Charge transfer complexes between pteridine substrates and the active center molybdenum of xanthine oxidase

Xanthine Oxidase-A Personal History

A personal perspective is provided regarding the work in several laboratories, including the author's, that has established the reaction mechanism of xanthine oxidase and related enzymes.

Resonance Raman spectroscopy of pyranopterin molybdenum enzymes

Resonance Raman spectroscopy (rR) is a powerful spectroscopic probe that is widely used for studying the geometric and electronic structure of metalloproteins. In this focused review, we detail how resonance Raman spectroscopy has contributed to a greater understanding of electronic structure, geometric structure, and the reaction mechanisms of pyranopterin molybdenum enzymes. The review focuses on the enzymes sulfite oxidase (SO), dimethyl sulfoxide reductase (DMSOR), xanthine oxidase (XO), and carbon monoxide dehydrogenase. Specifically, we highlight how Mo-Ooxo, Mo-Ssulfido, Mo-Sdithiolene, and dithiolene CC vibrational modes, isotope and heavy atom perturbations, resonance enhancement, and associated Raman studies of small molecule analogs have provided detailed insight into the nature of these metalloenzyme active sites.

Spectroscopic Studies of Mononuclear Molybdenum Enzyme Centers

A concise review is provided of the contributions that various spectroscopic methods have made to our understanding of the physical and electronic structures of mononuclear molybdenum enzymes. Contributions to our understanding of the structure and function of each of the major families of these enzymes is considered, providing a perspective on how spectroscopy has impacted the field.

Electron transfer through arsenite oxidase: Insights into Rieske interaction with cytochrome c

Arsenic is a widely distributed environmental toxin whose presence in drinking water poses a threat to >140 million people worldwide. The respiratory enzyme arsenite oxidase from various bacteria catalyses the oxidation of arsenite to arsenate and is being developed as a biosensor for arsenite. The arsenite oxidase from Rhizobium sp. str. NT-26 (a member of the Alphaproteobacteria) is a heterotetramer consisting of a large catalytic subunit (AioA), which contains a molybdenum centre and a 3Fe-4S cluster, and a small subunit (AioB) containing a Rieske 2Fe-2S cluster. Stopped-flow spectroscopy and isothermal titration calorimetry (ITC) have been used to better understand electron transfer through the redox-active centres of the enzyme, which is essential for biosensor development. Results show that oxidation of arsenite at the active site is extremely fast with a rate of >4000s-1 and reduction of the electron acceptor is rate-limiting. An AioB-F108A mutation results in increased activity with the artificial electron acceptor DCPIP and decreased activity with cytochrome c, which in the latter as demonstrated by ITC is not due to an effect on the protein-protein interaction but instead to an effect on electron transfer. These results provide further support that the AioB F108 is important in electron transfer between the Rieske subunit and cytochrome c and its absence in the arsenite oxidases from the Betaproteobacteria may explain the inability of these enzymes to use this electron acceptor.

Molybdenum-containing nitrite reductases: Spectroscopic characterization and redox mechanism

OBJECTIVES

This review summarizes the spectroscopic results, which will provide useful suggestions for future research. In addition, the fields that urgently need more information are also advised.

BACKGROUND

Nitrite-NO-cGMP has been considered as an important signaling pathway of NO in human cells. To date, all the four known human molybdenum-containing enzymes, xanthine oxidase, aldehyde oxidase, sulfite oxidase, and mitochondrial amidoxime-reducing component, have been shown to function as nitrite reductases under hypoxia by biochemical, cellular, or animal studies. Various spectroscopic techniques have been applied to investigate the structure and catalytic mechanism of these enzymes for more than 20 years.

METHODS

We summarize the published data on the applications of UV-vis and EPR spectroscopies, and X-ray crystallography in studying nitrite reductase activity of the four human molybdenum-containing enzymes.

RESULTS

UV-vis has provided useful information on the redox active centers of these enzymes. The utilization of EPR spectroscopy has been critical in determining the coordination and redox status of the Mo center during catalysis. Despite the lack of substrate-bound crystal structures of these nitrite reductases, valuable structural information has been obtained by X-ray crystallography.

CONCLUSIONS

To fully understand the catalytic mechanisms of these physiologically/pathologically important nitrite reductases, structural studies on substrate-redox center interaction are needed.

Molybdenum-containing hydroxylases

Unlike monooxygenases, molybdenum-containing hydroxylases catalyze the hydroxylation of carbon centers using oxygen derived ultimately from water, rather than O(2), as the source of the oxygen atom incorporated into the product, and do not require an external source of reducing equivalents. The mechanism by which this interesting chemistry takes place has been the subject of investigation for some time, and in the last several years the chemical course of the reaction has become increasingly well understood. The present minireview summarizes recent mechanistic and structure/function studies of members of this large and growing family of enzymes.

Superoxide anion scavenging and xanthine oxidase inhibition of (+)-catechin-aldehyde polycondensates. Amplification of the antioxidant property of (+)-catechin by polycondensation with aldehydes

In this study, the antioxidant property of (+)-catechin-aldehyde polycondensates has been examined. Superoxide anions are one of the most typical reactive oxygen species (ROS) and generated by xanthine oxidase (XO). The measurements of the superoxide anion scavenging and XO inhibition activity showed that catechin had pro-oxidant properties in lower concentrations and little XO inhibition. On the other hand, the polycondensates exhibited much higher effects compared to the catechin monomer, and their physiological activities were greatly affected by the structure of polycondensates. Steady-state analysis of the inhibition against XO showed that the inhibition type of the polycondensate was uncompetitive. Furthermore, the results of the circular dichroism and UV-visible measurements of a mixture of the polycondensate and XO were in good agreement with that of the steady-state analysis; the spectral changes due to the chelation of the polycondensate onto the Fe/S and/or the FAD center of XO were observed. These data strongly suggest that the polycondensates possess a great potential as antioxidant for various applications.

Chemical modification of milk xanthine oxidase with different modifiers

Xanthine oxidase (XO), purified from buttermilk was subjected to modification with N-phenylmaleimide, p-toluene-sulfonyl chloride, and 2-mercaptobenzimidazole. Spectrophotometric monitoring of the enzyme before and after treatment with these modifiers are presented. The results show that the interaction of XO with the modifiers was accompained by a change in UV absorption, as compared with untreated enzyme. The data indicate that these modifiers caused conformational changes in the polypeptide chain of milk XO due to interaction of these modifiers with sulfahydryl and/or hydroxyl groups. Moreover, the modifiers induce uptake inhibition of milk XO and appeared to be dependant upon either the concentration or incubation time.

Tautomeric energetics of xanthine oxidase substrates: xanthine, 2-oxo-6-methylpurine, and lumazine

The stability of the tautomers of each of the three important substrates of xanthine oxidase, xanthine, 2-oxo-6-methylpurine, and lumazine, was examined by quantum mechanical calculations. The geometries of these tautomers were optimized at the AM1, Hartree-Fock (HF/6-31G), and hybrid Hartree-Fock/density functional theory (B3LYP/6-31G(d)) levels of theory. The single point energies of some of the more stable tautomers for each of the substrates were calculated at the B3LYP/6-311 +G(2d,p) level of theory. The Conductor Polarized Continuum Model (CPCM) was used to evaluate the solvent effects on the relative stabilities of these tautomers. The calculations clearly identify the lowest energy tautomeric form for xanthine and lumazine. On the other hand, there appear to be three tautomers for 2-oxo-6-methylpurine, with only minor energetic differences in vacuo. In water, however, only one of them predominates. The lowest energy tautomers presumably represent the predominant tautomeric forms at the molybdenum center of xanthine oxidase during catalysis. Implications of these computational results are discussed in the context of enzyme catalysis.

Interaction of Cu2+ ion with milk xanthine oxidase

The interaction of Cu2+ ion with milk xanthine oxidase (XO) has been studied by optical spectroscopy, circular dichroism, ESR and transient kinetic techniques. It is observed that XO forms optically observable complexes with Cu2+ ion. The pH dependence studies of the formation of Cu2+-XO complex by optical spectroscopy and circular dichroism show that at least one ionizable group may be responsible for the formation of the complex. The EPR studies show that Cu2+ ion binds to XO with sulfur and nitrogenous ligands. The transient kinetic study of the interaction of Cu2+ with XO shows the existence of two Cu2+ bound XO complexes formed at two different time scales of the interaction, one at < or =5 ms and the other one at around 20 s. The complex formed at longer time scale may be responsible for the inhibition of the enzyme activity.

FTIR characterization of heterocycles lumazine and violapterin in solution: effects of solvent on anionic forms

Fourier transform infrared (FTIR) spectra have been obtained from solution samples of the heterocycles uracil, lumazine, and violapterin and reveal interpretable carbonyl stretching frequencies. Spectra of conjugate bases of lumazine and violapterin demonstrate decreases in these carbonyl stretching frequencies upon ionization. Based on isotopic shifts from amide deuterated analogs, semiempirical QCFF/PI calculations were used to assign the vibrational frequencies in the region 1100-1800 cm-1 observed from samples in dimethylsulfoxide (DMSO) and aqueous solutions to specific normal modes. The observed deuterium shifts and the calculations suggest that, in some cases, N-H bending motions are coupled to the C=O stretching motions of the pyrimidine ring. These data suggest that for lumazine anions a change in solvent can significantly change the mixing of the N-H bending and C=O stretching vibrational motions. This implies that vibrational analysis for lumazine species in relatively noninteracting media like nonpolar solvents, mulls or pellets cannot necessarily be transferred to the system when it is dissolved in a polar, hydrogen-bonding solvent such as water. Although other explanations can be offered, our vibrational analysis suggests that the changes in normal mode composition of the predominantly C=O stretching vibrations of lumazine anion on going from dimethylsulfoxide to water solution are consistent with a change in the predominant tautomer of the heterocycle. This change appears to correspond to a shifting of the location of the remaining acidic proton to a different ring nitrogen atom. This interpretation is of interest in view of recent ab initio calculations which suggest that proton shifts may occur during the hydroxylation of lumazine as mediated by the enzyme xanthine oxidase.

The molybdenum centers of xanthine oxidase and xanthine dehydrogenase. Determination of the spectral change associated with reduction from the Mo(VI) to the Mo(IV) state

The UV-visible absorbance change associated with reduction of the molybdenum centers of xanthine oxidase and xanthine dehydrogenase has been determined using a double-difference technique. At pH 8.5, the Mo(VI) minus Mo(IV) difference spectrum seen with xanthine oxidase exhibits a positive feature at 420 nm, having an extinction change of approximately 3,000 M-1 cm-1 as well as evidence for a negative feature below 340 nm. In xanthine oxidase this change is found to exhibit a marked pH dependence, implicating protonation/deprotonation events associated with changes in the molybdenum oxidation state. Application of the double-difference protocol to the respective circular dichroism spectra of xanthine oxidase and xanthine dehydrogenase reveals appreciable CD changes at 420 and 580 nm associated with the reduction of the molybdenum center. The present results demonstrate a direct spectroscopic handle on the molybdenum centers of both xanthine oxidase and xanthine dehydrogenase.

Kinetics and thermodynamics of the molecular mechanism of the reductive half-reaction of xanthine oxidase

The kinetics and thermodynamics of the reductive half-reaction of xanthine oxidase with xanthine as substrate have been investigated by stopped-flow kinetic measurements. The temperature dependence of the steady-state and transient kinetics of the reductive half-reaction reveals the existence of at least three molecular intermediates during this half-reaction. All the microscopic rate constants and the thermodynamic activation parameters of the elementary steps of the reductive half-reaction have been determined for the first time. The microscopic rate constants and the thermodynamic activation parameters of the individual steps show wide variations in their magnitudes. The present work provides the most detailed and incisive description of the reaction of xanthine oxidase with its physiological substrate xanthine.

Reductive half-reaction of xanthine oxidase: mechanistic role of the species giving rise to the "rapid type 1" molybdenum(V) electron paramagnetic resonance signal

The reaction of xanthine oxidase with xanthine, 1-methylxanthine, and 2-hydroxy-6-methylpurine has been reinvestigated with the aim of elucidating the mechanistic role of the species giving rise to the "rapid" Mo(V) electron paramagnetic resonance (EPR) signal. It is found that addition of 2.0 mM 1-methylxanthine or 2-hydroxy-6-methylpurine to partially reduced enzyme generates substantial amounts of the Type 1 form of the "rapid" EPR signal, characterized by superhyperfine coupling to one strongly interacting (aav = 13 G) and one weakly interacting (aav = 3 G) proton. The "rapid" signals observed with both substrates are identical to those observed in the course of the anaerobic reaction of enzyme with a stoichiometric excess of substrate. With 2-hydroxy-6-methylpurine at pH 10, a burst phase in the formation of the species giving rise to the "rapid Type 1" signal is observed that is fast relative to the rate of formation of the species giving rise to the "very rapid" EPR signal. At pH 8.5, partial reduction of enzyme prior to reaction with xanthine, 1-methylxanthine, or 2-hydroxy-6-methylpurine reverses the relative amounts of "rapid" and "very rapid" EPR signal observed at the shortest reaction times. The substantial amounts of "rapid Type 1" signal formed by addition of substrates to partially reduced enzyme or by reaction of oxidized enzyme with a stoichiometric excess of substrate contrasts with previous work, which has shown that under single-turnover conditions none of the substrates investigated generates an appreciable amount of "rapid" EPR signal.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer graphic study on models of the molybdenum cofactor of xanthine oxidase

Within the scope of our molecular modeling studies on xanthine oxidase (XOD) inhibition by purine analogs we were interested to build up a three-dimensional model of the molybdenum active site. Spectroscopic data indicated that a Mo (VI)atom which is coordinated to sulfur, oxygen and/or nitrogen is clearly involved in substrate binding. In the present study, those data and X-ray crystallography data were used to reconstruct molybdenum-organic complexes from models proposed in the literature. The computer graphic-assisted modeling and evaluation of the model complexes show that the description of the molybdenum center needs further refinement.