X-ray Crystal Structure of a Xanthine Oxidase Complex with the Flavonoid Inhibitor Quercetin

Abstract

Xanthine oxidase catalyzes the sequential hydroxylation of hypoxanthine to uric acid via xanthine as intermediate. Deposition of crystals of the catalytic product uric acid or its monosodium salt in human joints with accompanying joint inflammation is the major cause of gout. Natural flavonoids are attractive leads for rational design of preventive and therapeutic xanthine oxidase inhibitors due to their beneficial antioxidant, anti-inflammatory, and antiproliferative activities in addition to their micromolar inhibitory activities toward xanthine oxidase. We determined the first complex X-ray structure of mammalian xanthine oxidase with the natural flavonoid inhibitor quercetin at 2.0 Å resolution. The inhibitor adopts a single orientation with its benzopyran moiety sandwiched between Phe 914 and Phe 1009 and ring B pointing toward the solvent channel leading to the molybdenum active center. The favorable steric complementarity of the conjugated three-ring structure of quercetin with the active site and specific hydrogen-bonding interactions of exocyclic hydroxy groups with catalytically relevant residues Arg 880 and Glu 802 correlate well with a previously reported structure-activity relationship of flavonoid inhibitors of xanthine oxidase. The current complex provides a structural basis for the rational design of flavonoid-type inhibitors against xanthine oxidase useful for the treatment of hyperuricemia, gout, and inflammatory disease states.

Anti-Hyperuricemic, Anti-Inflammatory and Analgesic Effects of Siegesbeckia orientalis L. Resulting from the Fraction with High Phenolic Content

“…Taking into consideration both anti-inflammatory and XO inhibitory effects, we focused on flavonoids and other phenolic compounds which are extensively studied and well-known antioxidants as potential phytochemical agents for treating diseases mediated by free radicals, including inflammation and gout [19, 20]. …”

Inhibition of Human Aldehyde Oxidase Activity by Diet-Derived Constituents: Structural Influence, Enzyme-Ligand Interactions, and Clinical Relevance

“…These interactions are consistent with previous molecular modeling studies involving the AO substrates zoniporide, DACA, and substituted phthalazines (Dastmalchi and Hamzeh-Mivehrod, 2005;Dalvie et al, 2012;Choughule et al, 2013), supporting the hypothesis that the inhibitors are competing with substrates within the active site. A recent report describing the X-ray crystal structure of quercetin bound to the active site of xanthine oxidase, which is highly homologous to AO, further supports this contention (Cao et al, 2014).…”

Structural Analysis of Flavonoid/Drug Target Complexes: Natural Products as Lead Compounds for Drug Development

“…Natural flavonoids with 7-hydroxy groups have been found to bind tightly to xanthine oxidase and are thus current lead compounds for rational drug design as xanthine oxidase inhibitors. The inhibitory potency of the flavonoids towards xanthine oxidase is in the micromolar range [25]. In the crystal structure of quercetin and xanthine oxidase, the inhibitor benzopyran group is between Phe914 and Phe1009, and the B ring is pointed toward the solvent channel leading to the molybdenum binding site (active site).…”

“…As seen in several studies, the hydrophobic interactions of the conjugated 3 ring structure of quercetin and the active site residues, along with specific hydrogen bonding interactions of hydroxyl groups outside the ring of quercetin (7-hydroxy groups), which interact with Arg880 and Glu802 of the enzyme, promote tight binding. The tight binding of quercetinin, the xanthine oxidase enzyme crystal structure, provides structural evidence that flavonoid-type inhibitors of this enzyme are useful for the treatment of gout and other inflammatory disease states [25]. In Figure 7A bovine xanthine oxidase is shown complexed with Flavin-Adenine Dinucleotide (FAD) in PDB code 3NVY, and Figure 7B shows quercerin (QUE) and MTE (See Table 1 for IUPAC name).…”