。热力学分析表明，这种相关性并非由于两个过程中涉及类似的构象变化。提出三级结构的协同稳定作用来解释这种相关性。
