Computational Approaches Elucidate the Allosteric Mechanism of Human Aromatase Inhibition: A Novel Possible Route to Small-Molecule Regulation of CYP450s Activities?

Human aromatase (HA) is a P450 cytochrome (CYP) with an essential role
in estrogen biosynthesis. Since more than 70\% of breast cancers are
positive for estrogenic receptor (ER), the reduction of estrogen
physiological concentrations through HA inhibition is one of most
important therapeutic strategies against this cancer type. Recently,
experimental evidence showed that selected taxmoxifen metabolites, which
are typically used as estrogen receptor modulators (SERMs), inhibit HA
through an allosteric mechanism. In this work, we present a
computational protocol to (i) characterize the structural framework and
(ii) define the atomistic details of the determinants for the
noncompetitive inhibition mechanism. Our calculations identify two
putative binding sites able to efficiently bind all tamoxifen
metabolites. Analysis of long-scale molecular dynamics simulations
reveal that endoxifen, the most effective noncompetitive inhibitor,
induces significant enzyme rigidity by binding in one of the possible
peripheral sites. The consequence of this binding event is the
suppression of one of the functional enzymatic collective motions
associated with breathing of the substrate access channel. Moreover, an
internal dynamics-based alignment of HA with six other human cytochromes
shows that this collective motion is common to other members of the
CYP450 protein family. On this basis, our findings may thus be of help
for the development of new (pan)inhibitors for the therapeutic treatment
of cancer, targeting and modulating the activity of HA and of estrogen
receptor, and may also stimulate the development of new drug design
strategies for chemoprevention and chemoprotection via allosteric
inhibition of CYP450 proteins