Manchester Institute of Biotechnology, The University of Manchester, 131 Princess Street, Manchester M1 7DN, UK.
Department of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester M13 9PL, UK.
Int J Mol Sci. 2020 Sep 27;21(19):7133. doi: 10.3390/ijms21197133.
Heme peroxidases have important functions in nature related to the detoxification of HO. They generally undergo a catalytic cycle where, in the first stage, the iron(III)-heme-HO complex is converted into an iron(IV)-oxo-heme cation radical species called Compound I. Cytochrome peroxidase Compound I has a unique electronic configuration among heme enzymes where a metal-based biradical is coupled to a protein radical on a nearby Trp residue. Recent work using the engineered N-methyl histidine-ligated cytochrome peroxidase highlighted changes in spectroscopic and catalytic properties upon axial ligand substitution. To understand the axial ligand effect on structure and reactivity of peroxidases and their axially N-methyl histidine engineered forms, we did a computational study. We created active site cluster models of various sizes as mimics of horseradish peroxidase and cytochrome peroxidase Compound I. Subsequently, we performed density functional theory studies on the structure and reactivity of these complexes with a model substrate (styrene). Thus, the work shows that the N-methyl histidine group has little effect on the electronic configuration and structure of Compound I and little changes in bond lengths and the same orbital occupation is obtained. However, the N-methyl histidine modification impacts electron transfer processes due to a change in the reduction potential and thereby influences reactivity patterns for oxygen atom transfer. As such, the substitution of the axial histidine by N-methyl histidine in peroxidases slows down oxygen atom transfer to substrates and makes Compound I a weaker oxidant. These studies are in line with experimental work on N-methyl histidine-ligated cytochrome peroxidases and highlight how the hydrogen bonding network in the second coordination sphere has a major impact on the function and properties of the enzyme.
血红素过氧化物酶在与 HO 解毒相关的自然界中具有重要功能。它们通常经历一个催化循环，在第一阶段，铁(III)-血红素-HO 配合物转化为称为化合物 I 的铁(IV)-氧合血红素阳离子自由基物种。细胞色素 c 过氧化物酶化合物 I 在血红素酶中具有独特的电子构型，其中金属基双自由基与附近 Trp 残基上的蛋白质自由基偶联。最近使用工程化的 N-甲基组氨酸配位的细胞色素 c 过氧化物酶的工作强调了轴向配体取代对光谱和催化性质的影响。为了了解轴向配体对过氧化物酶及其轴向 N-甲基组氨酸工程化形式的结构和反应性的影响，我们进行了计算研究。我们创建了各种大小的活性位点簇模型，作为辣根过氧化物酶和细胞色素 c 过氧化物酶化合物 I 的模拟物。随后，我们使用模型底物（苯乙烯）对这些复合物的结构和反应性进行了密度泛函理论研究。因此，这项工作表明，N-甲基组氨酸基团对化合物 I 的电子构型和结构几乎没有影响，并且键长几乎没有变化，并且获得了相同的轨道占据。然而，N-甲基组氨酸修饰由于还原电位的变化而影响电子转移过程，从而影响氧原子转移的反应性模式。因此，过氧化物酶中轴向组氨酸被 N-甲基组氨酸取代会减缓氧原子向底物的转移，使化合物 I 成为较弱的氧化剂。这些研究与 N-甲基组氨酸配位的细胞色素 c 过氧化物酶的实验工作一致，并强调了第二配位球中的氢键网络如何对酶的功能和性质产生重大影响。
