State key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.
J Biol Chem. 2012 Nov 9;287(46):38790-9. doi: 10.1074/jbc.M112.368050. Epub 2012 Sep 6.
In the human body, arsenic is metabolized by methylation. Understanding this process is important and provides insight into the relationship between arsenic and its related diseases. We used the rapid equilibrium kinetic model to study the reaction sequence of arsenite methylation. The results suggest that the mechanism for arsenite methylation is a completely ordered mechanism that is also of general interest in reaction systems with different reductants, such as tris(2-carboxyethyl)phosphine, cysteine, and glutathione. In the reaction, cysteine residues of recombinant human arsenic (+3 oxidation state) methyltransferase (hAS3MT) coordinate with arsenicals and involve the methyl transfer step. S-Adenosyl-l-methionine (AdoMet) is the first-order reactant, which modulates the conformation of hAS3MT to a best matched state by hydrophobic interaction. As the second-order reactant, reductant reduces the disulfide bond, most likely between Cys-250 and another cysteine residue of hAS3MT, and exposes the active site cysteine residues for binding trivalent inorganic arsenic (iAs(3+)) to give monomethylarsonic dicysteine (MADC(3+)). In addition, the reaction can be extended to further methylate MADC(3+) to dimethylarsinic cysteine (DAMC(3+)). In the methylation reaction, the β-pleated sheet content of hAS3MT is increased, and the hydrophobicity of the microenvironment around the active sites is decreased. Similarly, we confirm that both the high β-pleated sheet content of hAS3MT and the high dissociation ability of the enzyme-AdoMet-reductant improve the yield of dimethylated arsenicals.
在人体中，砷通过甲基化作用进行代谢。了解这一过程非常重要，它为砷及其相关疾病之间的关系提供了深入的认识。我们使用快速平衡动力学模型研究了亚砷酸盐甲基化的反应顺序。结果表明，亚砷酸盐甲基化的机制是一种完全有序的机制，对于不同还原剂（如三（2-羧乙基）膦、半胱氨酸和谷胱甘肽）的反应体系也具有普遍意义。在反应中，重组人砷（+3 氧化态）甲基转移酶（hAS3MT）的半胱氨酸残基与砷化物配位，并涉及甲基转移步骤。S-腺苷甲硫氨酸（AdoMet）是一级反应物，通过疏水相互作用调节 hAS3MT 的构象以达到最佳匹配状态。作为二级反应物，还原剂还原二硫键，很可能是 Cys-250 和 hAS3MT 中的另一个半胱氨酸残基之间的二硫键，暴露出活性位点的半胱氨酸残基，用于结合三价无机砷（iAs(3+)）形成单甲基砷酸二半胱氨酸（MADC(3+)）。此外，反应可以进一步将 MADC(3+)甲基化为二甲基砷酸半胱氨酸（DAMC(3+)）。在甲基化反应中，hAS3MT 的β-折叠片含量增加，活性位点周围微环境的疏水性降低。同样，我们证实 hAS3MT 的高β-折叠片含量和酶-AdoMet-还原剂的高解离能力均提高了二甲基砷化物的产量。
