On how the binding cavity of AsqJ dioxygenase controls the desaturation reaction regioselectivity: a QM/MM study

Zuzanna Wojdyla,Tomasz Borowski

doi:10.1007/s00775-018-1575-3

Abstract

The Fe(II)/2-oxoglutarate-dependent dioxygenase AsqJ from Aspergillus nidulans catalyses two pivotal steps in the synthesis of quinolone antibiotic 4′-methoxyviridicatin, i.e., desaturation and epoxidation of a benzodiazepinedione. The previous experimental results signal that, during the desaturation reaction, hydrogen atom transfer (HAT) from the benzylic carbon atom (C10) is a more likely step to initiate the reaction than the alternative HAT from the ring moiety (C3 atom). To unravel the origins of this regioselectivity and to explain why the observed reaction is desaturation and not the “default” hydroxylation, we performed a QM/MM study on the reaction catalysed by AsqJ. Herein, we report results that complement the experimental findings and suggest that HAT at the C10 position is the preferred reaction due to favourable interactions between the substrate and the binding cavity that compensate for the relatively high intrinsic barrier associated with the process. For the resultant radical intermediate, the desaturation/hydroxylation selectivity is governed by electronic properties of the reactants, i.e., the energy gap between the orbital that hosts the unpaired electron and the sigma orbital for the C–H bond as well as the gap between the orbitals mixing in transition state structures for each elementary step.Graphical abstractRegiospecificity of the AsqJ dehydrogenation reaction is dictated by substrate–protein interactions. 82 × 44 mm (300 × 300 dpi)

Highlights

The Fe(II)/2-oxoglutarate-dependent dioxygenases (ODD) are mononuclear enzymes that catalyse a wide variety of reactions, i.e., hydroxylations, desaturations, halogenations, or cyclisations [1, 2]
This result is at odds with the experimental findings, which indicate the opposite regioselectivity of the initial hydrogen atom transfer (HAT) process [8]
The differences could stem from the choice of Fe(IV) coordination geometry that was assumed in the computational study

Summary

Introduction

The Fe(II)/2-oxoglutarate-dependent dioxygenases (ODD) are mononuclear enzymes that catalyse a wide variety of reactions, i.e., hydroxylations, desaturations, halogenations, or cyclisations [1, 2]. Results of a previous QM/MM study on the reaction mechanism of AsqJ suggested that the energetically favourable desaturation pathway involves HAT at the C3 position (with energy barrier of 19.3 kcal mol−1), whereas the alternative HAT from C10 is hindered by a high barrier of 30 kcal mol−1 [9] This result is at odds with the experimental findings, which indicate the opposite regioselectivity of the initial HAT process [8]. In the QM/MM study on the desaturation mechanism, a pentacoordinated complex in trigonal bipyramidal geometry was used In such a model, the oxo ligand was located far from the substrate (trans to His-134) and, the computed reaction energy profiles might not reflect the actual interactions within the AsqJ active site.

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