Stellwagen E, Babul J
Biochemistry. 1975 Nov 18;14(23):5135-40. doi: 10.1021/bi00694a018.
Increasing concentrations of chloride were found to increase the resolution between two visible absorbance spectral transitions associated with acidification of ferricytochrome c. Analysis of a variety of spectral and viscosity measurements indicates that protonation of a single group having an apparent pK of 2.1 +/- 0.2 and an intrinsic pK of about 5.3 displaces the methionine ligand without significantly perturbing the native globular conformation. Analysis of methylated ferricytochrome c suggests that protonation of a carboxylate ion, most likely a heme propionate residue, is responsible for displacement of the methionine ligand. Addition of a proton to a second group having an apparent pK of 1.2 +/- 0.1 displaces the histidine ligand and unfolds the protein from a globular conformation into a random coil. It is most likely that the second protonation occurs on the imidazole ring of the histidine ligand itself. Chloride is proposed to perturb these transitions by ligation in the fifth coordination position of the heme ion. Such ligation stabilizes a globular conformation of ferricytochrome c at pH 0.0 and 25 degrees.
研究发现，随着氯化物浓度的增加，与高铁细胞色素c酸化相关的两个可见吸收光谱跃迁之间的分辨率提高。对各种光谱和粘度测量结果的分析表明，表观pK为2.1±0.2、固有pK约为5.3的单个基团的质子化会取代甲硫氨酸配体，而不会显著干扰天然球状构象。对甲基化高铁细胞色素c的分析表明，羧酸根离子（很可能是血红素丙酸酯残基）的质子化是导致甲硫氨酸配体被取代的原因。向表观pK为1.2±0.1的第二个基团添加质子会取代组氨酸配体，并使蛋白质从球状构象展开为无规卷曲。最有可能的是，第二次质子化发生在组氨酸配体自身的咪唑环上。有人提出，氯化物通过与血红素离子的第五个配位位置配位来干扰这些跃迁。这种配位在pH 0.0和25℃时稳定了高铁细胞色素c的球状构象。
