Tollin G, Cheddar G, Watkins J A, Meyer T E, Cusanovich M A
Biochemistry. 1984 Dec 18;23(26):6345-9. doi: 10.1021/bi00321a009.
We have measured the ionic strength dependence of the rate constants for electron transfer from the semiquinone of Clostridium pasteurianum flavodoxin to 12 c-type cytochromes and several inorganic oxidants using stopped-flow methodology. The experimental data were fit quite well by an electrostatic model that represents the interaction domains as parallel disks with a point charge equal to the charge within this region of the protein. The analysis provides an evaluation of the electrostatic interaction energy and the rate constant at infinite ionic strength (k affinity). The electrostatic charge on the oxidant within the interaction site can be obtained from the electrostatic energy, and for most of those reactants for which structures are available, the results are in good agreement with expectation. The k affinity values were found to correlate with redox potential differences, as expected from the theory of adiabatic (or nonadiabatic) outer-sphere electron-transfer reactions. Deviations from the theoretical curves are interpreted in terms of the influence of surface topology on reaction rate constants. In general, we find that electrostatic effects, steric influences, and redox potential all exert a much larger effect on reaction rate constants for the flavodoxin-cytochrome system than has been previously observed for free flavin-cytochrome interactions. The implications of this for determining biological specificity are discussed.
我们采用停流技术，测定了从巴氏芽孢杆菌黄素氧还蛋白半醌到12种c型细胞色素以及几种无机氧化剂的电子转移速率常数对离子强度的依赖性。实验数据通过静电模型得到了很好的拟合，该模型将相互作用域表示为平行盘，其点电荷等于蛋白质该区域内的电荷。该分析提供了对静电相互作用能以及无限离子强度下速率常数（亲和力常数k）的评估。相互作用位点内氧化剂上的静电荷可从静电能中获得，对于大多数有结构信息的反应物，结果与预期相符。正如绝热（或非绝热）外层电子转移反应理论所预期的那样，发现亲和力常数k值与氧化还原电位差相关。从表面拓扑结构对反应速率常数的影响角度解释了与理论曲线的偏差。总体而言，我们发现静电效应、空间位阻影响和氧化还原电位对黄素氧还蛋白 - 细胞色素系统反应速率常数的影响，比之前在游离黄素 - 细胞色素相互作用中观察到的要大得多。讨论了这对确定生物学特异性的意义。
