Fakultät für Chemie, Institut für Organische Chemie, Universität Duisburg-Essen, Essen, Germany.
Chemistry. 2012 Aug 27;18(35):10937-48. doi: 10.1002/chem.201103477. Epub 2012 Jul 24.
The mammalian heme enzyme myeloperoxidase (MPO) catalyzes the reaction of Cl(-) to the antimicrobial-effective molecule HOCl. During the catalytic cycle, a reactive intermediate "Compound I" (Cpd I) is generated. Cpd I has the ability to destroy the enzyme. Indeed, in the absence of any substrate, Cpd I decays with a half-life of 100 ms to an intermediate called Compound II (Cpd II), which is typically the one-electron reduced Cpd I. However, the nature of Cpd II, its spectroscopic properties, and the source of the additional electron are only poorly understood. On the basis of DFT and time-dependent (TD)-DFT quantum chemical calculations at the PBE0/6-31G* level, we propose an extended mechanism involving a new intermediate, which allows MPO to protect itself from self-oxidation or self-destruction during the catalytic cycle. Because of its similarity in electronic structure to Cpd II, we named this intermediate Cpd II'. However, the suggested mechanism and our proposed functional structure of Cpd II' are based on the hypothesis that the heme is reduced by charge separation caused by reaction with a water molecule, and not, as is normally assumed, by the transfer of an electron. In the course of this investigation, we found a second intermediate, the reduced enzyme, towards which the new mechanism is equally transferable. In analogy to Cpd II', we named it Fe(II'). The proposed new intermediates Cpd II' and Fe(II') allow the experimental findings, which have been well documented in the literature for decades but not so far understood, to be explained for the first time. These encompass a) the spontaneous decay of Cpd I, b) the unusual (chlorin-like) UV/Vis, circular dichroism (CD), and resonance Raman spectra, c) the inability of reduced MPO to bind CO, d) the fact that MPO-Cpd II reduces SCN(-) but not Cl(-), and e) the experimentally observed auto-oxidation/auto-reduction features of the enzyme. Our new mechanism is also transferable to cytochromes, and could well be viable for heme enzymes in general.
哺乳动物血红素酶髓过氧化物酶 (MPO) 催化 Cl(-) 反应生成具有抗菌作用的分子 HOCl。在催化循环中，生成一种活性中间体“复合物 I”(Cpd I)。Cpd I 具有破坏酶的能力。事实上，在没有任何底物的情况下，Cpd I 会以半衰期为 100ms 的速度衰减为称为复合物 II (Cpd II) 的中间产物，通常是 Cpd I 的单电子还原产物。然而，Cpd II 的性质、其光谱特性以及额外电子的来源仅知之甚少。基于密度泛函理论和时间相关 (TD)-DFT 量子化学计算，在 PBE0/6-31G* 水平上，我们提出了一种扩展的机制，该机制涉及一种新的中间产物，使 MPO 能够在催化循环中保护自身免受自氧化或自破坏。由于其电子结构与 Cpd II 相似，我们将这种中间产物命名为 Cpd II'。然而，所提出的机制和我们建议的 Cpd II'功能结构基于这样的假设，即血红素通过与水分子反应引起的电荷分离而被还原，而不是像通常假设的那样通过电子转移。在这项研究中，我们发现了第二种中间产物，即还原酶，新机制同样适用于这种中间产物。与 Cpd II'类似，我们将其命名为 Fe(II')。所提出的新中间产物 Cpd II'和 Fe(II')使得数十年来文献中已有充分记录但迄今仍未得到理解的实验发现能够首次得到解释。这些发现包括：a) Cpd I 的自发衰变；b) 异常的（氯仿样）紫外/可见、圆二色性 (CD) 和共振拉曼光谱；c) 还原 MPO 不能结合 CO 的事实；d) MPO-Cpd II 还原 SCN(-)但不还原 Cl(-)的事实；以及 e) 实验观察到的酶的自动氧化/自动还原特征。我们的新机制也可转移到细胞色素中，并且很可能适用于一般的血红素酶。
