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Update of the NAD(P)H:quinone oxidoreductase (NQO) gene family.

ABSTRACT: The NAD(P)H:quinone acceptor oxidoreductase (NQO) gene family belongs to the flavoprotein clan and, in the human genome, consists of two genes (NQO1 and NQO2). These two genes encode cytosolic flavoenzymes that catalyse the beneficial two-electron reduction of quinones to hydroquinones. This reaction prevents the unwanted one-electron reduction of quinones by other quinone reductases; one-electron reduction results in the formation of reactive oxygen species, generated by redox cycling of semiquinones in the presence of molecular oxygen. Both the mammalian NQO1 and NQO2 genes are upregulated as a part of the oxidative stress response and are inexplicably overexpressed in particular types of tumours. A non-synonymous mutation in the NQO1 gene, leading to absence of enzyme activity, has been associated with an increased risk of myeloid leukaemia and other types of blood dyscrasia in workers exposed to benzene. NQO2 has a melatonin-binding site, which may explain the anti-oxidant role of melatonin. An ancient NQO3 subfamily exists in eubacteria and the authors suggest that there should be additional divisions of the NQO family to include the NQO4 subfamily in fungi and NQO5 subfamily in archaebacteria. Interestingly, no NQO genes could be identified in the worm, fly, sea squirt or plants; because these taxa carry quinone reductases capable of one- and two-electron reductions, there has been either convergent evolution or redundancy to account for the appearance of these enzyme functions whenever they have been needed during evolution.

SUBMITTER: Vasiliou V

PROVIDER: S-EPMC3500182 | biostudies-literature | 2006 Mar

REPOSITORIES: biostudies-literature

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Update of the NAD(P)H:quinone oxidoreductase (NQO) gene family.

Vasiliou Vasilis V Ross David D Nebert Daniel W DW

Human genomics 20060301 5

The NAD(P)H:quinone acceptor oxidoreductase (NQO) gene family belongs to the flavoprotein clan and, in the human genome, consists of two genes (NQO1 and NQO2). These two genes encode cytosolic flavoenzymes that catalyse the beneficial two-electron reduction of quinones to hydroquinones. This reaction prevents the unwanted one-electron reduction of quinones by other quinone reductases; one-electron reduction results in the formation of reactive oxygen species, generated by redox cycling of semiqu ...[more]

PMID: 16595077

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Project description:NAD(P)H:quinone oxidoreductase type I (NQO1) is a target enzyme for triggered delivery of drugs at inflamed tissue and tumor sites, particularly those that challenge traditional therapies. Prodrugs, macromolecules, and molecular assemblies possessing trigger groups that can be cleaved by environmental stimuli are vehicles with the potential to yield active drug only at prescribed sites. Furthermore, quinone propionic acids (QPAs) covalently attached to prodrugs or liposome surfaces can be removed by application of a reductive trigger stimulus, such as that from NQO1; their rates of reductive activation should be tunable via QPA structure. We explored in detail the recombinant human NAD(P)H:quinone oxidoreductase type I (rhNQO1)-catalyzed NADH reduction of a family of substituted QPAs and obtained high precision kinetic parameters. It is found that small changes in QPA structure-in particular, single atom and function group substitutions on the quinone ring at R(1)-lead to significant impacts on the Michaelis constant (K(m)), maximum velocity (V(max)), catalytic constant (k(cat)), and catalytic efficiency (k(cat)/K(m)). Molecular docking simulations demonstrate that alterations in QPA structure result in large changes in QPA alignment and placement with respect to the flavin isoalloxazine ring in the active site of rhNQO1; a qualitative relationship exists between the kinetic parameters and the depth of QPA penetration into the rhNQO1 active site. From a quantitative perspective, a very good correlation is observed between log(k(cat)/K(m)) and the molecular-docking-derived distance between the flavin hydride donor site and quinone hydride acceptor site in the QPAs, an observation that is in agreement with developing theories. The comprehensive kinetic and molecular modeling knowledge obtained for the interaction of recombinant human NQO1 with the quinone propionic acid analogues provides insight into the design and implementation of the QPA trigger groups for drug delivery applications.

Project description:The E. coli protein WrbA is an FMN-dependent NAD(P)H:quinone oxidoreductase that has been implicated in oxidative defense. Three subunits of the tetrameric enzyme contribute to each of four identical, cavernous active sites that appear to accommodate NAD(P)H or various quinones, but not simultaneously, suggesting an obligate tetramer with a ping-pong mechanism in which NAD departs before oxidized quinone binds. The present work was undertaken to evaluate these suggestions and to characterize the kinetic behavior of WrbA. Steady-state kinetics results reveal that WrbA conforms to a ping-pong mechanism with respect to the constancy of the apparent Vmax to Km ratio with substrate concentration. However, the competitive/non-competitive patterns of product inhibition, though consistent with the general class of bi-substrate reactions, do not exclude a minor contribution from additional forms of the enzyme. NMR results support the presence of additional enzyme forms. Docking and energy calculations find that electron-transfer-competent binding sites for NADH and benzoquinone present severe steric overlap, consistent with the ping-pong mechanism. Unexpectedly, plots of initial velocity as a function of either NADH or benzoquinone concentration present one or two Michaelis-Menten phases depending on the temperature at which the enzyme is held prior to assay. The effect of temperature is reversible, suggesting an intramolecular conformational process. WrbA shares these and other details of its kinetic behavior with mammalian DT-diaphorase, an FAD-dependent NAD(P)H:quinone oxidoreductase. An extensive literature review reveals several other enzymes with two-plateau kinetic plots, but in no case has a molecular explanation been elucidated. Preliminary sedimentation velocity analysis of WrbA indicates a large shift in size of the multimer with temperature, suggesting that subunit assembly coupled to substrate binding may underlie the two-plateau behavior. An additional aim of this report is to bring under wider attention the apparently widespread phenomenon of two-plateau Michaelis-Menten plots.

Dataset Information

Update of the NAD(P)H:quinone oxidoreductase (NQO) gene family.

Publications

Update of the NAD(P)H:quinone oxidoreductase (NQO) gene family.

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