van Severen Marie-Céline, Andrejić Milica, Li Jilai, Starke Kerstin, Mata Ricardo A, Nordlander Ebbe, Ryde Ulf
Theoretical Chemistry, Department of Chemistry, Chemical Centre, Lund University, P. O. Box 124, 221 00, Lund, Sweden.
J Biol Inorg Chem. 2014 Oct;19(7):1165-79. doi: 10.1007/s00775-014-1172-z. Epub 2014 Jun 24.
The oxidation of sulfite to sulfate by two different models of the active site of sulfite oxidase has been studied. Both protonated and deprotonated substrates were tested. Geometries were optimized with density functional theory (TPSS/def2-SV(P)) and energies were calculated either with hybrid functionals and large basis sets (B3LYP/def2-TZVPD) including corrections for dispersion, solvation, and entropy, or with coupled-cluster theory (LCCSD(T0)) extrapolated toward a complete basis set. Three suggested reaction mechanisms have been compared and the results show that the lowest barriers are obtained for a mechanism where the substrate attacks a Mo-bound oxo ligand, directly forming a Mo-bound sulfate complex, which then dissociates into the products. Such a mechanism is more favorable than mechanisms involving a Mo-sulfite complex with the substrate coordinating either by the S or O atom. The activation energy is dominated by the Coulomb repulsion between the Mo complex and the substrate, which both have a negative charge of -1 or -2.
研究了亚硫酸盐氧化酶活性位点的两种不同模型将亚硫酸盐氧化为硫酸盐的过程。对质子化和去质子化的底物都进行了测试。采用密度泛函理论（TPSS/def2-SV(P)）对几何结构进行了优化，并使用包含色散、溶剂化和熵校正的杂化泛函和大基组（B3LYP/def2-TZVPD）或外推至完整基组的耦合簇理论（LCCSD(T0)）计算能量。比较了三种提出的反应机制，结果表明，底物攻击与钼结合的氧代配体，直接形成与钼结合的硫酸盐配合物，然后分解为产物的机制具有最低的势垒。这种机制比涉及底物通过硫或氧原子配位的钼-亚硫酸盐配合物的机制更有利。活化能主要由钼配合物与底物之间的库仑排斥作用决定，它们都带有-1或-2的负电荷。
