Characterization of cofactors, substrates and inhibitor binding to flavoenzyme quinone reductase 2 by automated supramolecular nano-electrospray ionization mass spectrometry

Abstract

Quinone reductase 2 (QR2) is a cytosolic homodimeric enzyme implicated in the reduction of quinone in the presence of natural derivatives of NADH such as N-ribosyldihydronicotinamide. QR2 does not recognize NADH or NADPH as co-substrates, unlike quinone reductase 1 (QR1). This feature is not the only unusual one of this enzyme. Although it resembles quinone reductase 1, the well-described detoxifying enzyme, QR2 does not share many features with QR1. Particularly, it does not seem to have a similar detoxifying function in cells. Therefore, starting from basic knowledge on QR2 and adding up to previous works published on the enzyme, we wanted to rebuild its biochemical description because some of the recently described characteristics are surprising, and merit further explorations. For example, QR2 seems to be over-expressed in neurodegenerative diseases, and this over-expression seems to be linked to a worsening of the pathological conditions. Indeed, our specific inhibitors of QR2, tested in vivo, show outstanding properties impairing memory loss. These observations led us to further describe, at the molecular level, the relationship between QR2 and some of its inhibitors and co-substrates. In the present paper, we address this question using non-denaturing supramolecular nano-electrospray ionization mass spectrometry. This characterization helps understand the physical relationship between inhibitors such as resveratrol or melatonin and the enzyme.