Ohmura T, Nakamura H, Niki K, Cusanovich M A, Akutsu H
Advanced Technology Research Center, Mitsubishi Heavy Industries, Ltd., Yokohama, Japan.
Biophys J. 1998 Sep;75(3):1483-90. doi: 10.1016/S0006-3495(98)74067-6.
The effect of ionic strength on the macroscopic and microscopic redox potentials and the heme environment of cytochrome c3 from Desulfovibrio vulgaris Miyazaki F have been investigated by NMR and electrochemical methods. The redox potentials of this tetraheme protein are found to be ionic strength-dependent. Especially, the microscopic redox potentials of hemes 2 and 3 at the fourth reduction step increase significantly with increasing ionic strength, which is in contraction to the theoretical expectation. The coordinated imidazole proton signals are unaffected by ionic strength. However, the methyl and propionate proton signals of hemes 1 and 4 showed significant ionic strength dependencies that are distinct from those for hemes 2 and 3. This heme classification is the same as that found in the ionic strength dependencies of the microscopic redox potentials at the fourth reduction step. Furthermore, the effect of ionic strength on the electrostatic potentials at the heme irons has been examined on the theoretical basis. The electrostatic potential at heme 4 changes up to 1 M ionic strength, which was not expected from the observations reported on cytochromes so far. These results are discussed in connection with the reported anomalous ionic strength dependency of the reduction rate of cytochrome c3.
通过核磁共振（NMR）和电化学方法研究了离子强度对来自普通脱硫弧菌宫崎F株的细胞色素c3的宏观和微观氧化还原电位以及血红素环境的影响。发现这种四血红素蛋白的氧化还原电位依赖于离子强度。特别是，在第四次还原步骤中，血红素2和3的微观氧化还原电位随着离子强度的增加而显著增加，这与理论预期相反。配位的咪唑质子信号不受离子强度的影响。然而，血红素1和4的甲基和丙酸质子信号显示出明显的离子强度依赖性，这与血红素2和3的不同。这种血红素分类与第四次还原步骤中微观氧化还原电位的离子强度依赖性相同。此外，还在理论基础上研究了离子强度对血红素铁静电势的影响。血红素4处的静电势在离子强度达到1 M时发生变化，这是迄今为止关于细胞色素的报道中未曾预期到的。结合报道的细胞色素c3还原速率的异常离子强度依赖性对这些结果进行了讨论。
