NADH quinone oxidoreductase (NQO1) with inhibitor dicoumarol

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The crystal structure of NADH quinone oxidoreductase (NQO1) in complex with its potent inhibitor dicoumarol

NAD(P)H quinone oxidoreductase 1 (NQO1) is a ubiquitous flavoenzyme that catalyzes two electron reduction of quinones to hydroquinones utilizing NAD(P)H as an electron donor. NQO1 is a homo-dimer that functions via a “ping pong” mechanism. NAD(P)H binds to NQO1, reduces the FAD co-factor and is then released, allowing the quinone substrate to bind the enzyme and to be reduced. The NAD(P)H and the quinone binding sites of NQO1 have a significant overlap, thus providing a molecular basis for this “ping pong” mechanism.

Certain coumarins, flavones and the reactive dye cibacron blue are competitive inhibitors of NQO1 activity, which compete with NAD(P)H for binding to NQO1. Dicoumarol (3-3’–methylene-bis (4-hydroxycoumarin)), is the most potent competitive inhibitor of NQO1. Dicoumarol competes with NAD(P)H for binding to NQO1 and prevents the electron transfer to FAD.

In addition to its role in the detoxification of quinones, NQO1 is also a 20S proteasome-associated protein that plays an important role in the stability of the tumor suppressor p53 and several other short-lived proteins including p73α and ornithine decarboxylase (ODC). NQO1 binds and stabilizes p53, protecting p53 from ubiquitin-independent 20S proteasomal degradation. Dicoumarol and several other inhibitors of NQO1 activity, which compete with NADH for binding to NQO1, disrupt the binding of NQO1 to p53 and induce ubiquitin-independent p53 degradation.

NQO1 binds and stabilizes several short-lived proteins including the tumor suppressors p53 and p73 and the enzyme ornithine decarboxylase (ODC). Dicoumarol is a widely used potent competitive inhibitor of NQO1 enzymatic activity, which competes with NAD(P)H for binding to NQO1. Dicoumarol also disrupts the binding of NQO1 to p53, p73 and ODC and induces their ubiquitin-independent proteasomal degradation. The crystal structure of human NQO1 in complex with dicoumarol was determine at 2.75 Å resolution. The different residues of NQO1 interacting with dicoumarol with and the conformational changes imposed upon dicoumarol binding. The most prominent conformational changes that occur in the presence of dicoumarol involve Tyr 128 and Phe 232 that are present on the surface of the NQO1 catalytic pocket. Based on the comparison of NQO1 structure in complex with different NQO1 inhibitors and our previous analysis of NQO1 mutations that affect NQO1 interactions we propose that the specific conformation of Tyr 128 and Phe 232 is important for NQO1 interaction with p53 and other client proteins.

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