Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
J Am Chem Soc. 2010 Dec 1;132(47):17032-40. doi: 10.1021/ja107876p. Epub 2010 Nov 5.
The acceleration of electron transfer (ET) rates in redox proteins relative to aqueous solutes can be attributed to the protein's ability to reduce the nuclear response or reorganization upon ET, while maintaining sufficiently high electronic coupling. Quantitative predictions of reorganization free energy remain a challenge, both experimentally and computationally. Using density functional calculations and molecular dynamics simulation with an electronically polarizable force field, we report reorganization free energies for intraprotein ET in four heme-containing ET proteins that differ in their protein fold, hydrophilicity, and solvent accessibility of the electron-accepting group. The reorganization free energies for ET from the heme cofactors of cytochrome c and b(5) to solvent exposed Ru-complexes docked to histidine residues at the surface of these proteins fall within a narrow range of 1.2-1.3 eV. Reorganization free energy is significantly lowered in a designed 4-helix bundle protein where both redox active cofactors are protected from the solvent. For all ET reactions investigated, the major components of reorganization are the solvent and the protein, with the solvent contributing close to or more than 50% of the total. In three out of four proteins, the protein reorganization free energy can be viewed as a collective effect including many residues, each of which contributing a small fraction. These results have important implications for the design of artificial electron transport proteins. They suggest that reorganization free energy may in general not be effectively controlled by single point mutations, but to a large extent by the degree of solvent exposure of the ionizable cofactors.
电子转移（ET）在氧化还原蛋白中相对于水溶液溶质的速率的加速可以归因于蛋白质在保持足够高的电子耦合的情况下降低 ET 时的核响应或重组。重组自由能的定量预测在实验和计算上都是一个挑战。我们使用密度泛函计算和带有电子极化力场的分子动力学模拟，报告了四个含血红素的 ET 蛋白中蛋白质内 ET 的重组自由能，这些蛋白在其蛋白质折叠、亲水性和电子接受基团的溶剂可及性方面存在差异。细胞色素 c 和 b(5)的血红素辅因子与溶剂暴露的 Ru 配合物之间 ET 的重组自由能，在与这些蛋白质表面的组氨酸残基对接的范围内，落在 1.2-1.3eV 的狭窄范围内。在一个设计的 4 螺旋束蛋白中，重组自由能显著降低，其中两个氧化还原活性辅因子都受到溶剂的保护。对于所有研究的 ET 反应，重组的主要成分是溶剂和蛋白质，其中溶剂的贡献接近或超过总贡献的 50%。在四个蛋白中的三个中，蛋白质重组自由能可以看作是包括许多残基的集体效应，每个残基贡献一小部分。这些结果对人工电子传输蛋白的设计具有重要意义。它们表明，重组自由能一般可能不能通过单点突变有效控制，而是在很大程度上取决于可离子化辅因子的溶剂暴露程度。
