Reorganization Energy for Internal Electron Transfer in Multicopper Oxidases

We have calculated the reorganization energy for the intramolecular electron transfer between the reduced type 1 copper site and the peroxy intermediate of the trinuclear cluster in the multicopper oxidase CueO. The calculations are performed at the combined quantum mechanics and molecular mechanics (QM/MM) level, based on molecular dynamics simulations with tailored potentials for the two copper sites. We obtain a reorganization energy of 91–133 kJ/mol, depending on the theoretical treatment. The two Cu sites contribute by 12 and 22 kJ/mol to this energy, whereas the solvent contribution is 34 kJ/mol. The rest comes from the protein, involving small contributions from many residues. We have also estimated the energy difference between the two electron-transfer states and show that the reduction of the peroxy intermediate is exergonic by 43–87 kJ/mol, depending on the theoretical method. Both the solvent and the protein contribute to this energy difference, especially charged residues close to the two Cu sites. We compare these estimates with energies obtained from QM/MM optimizations and QM calculations in a vacuum and discuss differences between the results obtained at various levels of theory.

本研究针对多铜氧化酶CueO（multicopper oxidase CueO）中还原态1型铜位点（type 1 copper site）与三核簇过氧中间体之间的分子内电子转移，计算了其重组能（reorganization energy）。计算基于采用针对两个铜位点定制的势能开展的分子动力学模拟，采用量子力学与分子力学联用（QM/MM）理论级别完成。根据不同的理论处理方式，我们得到的重组能范围为91–133 kJ/mol。其中两个铜位点分别贡献12和22 kJ/mol，溶剂贡献为34 kJ/mol，剩余部分则来自蛋白质，涉及多个残基的小幅贡献。我们还估算了两种电子转移态之间的能量差，结果表明：依据不同理论方法，过氧中间体的还原过程为放能反应，自由能变化范围为43–87 kJ/mol。溶剂与蛋白质均对该能量差存在贡献，尤其是靠近两个铜位点的带电残基。我们将上述估算结果与通过QM/MM优化以及真空量子力学计算得到的能量进行对比，并讨论了不同理论级别下所得结果之间的差异。