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

Abstract

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 the sexual and reproductive development. However, these also are involved in the development and growth of hormonedependent 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 the 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.3

Regio- and stereoselectivity in the CYP450BM3-catalyzed hydroxylation of complex terpenoids: a QM/MM study

“…The relative free energy profile shows that the barrier associated with the rebound step is lower than that of the hydrogen abstraction step, indicating the hydrogen abstraction is the rate limiting step. This is in agreement with previous literature, [43][44][45][46] so the later discussion is focused on the hydrogen abstraction, the rate limiting step of the P450catalyzed hydroxylation.…”

“…The hydrogen abstraction has been shown to be rate-limiting step of the P450-catalyzed hydroxylation reaction. [42][43][44][45][46] The rebound step calculations were run on two representative clusters C1 and C2 (Figure S18). The relative free energy profile shows that the barrier associated with the rebound step is lower than that of the hydrogen abstraction step, indicating the hydrogen abstraction is the rate limiting step.…”

The effect of third-generation aromatase inhibitors on aromatase avtivity in visceral adipose tissue

“…Незважаючи на досить інваріативний механізм дії, інгібітори ароматази третього покоління відзначаються значущою клінічною ефективністю та успішно застосовуються в ад'ювантній терапії естроген-рецептор позитивного раку молочної залози (здебільшого у пацієнток постменопаузного віку), навіть у випадках відсутності відповіді на лікування антиестрогенними препаратами першої лінії терапії. Антиароматазні препарати третього покоління характеризуються високою селективністю до відповідного ензиму та володіють задовільним профілем безпеки (Mantas et al, 2016;Viciano & Martí, 2016;Mojaddami et al, 2017).…”

In vitro metabolism of exemestane by hepatic cytochrome P450s: impact of nonsynonymous polymorphisms on formation of the active metabolite 17β‐dihydroexemestane

“…At present, a lack of consensus exists regarding the exact mechanism by which EXE irreversibly inhibits the enzyme (Brueggemeier ; Lombardi ; Ghosh et al. , ; Viciano and Marti ). However, its efficacy in suppressing aromatase‐mediated synthesis of estrogens from adrenal‐supplied androgens (aromatization) is well established.…”

“…EXE is a third-generation steroidal aromatase inhibitor used in the treatment and prevention of breast cancer in postmenopausal women (Geisler et al 1998;Goss et al 2011). At present, a lack of consensus exists regarding the exact mechanism by which EXE irreversibly inhibits the enzyme (Brueggemeier 2002;Lombardi 2002;Ghosh et al 2009Ghosh et al , 2012Viciano and Marti 2016). However, its efficacy in suppressing aromatase-mediated synthesis of estrogens from adrenal-supplied androgens (aromatization) is well established.…”