Coupled electron transfer and proton hopping in the final step of CYP19-catalyzed androgen aromatization.

Aromatase (CYP19) catalyzes the terminal step in estrogen biosynthesis, which requires three separate oxidation reactions, culminating in an enigmatic aromatization that converts an androgen to an estrogen. A stable ferric peroxo (Fe(3+)O(2)(2-)) intermediate is seen by electron paramagnetic resonance, but its role in this complex reaction remains controversial. Combining molecular dynamics simulation and hybrid quantum mechanics/molecular mechanics, we show that ferric peroxo addition to the 19-aldehyde initiates the reaction. Stepwise cleavage of the C10-C19 and O-O bonds of the peroxohemiacetal extrudes formate and yields Compound II, which in turn desaturates the steroid through successive abstraction of the 1β-hydrogen atom and deprotonation of the 2β-position. Throughout the transformation, a proton is cyclically relayed between D309 and the substrate to stabilize reaction intermediates. This mechanism invokes novel oxygen intermediates and provides a unifying interpretation of past experimental mechanistic studies.