How Does Replacement of the Axial Histidine Ligand in Cytochrome c Peroxidase by N[delta]-Methyl Histidine Affect Its Properties and Functions? A Computational Study

Heme peroxidases have important functions in nature related to the detoxification of [H.sub.2][O.sub.2]. They generally undergo a catalytic cycle where, in the first stage, the iron(III)-heme-[H.sub.2][O.sub.2] complex is converted into an iron(IV)-oxo-heme cation radical species called Compound I. Cytochrome c 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[delta]-methyl histidine-ligated cytochrome c 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[delta] -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 c 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[delta]-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[delta]-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[delta]-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[delta]-methyl histidine-ligated cytochrome c 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. Keywords: density functional theory; enzyme models; epoxidation; hydroxylation; heme enzymes; peroxidases; enzyme engineering