Directional electron transfer in ruthenium-modified horse heart cytochrome c.
Directional electron transfer in ruthenium-modified horse heart cytochrome c.
复制标题
钌修饰的马心细胞色素 c 中的定向电子转移。
作者：
Bechtold,R;Kuehn,C;Lepre,C;Isied,SS
Cytochromec可以被[（NH3）5 RuII/III-]修饰，特别是在组氨酸33的咪唑部分，我们最近讨论了这种修饰蛋白质内电子转移的热力学和动力学1 -5。金枪鱼细胞色素6的氧化和还原形式的X射线晶体结构表明，细胞色素6的血红素基团与钌标记之间的分离为12-16 π。从[（NH3）5 RuII-]中心到Fe（III）血红素中心的内部电子转移速率常数为K <$53s −1（25 °C）（ΔH <$=3.5kcal mol−1，ΔS <$= − 39 EU），通过脉冲辐解测量。测得的单分子速率常数1，k = 53 s−1，与细胞色素分子内发生的许多构象变化处于相同的时间尺度7 -9。这些结果提出了一个问题，是否电子转移或蛋白质构象变化是在这个过程中的限速步骤。我们在这里描述了一个实验，进一步探测这个分子内电子转移步骤。它涉及通过改变钌标记的氧化还原电位来逆转电子转移的方向。本文所述的新的钌-细胞色素衍生物中的电子转移是从血红素（II）到RU（III）标记，而在（NH_3）_5Ru-细胞色素衍生物中，电子转移是从RU（II）到血红素（III）。本文所述的新型钌-细胞色素中血红素（II）到钌（III）的分子内电子转移比（NH_3）_5Ru-细胞色素中钌（II）到血红素（III）的电子转移慢得多（> 105倍）。因此，我们得出结论，电子转移在细胞色素是方向性的，与蛋白质包膜可能参与这种方向性。
Cytochromeccan be modified by [(NH3)5RuII/III-] specifically at the imidazole moiety of histidine 33, and we have recently discussed the thermodynamics and kinetics of electron transfer within this modified protein1–5. X-ray crystal structures of the oxidized and reduced forms of tuna cytochromec6indicate that the separation between the haem group of cytochromecand the ruthenium label is 12–16 Å. Internal electron transfer from the [(NH3)5RuII-] centre to the Fe(III) haem centre occurs with a rate constantK≃ 53s−1(25 °C) (ΔH‡=3.5kcal mol−1, ΔS‡= −39EU), as measured by pulse radiolysis. The measured unimolecular rate constant1, k ≃ 53 s−1, is on the same timescale as a number of conformational changes that occur within the cytochromecmolecule7–9. These results raise the question of whether electron transfer or protein conformational change is the rate limiting step in this process. We describe here an experiment that probes this intramolecular electron transfer step further. It involves reversing the direction of electron transfer by changing the redox potential of the ruthenium label. Electron transfer in the new ruthenium–cytochromecderivative described here is from haem(II) to the RU(III) label, whereas in (NH3)5Ru–cytochromecthe electron transfer is from RU(II) to haem(III). Intramolecular electron transfer from haem(II) to RU(III) in the new ruthenium–cytochromecdescribed here proceeds much slower (> 105times) than the electron transfer from Ru(II) to haem(III) in the (NH3)5Ru–cytochromec. We therefore conclude that electron transfer in cytochromecis directional, with the protein envelope presumably involved in this directionality.