Mammalian xanthine oxidoreductases, which catalyze the last two steps in the formation of urate, are synthesized as the dehydrogenase form xanthine dehydrogenase (XDH) but can be readily converted to the oxidase form xanthine oxidase (XO) by oxidation of sulfhydryl residues or by proteolysis. Here, we present the crystal structure of the dimeric (Mr, 290,000) bovine milk XDH at 2.1-Å resolution and XO at 2.5-Å resolution and describe the major changes that occur on the proteolytic transformation of XDH to the XO form. Each molecule is composed of an N-terminal 20-kDa domain containing two iron sulfur centers, a central 40-kDa flavin adenine dinucleotide domain, and a C-terminal 85-kDa molybdopterin-binding domain with the four redox centers aligned in an almost linear fashion. Cleavage of surface-exposed loops of XDH causes major structural rearrangement of another loop close to the flavin ring (Gln 423OLys 433). This movement partially blocks access of the NAD substrate to the flavin adenine dinucleotide cofactor and changes the electrostatic environment of the active site, reflecting the switch of substrate specificity observed for the two forms of this enzyme. Milk xanthine oxidase is an archetypal enzyme, which was originally described as aldehyde oxidase in 1902 (1) and has since served as a benchmark for the whole class of complex metalloflavoproteins (2). Xanthine oxidoreductase enzymes have been isolated from a wide range of organisms, from bacteria to man, and catalyze the hydroxylation of a wide variety of purine, pyrimidine, pterin, and aldehyde substrates. All of these proteins have similar molecular weights and composition of redox centers (3, 4). The mammalian enzymes, which catalyze the hydroxylation of hypoxanthine and xanthine, the last two steps in the formation of urate, are synthesized as the dehydrogenase form xanthine dehydrogenase (XDH) and exist mostly as such in the cell but can be readily converted to the oxidase form xanthine oxidase (XO) by oxidation of sulfhydryl residues or by proteolysis. XDH shows a preference for NAD ϩ reduction at the flavin adenine dinucleotide (FAD) reaction site, whereas XO fails to react with NAD ϩ and exclusively uses dioxygen as its substrate, leading to the formation of superoxide anion and hydrogen peroxide (3). The enzyme is a target of drugs against gout and hyperuricemia (5), and the conversion of XDH to XO is of major interest as it has been implicated in diseases characterized by oxygen-radical-induced tissue damage, such as postischemic reperfusion injury (6). Recent work suggests that XO also might be associated with blood pressure regulation (7).The active form of the enzyme is that of a homodimer of molecular mass 290 kDa, with each of the monomers acting independently in catalysis. Each subunit contains one molybdopterin cofactor, two spectroscopically distinct [2Fe-2S] centers, and one FAD cofactor. The oxidation of xanthine takes place at the molybdopterin center (Mo-pt) and the electrons thus introduced are rapidly distributed to the other c...

Xanthine oxidase and xanthine dehydrogenase are enzymes involved in the metabolism of purines and pyrimidines in various organisms. Their relationship to one another has been the subject of considerable debate, primarily because of their proposed roles in ischemia/reperfusion damage in tissues. Differences in the kinetics and oxidation-reduction behavior of the two forms are accounted for by the presence in the dehydrogenase of a binding site for NAD+, as well as a substantially lower reduction potential for the flavin FADH./FADH2 couple of the dehydrogenase relative to the oxidase. This review presents recent advances of our understanding of the biochemistry and molecular biology of these systems, including a model for the overall morphology of xanthine oxidizing enzymes. The evidence that the two enzymes represent alternate forms of the same gene product, in some cases reversibly interconvertible between one another, is discussed.

Molybdenum is widely distributed in biology and is usually found as a mononuclear metal center in the active sites of many enzymes catalyzing oxygen atom transfer. The molybdenum hydroxylases are distinct from other biological systems catalyzing hydroxylation reactions in that the oxygen atom incorporated into the product is derived from water rather than molecular oxygen. Here, we present the crystal structure of the key intermediate in the hydroxylation reaction of xanthine oxidoreductase with a slow substrate, in which the carbon-oxygen bond of the product is formed, yet the product remains complexed to the molybdenum. This intermediate displays a stable broad charge-transfer band at Ϸ640 nm. The crystal structure of the complex indicates that the catalytically labile MoOOH oxygen has formed a bond with a carbon atom of the substrate. In addition, the MoAS group of the oxidized enzyme has become protonated to afford MoOSH on reduction of the molybdenum center. In contrast to previous assignments, we find this last ligand at an equatorial position in the square-pyramidal metal coordination sphere, not the apical position. A water molecule usually seen in the active site of the enzyme is absent in the present structure, which probably accounts for the stability of this intermediate toward ligand displacement by hydroxide.

Reactive oxygen species are generated by various biological systems, including NADPH oxidases, xanthine oxidoreductase, and mitochondrial respiratory enzymes, and contribute to many physiological and pathological phenomena. Mammalian xanthine dehydrogenase (XDH) can be converted to xanthine oxidase (XO), which produces both superoxide anion and hydrogen peroxide. Recent X‐ray crystallographic and site‐directed mutagenesis studies have revealed a highly sophisticated mechanism of conversion from XDH to XO, suggesting that the conversion is not a simple artefact, but rather has a function in mammalian organisms. Furthermore, this transition seems to involve a thermodynamic equilibrium between XDH and XO; disulfide bond formation or proteolysis can then lock the enzyme in the XO form. In this review, we focus on recent advances in our understanding of the mechanism of conversion from XDH to XO.

Heme-binding protein 23 kDa (HBP23), a rat isoform of human proliferation-associated gene product (PAG), is a member of the peroxiredoxin family of peroxidases, having two conserved cysteine residues. Recent biochemical studies have shown that HBP23/ PAG is an oxidative stress-induced and proliferation-coupled multifunctional protein that exhibits specific bindings to c-Abl protein tyrosine kinase and heme, as well as a peroxidase activity. A 2.6-Å resolution crystal structure of rat HBP23 in oxidized form revealed an unusual dimer structure in which the active residue Cys-52 forms a disulfide bond with conserved Cys-173 from another subunit by C-terminal tail swapping. The active site is largely hydrophobic with partially exposed Cys-173, suggesting a reduction mechanism of oxidized HBP23 by thioredoxin. Thus, the unusual cysteine disulfide bond is involved in peroxidation catalysis by using thioredoxin as the source of reducing equivalents. The structure also provides a clue to possible interaction surfaces for c-Abl and heme. Several significant structural differences have been found from a 1-Cys peroxiredoxin, ORF6, which lacks the C-terminal conserved cysteine corresponding to Cys-173 of HBP23.