What Factors Influence the Rate Constant of Substrate Epoxidation by Compound I of Cytochrome P450 and Analogous Iron(IV)-Oxo Oxidants?

Abstract

The cytochromes P450 are a versatile range of mono-oxygenase enzymes that catalyze a variety of different chemical reactions, of which the key reactions include aliphatic hydroxylation and C=C double bond epoxidation. To establish the fundamental factors that govern substrate epoxidation by these enzymes we have done a systematic density functional theory study on substrate epoxidation by the active species of P450 enzymes, namely the iron(IV)-oxo porphyrin cation radical oxidant or Compound I. We show here, for the first time, that the rate constant of substrate epoxidation, and hence the activation energy, correlates with the ionization potential of the substrate as well as with intrinsic electronic properties of the active oxidant such as the polarizability volume. To explain these findings we present an electron-transfer model for the reaction mechanism that explains the factors that determine the barrier heights and developed a valence bond (VB) curve crossing mechanism to rationalize the observed trends. In addition, we have found a correlation for substrate epoxidation reactions catalyzed by a range of heme and nonheme iron(IV)-oxo oxidants with the strength of the O-H bond in the iron-hydroxo complex, i.e. BDE(OH), which is supported by the VB model. Finally, the fundamental factors that determine the regioselectivity change between substrate hydroxylation and epoxidation are discussed. It is shown that the regioselectivity of aliphatic hydroxylation versus double bond epoxidation is not influenced by the choice of the oxidant but is purely substrate dependent.

Role of Substrate Positioning in the Catalytic Reaction of 4-Hydroxyphenylpyruvate Dioxygenase—A QM/MM Study

“…Consistent with the results of the previous DFT studies, 8,49,50 In SOSP1 the Fe-O1 bond is elongated to 2.01 from 1.67Å, whereas the O1-C1 and O1-C2 distances are rather short, i.e. 2.02 and 1.85Å, respectively ( (Fig.…”

Synthesis of 1‐Naphthol by a Natural Peroxygenase Engineered by Directed Evolution

“…Naphthalene ionization energies were computed for both PaDa-I and JaWa variants. Results show that the increase in the substrate's positive electrostatic environment increases naphthalene's ionization energy (from 213.55 kcal/mol in PaDa-I to 214.57 kcal/mol in JaWa), leading to a higher activation barrier, as shown in previous studies [30,31] and in agreement with JaWa lower kcat observed experimentally. On the other hand, The R257K mutation was not expected to affect the hydroxylation of naphthalene but further PELE simulations with DMP show an additional access to the active heme site, close to this residue (not present in naphthalene), which could affect both the Km and kcat.…”

A Systematic Account on Aromatic Hydroxylation by a Cytochrome P450 Model Compound I: A Low‐Pressure Mass Spectrometry and Computational Study

“…These diagrams have been used previously to rationalize barrier heights of chemical reactions and explain what features of substrate and oxidant affect the value of the barrier height. [21,27] The diagram starts on the bottom-left with the reactant configuration of [Fe IV (O)(Por +• )] + and arene. Thus, in the ground state the system has three unpaired electrons distributed over the *xz, *yz and a1u molecular orbitals.…”

“…Clearly, the axial cysteinate ligand in P450 enzymes affects the reactivity properties of the oxo group and, thereby makes it a better oxidant. Electrophilic reactions, such as aromatic hydroxylation and epoxidation, however, tend to connect with the ionization energy of the substrate and the electron affinity of the oxidant, [21] which is appropriate for [Fe IV (O)(Por +• )] + to perform these reactions.…”