Szilagyi R K, Musaev D G, Morokuma K
Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
Inorg Chem. 2001 Feb 12;40(4):766-75. doi: 10.1021/ic000188n.
The Mo-site and its ligand environment of the FeMo-cofactor (FeMo-co) were studied using the hybrid density functional method B3LYP. The structure and stability of the model complex (S-ligand)3(N-ligand)Mo[(S)-OCH(CH3)C(O)O-] along with its various protonated and reduced/oxidized forms were calculated. Several hypotheses were tested: (i) ligand environment of the Mo-site, (ii) monodentate vs bidentate coordination of the Mo-bound homocitrate ligand, (iii) substrate coordination to the Mo center, and (iv) Mo-His interaction. It was found that the decoordination of one of the homocitrate (lactate in the model) "legs", the bidentate-->monodentate rearrangement, does not occur spontaneously upon either single/double protonation or one-electron reduction. However, it could occur only upon substrate coordination to the Mo-center of the single-protonated forms of the complex. It was shown that one-electron reduction, single-protonation, and substrate coordination facilitate the bidentate<-->monodentate rearrangement of the homocitrate (lactate) ligand of FeMo-co. It was demonstrated that the smallest acceptable model of His ligand in FeMo-co is methylimidazolate (MeIm-). Our studies suggest that the epsilon-N of the FeMo-co-bound His residue is not protonated, and as a consequence the cluster is tightly bound to the protein matrix via a strong Mo-N delta bond.
采用杂化密度泛函方法B3LYP研究了铁钼辅因子（FeMo-co）的钼位点及其配体环境。计算了模型配合物（S-配体）3（N-配体）Mo[(S)-OCH(CH3)C(O)O-]及其各种质子化和还原/氧化形式的结构和稳定性。测试了几个假设：（i）钼位点的配体环境，（ii）与钼结合的高柠檬酸配体的单齿与双齿配位，（iii）底物与钼中心的配位，以及（iv）钼-组氨酸相互作用。结果发现，高柠檬酸（模型中的乳酸）“支链”之一的去配位，即双齿到单齿的重排，在单质子化/双质子化或单电子还原时不会自发发生。然而，它只能在底物与配合物单质子化形式的钼中心配位时发生。结果表明，单电子还原、单质子化和底物配位促进了FeMo-co的高柠檬酸（乳酸）配体的双齿<-->单齿重排。结果表明，FeMo-co中组氨酸配体的最小可接受模型是甲基咪唑盐（MeIm-）。我们的研究表明，与FeMo-co结合的组氨酸残基的ε-N没有质子化，因此该簇通过强Mo-Nδ键与蛋白质基质紧密结合。
