Theoretical Study of the Mechanism of Exemestane Hydroxylation Catalyzed by Human Aromatase Enzyme

Human aromatase (CYP19A1) aromatizes the androgens to form estrogens via a three-step oxidative process. The estrogens are necessary in humans, mainly in women, because of the role they play in sexual and reproductive development. However, these also are involved in the development and growth of hormone-dependent breast cancer. Therefore, inhibition of the enzyme aromatase, by means of drugs known as aromatase inhibitors, is the frontline therapy for these types of cancers. Exemestane is a suicidal third-generation inhibitor of aromatase, currently used in breast cancer treatment. In this study, the hydroxylation of exemestane catalyzed by aromatase has been studied by means of hybrid QM/MM methods. The Free Energy Perturbation calculations provided a free energy of activation for the hydrogen abstraction step (rate-limiting step) of 17 kcal/ mol. The results reveal that the hydroxylation of exemestane is not the inhibition stage, suggesting a possible competitive mechanism between the inhibitor and the natural substrate androstenedione in the first catalytic subcycle of the enzyme. Furthermore, the analysis of the interaction energy for the substrate and the cofactor in the active site shows that the role of the enzymatic environment during this reaction consists of a transition state stabilization by means of electrostatic effects

A theoretical study on the mechanism of the oxidation of substrates by human aromatase enzyme (CYP19A1)

The enzyme Cytochrome P450 aromatase plays an essential role in the biosynthesis of estrogens, and its inhibition is an important target for the development of drugs for the treatment of breast cancer. The main purpose of the present thesis is to improve the understanding of the catalytic mechanism and the biochemistry of this enzyme from the standpoint of theoretical chemistry. The results of this thesis have been divided into three main sections: (1) Study of the reactive species of the enzyme aromatase: Compound I; (2) Study of the hydroxylation of the natural substrate androstenedione, during the first catalytic subcycle of the enzyme aromatase; and (3) Study of the hydroxylation of Exemestane, an esteroidal third generation aromatase inhibitor, currently used in hormone dependent breast cancer therapy.La enzima citocromo P450 aromatasa juega un papel esencial en la biosíntesis de estrógenos, y su inhibición es un objetivo importante para el desarrollo de medicamentos para el tratamiento del cáncer de mama. El objetivo principal de la esta Tesis ha sido arrojar luz sobre el mecanismo catalítico y sobre la bioquímica de esta enzima, desde el punto de vista de la química teórica. Los resultados que se presentan en esta Tesis se han dividido en tres secciones principales: (1) Estudio de la especie reactiva de la enzima aromatasa: "Compound I"; (2) Estudio de la hidroxilación del substrato natural androstenediona, a lo largo del primer subciclo catalítico de esta enzima; y (3) Estudio de la hidroxilación del Exemestano, un inhibidor esteroideo de tercera generación de la enzima aromatasa, que se utiliza actualmente en el tratamiento del cáncer de mama hormonodependiente

New insight into the electronic structure of iron(IV)-oxo porphyrin compound I. A quantum chemical topological analysis

The electronic structure of iron-oxo porphyrin π-cation radical complex Por·+FeIV[DOUBLE BOND]O (S[BOND]H) has been studied for doublet and quartet electronic states by means of two methods of the quantum chemical topology analysis: electron localization function (ELF) η(r) and electron density ρ(r). The formation of this complex leads to essential perturbation of the topological structure of the carbon–carbon bonds in porphyrin moiety. The double C[DOUBLE BOND]C bonds in the pyrrole anion subunits, represented by pair of bonding disynaptic basins Vi=1,2(C,C) in isolated porphyrin, are replaced by single attractor V(C,C)i=1–20 after complexation with the Fe cation. The iron–nitrogen bonds are covalent dative bonds, N→Fe, described by the disynaptic bonding basins V(Fe,N)i=1–4, where electron density is almost formed by the lone pairs of the N atoms. The nature of the iron–oxygen bond predicted by the ELF topological analysis, shows a main contribution of the electrostatic interaction, Feδ+···Oδ−, as long as no attractors between the C(Fe) and C(O) core basins were found, although there are common surfaces between the iron and oxygen basines and coupling between iron and oxygen lone pairs, that could be interpreted as a charge-shift bond. The Fe[BOND]S bond, characterized by the disynaptic bonding basin V(Fe,S), is partially a dative bond with the lone pair donated from sulfur atom. The change of electronic state from the doublet (M = 2) to quartet (M = 4) leads to reorganization of spin polarization, which is observed only for the porphyrin skeleton (−0.43e to 0.50e) and S[BOND]H bond (−0.55e to 0.52e)