Formation and structure of the ferryl [FeO] intermediate in the non-haem iron halogenase SyrB2: Classical and QM/MM modelling agree

Abstract

To rationalise mechanistically the intriguing regio- and chemoselectivity patterns for different substrates of the non-haem iron/2-oxoglutarate dependent halogenase SyrB2, it is crucial to elucidate the structure of the pivotal [FeIVO] intermediate. We have approached the problem by a combination of classical and QM/MM modelling. We present complete atomistic models of SyrB2 in complex with its native substrate l-threonine as well as l-α-amino butyric acid and l-norvaline (all conjugated to the pantetheine tether), constructed by molecular docking and extensive MD simulations. We evaluate five isomers of the [FeO] intermediate in these simulations, with a view to identifying likely structures based on a simple "reaction distance" measure. Starting from models of the resting state, we then use QM/MM calculations to investigate the formation of the [FeO] species for all three substrates, identifying the intermediates along the O2 activation/decarboxylation pathway on the S = 1, 2, and 3 potential-energy surfaces. We find that, despite differences in the detailed course of the reaction, essentially all pathways produce the same [FeO] structure, in which the oxido is directed away from the substrate.