The reaction mechanism of xanthine oxidase: evidence for two-electron chemistry rather than sequential one-electron steps

Current research on xanthine oxidase has favored a mechanism involving base-catalyzed proton abstraction from a Mo-OH group, allowing nucleophilic attack on the substrate and hydride transfer from the substrate to Mo=S group in the active site. During the course of this reaction mechanism, the molybdenum redox cycles from MoVI to MoIV, with reoxidation of the MoIV speices to form the EPR active MoV intermediate. However, it has also been suggested that the reaction occurs in two subsequent one-electron steps. We have determined kinetic parameters kred and kred/Kd for a variety of plausible substrates as well as the one-electron reduction potentials for these substrates. Our data indicate no correlation between these kinetic parameters and their one-electron reduction potentials, as would be expected if the enzyme were using two subsequent one-electron reduction steps. Our results provide additional support to current evidence for the favored two-electron reduction mechanism.

The active site of arsenite oxidase from Alcaligenes faecalis

Arsenite oxidase, a member of the DMSO reductase family of molybdenum enzymes, has two molecules of guanosine dinucleotide molybdenum cofactor coordinating the molybdenum at the active site. X-ray absorption spectroscopy indicates that the Mo-S bonds shorten from 2.47 to 2.37 A upon reduction with the physiological substrate. It also indicates the presence of an oxo ligand at 1.70 A in both oxidized and reduced forms of the enzyme, together with a short, 1.83 A, Mo-O bond in the oxidized form that is lost upon reduction. Resonance Raman spectroscopy indicates that the two pterin dithiolene moieties have different aromaticities, with one, the Q-pterin, having a more discrete dithiolate structure while the other, the P-pterin, has considerable pi-delocalization. Our results indicate that the structure of arsenite oxidase is intermediate between that seen in other molybdenum enzymes, in which one ligand to the metal is provided by the polypeptide (serine, cysteine, or selenocysteine), and tungsten enzymes that lack a peptide ligand.

Molybdenum and tungsten in biology

Molybdenum is the only second-row transition metal that is required by most living organisms, and the few species that do not require molybdenum use tungsten, which lies immediately below molybdenum in the periodic table. Because of their unique chemical versatility and unusually high bioavailability these two transition metals have been incorporated into the active sites of enzymes over the course of evolution. Enzymes that contain molybdenum or tungsten continue to be discovered and several crystal structures have become available recently. This new structural information has been complemented by spectroscopic and kinetic methods, as well as computational approaches. Together, these studies provide an increasingly detailed view of the reaction mechanisms and the correlation between the electronic structure of the active site and catalytic function, one of the fundamental goals in metallobiochemistry.

Oxo, sulfido, and tellurido Mo-enedithiolate models for xanthine oxidase: understanding the basis of enzyme reactivity

The active site of the mononuclear molybdenum enzyme xanthine oxidase has an LMoOS(OH) center that catalyzes the hydroxylation of substrate (L representing an enedithiolate ligand contributed by a pterin cofactor in the enzyme). Reaction of the enzyme with cyanide results in the replacement of the Mo=S group with a second Mo=O group, which results in loss of enzyme activity. To understand the basis for this loss of activity, we have computationally examined the interaction of a model for the LMoO2(OH) as well the LMoOTe(OH) congener of the enzyme with formamide (a substrate for the enzyme). Our electronic structure calculations for the oxo congener indicate a reduced electron density on the hydrogen being transferred from substrate in the course of the reaction, a shorter O-H bond in the transition state, and a longer nascent O-C bond of product, factors which combine to account for the loss of reactivity in the LMoO2(OH) species. Interestingly, our calculations indicate that the Te congener is characterized by an increased electron density on the hydrogen species being transferred, a longer Te-H bond in the transition state, and a shorter O-C nascent bond in the product and suggest that a Te congener of xanthine oxidase, were it to be prepared experimentally, should exhibit catalytic activity.

Iron-sulfur cluster biosynthesis: characterization of Schizosaccharomyces pombe Isa1

Eukaryotic Isa1 is one of several mitochondrial proteins that have been implicated in Fe-S cluster assembly paths in vivo. We report the first biochemical characterization of an eukaryotic member of this family and discuss this in the context of results from in vivo studies and studies of bacterial homologues. Schizosaccharomyces pombe Isa1 is a multimeric protein carrying [2Fe-2S](2+) clusters that have been characterized by Mössbauer and optical spectroscopic studies. Complex formation with a redox-active ferredoxin has been identified through crosslinking experiments and the coordination chemistry and stability of the native clusters has been investigated through site-directed mutagenesis and spectroscopic analysis. Electronic supplementary material to this paper, containing Mössbauer and UV-visible spectra for mutant Isa1 proteins, can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00775-001-0330-2.

Functional asymmetry of photosystem II D1 and D2 peripheral chlorophyll mutants of Chlamydomonas reinhardtii

The peripheral accessory chlorophylls (Chls) of the photosystem II (PSII) reaction center (RC) are coordinated by a pair of symmetry-related histidine residues (D1-H118 and D2-H117). These Chls participate in energy transfer from the proximal antennae complexes (CP43 and CP47) to the RC core chromophores. In addition, one or both of the peripheral Chls are redox-active and participate in a low-quantum-yield electron transfer cycle around PSII. We demonstrate that conservative mutations of the D2-H117 residue result in decreased Chl fluorescence quenching efficiency attributed to reduced accumulation of the peripheral accessory Chl cation, Chl(Z)(+). In contrast, identical symmetry-related mutations at residue D1-H118 had no effect on Chl fluorescence yield or quenching kinetics. Mutagenesis of the D2-H117 residue also altered the line width of the Chl(Z)(+) EPR signal, but the line shape of the D1-H118Q mutant remained unchanged. The D1-H118 and D2-H117 mutations also altered energy transfer properties in PSII RCs. Unlike wild type or the D1-H118Q mutant, D2-H117N RCs exhibited a reduced CD doublet in the red region of Chl absorbance band, indicative of reduced energetic coupling between P680 and the peripheral accessory Chl. In addition, transient absorption measurements of D2-H117N RCs, excited on the blue side of the Chl absorbance band, exhibited a ( approximately 400 fs) pheophytin Q(X) band bleach lifetime component not seen in wild-type or D1-H118Q RCs. The origin of this component may be related to delayed fast-energy equilibration of the excited state between the core pigments of this mutant.

Crystal structure and stability studies of C77S HiPIP: a serine ligated [4Fe-4S] cluster

The crystal structure of Chromatium vinosum C77S HiPIP has been determined and is compared with that of wild type. This is the first reported crystal structure of a Ser ligated [4Fe-4S] cluster and reveals a 0.11 A shortening of the Fe-O bond (relative to Fe-S), but only minor structural alterations of the overall tertiary structure. Coordination changes are corroborated by resonance Raman spectroscopy. Comparison of the crystal and solution structures for HiPIPs identifies Phe48 as the main controller of solvent access to the Fe-S cluster; however, there is no significant change in cluster solvation of the C77S mutant relative to WT HiPIP. Ser ligation ultimately results in decreased cluster stability due to increased sensitivity to proton mediated degradation.

The use of TLD-700H dosemeters in the assessment of external doses at the former Semipalatinsk nuclear test site

The joint projects performed since 1995 by the Jülich Research Centre in co-operation with the Kazakh National Nuclear Centre in the area of the former nuclear test site near Semipalatinsk, in eastern Kazakhstan, have assessed the current dose rate of the population at and around the test site, as well as determining retrospectively the dose rate of persons affected by the atmospheric tests. Measurements of the population by personal dosemeters depend on reliably wearing these dosemeters over prolonged periods of time, and of a sufficient dosemeter return. In the past, such measurements have been particularly successful whenever short wearing times were possible. This requires high sensitivity of the dosemeters. The suitability of the highly sensitive TLD material of the BICRON TLD 700H type for such personal dosimetry measurements was investigated. It was tested in practical field application at the Semipalatinsk nuclear test site in September 2000. Initial results are available from individual doses received by a group of geologists and a group of herdsmen at the test site. For the first time, the individual dose was measured directly in these population groups. Detection limits below 1 microSv permit informative measurements for wearing times of less than two weeks. Most individual doses did not arise significantly out of local fluctuations of natural background. A conservative assessment from the aspect of practical health physics yielded a mean personal dose of 0.55 microSv per day for the herdsmen, whereas the geologists received a mean personal dose of 0.45 microSv per day. For an annual exposure period of typically, about three months, the radiation dose received by the persons investigated, in addition to the natural radiation exposure, is thus well below the international limit value of 1 mSv x a(-1) for the population dose.

Ratio of complex double strand break damage induced by 125IUdR and 123IUdR correlates with experimental in vitro cell killing effectiveness

The overall cellular damage induced by ionising radiation is determined by the number and spatial distribution of initial ionisations and excitations within the critical volume. This paper focuses on the physical and chemical phase of the radiation action chain following the decay of DNA-bound 123I and 125I. Monte Carlo simulations of these nuclides' decay provide electron emission spectra which are used as input data for track structure calculations. In combination with DNA models, these calculations allow the specific radiation source to be characterised in terms of DNA strand break patterns. The distribution of these patterns indicates that 125I produces much more severe breaks than 123I. The ratio of complex DSBs induced by both iodine isotopes correlates with the differences in cell killing effectiveness reported from in vitro survival experiments.

Intercomparison of whole-body counters by using a subject who had incorporated 137Cs into the body

During the years 1996-2000, eight whole-body counting facilities (WBC) from Finland, Germany, Japan and Russia took part in an intercomparison using a resident of the Russian town of Novozybkov who had been seriously contaminated as a result of the Chernobyl accident. The subject R (adult male, height 172 cm average body mass 64 kg; and 137Cs body burden within the range of 1-15 kBq) was investigated in the participating institutions during his business trips. The experimentally obtained data for his 137Cs body burden were compared with the predicted values, which had been deduced from the measurements of subject R using the reference WBC (St Petersburg Institute of Radiation Hygiene) and from his effective half-time of 137Cs in the body (68 days). The obtained results did not deviate more than 20% from reference activities. Four facilities were able to quantity the 40K in the subject's body. The differences between reported values of potassium did not exceed 10%. For subject R, the average annual effective dose from radiocaesium was 0.25 mSv and it was 0.18 mSv from 40K in the years 1996/97. The reliability of using a subject with naturally incorporated artificial radionuclides ('walking standard') instead of an anthropomorphous phantom for calibration and intercomparison of whole-body counters in a large-scale nuclear accident is discussed.

Identification and biochemical characterization of Arabidopsis thaliana sulfite oxidase. A new player in plant sulfur metabolism

In mammals and birds, sulfite oxidase (SO) is a homodimeric molybdenum enzyme consisting of an N-terminal heme domain and a C-terminal molybdenum domain (EC ). In plants, the existence of SO has not yet been demonstrated, while sulfite reductase as part of sulfur assimilation is well characterized. Here we report the cloning of a plant sulfite oxidase gene from Arabidopsis thaliana and the biochemical characterization of the encoded protein (At-SO). At-SO is a molybdenum enzyme with molybdopterin as an organic component of the molybdenum cofactor. In contrast to homologous animal enzymes, At-SO lacks the heme domain, which is evident both from the amino acid sequence and from its enzymological and spectral properties. Thus, among eukaryotes, At-SO is the only molybdenum enzyme yet described possessing no redox-active centers other than the molybdenum. UV-visible and EPR spectra as well as apparent K(m) values are presented and compared with the hepatic enzyme. Subcellular analysis of crude cell extracts showed that SO was mostly found in the peroxisomal fraction. In molybdenum cofactor mutants, the activity of SO was strongly reduced. Using antibodies directed against At-SO, we show that a cross-reacting protein of similar size occurs in a wide range of plant species, including both herbacious and woody plants.

Spectroscopic and functional properties of novel 2[4Fe-4S] cluster-containing ferredoxins from the green sulfur bacterium Chlorobium tepidum

Two distinct ferredoxins, Fd I and Fd II, were isolated and purified to homogeneity from photoautotrophically grown Chlorobium tepidum, a moderately thermophilic green sulfur bacterium that assimilates carbon dioxide by the reductive tricarboxylic acid cycle. Both ferredoxins serve a crucial role as electron donors for reductive carboxylation, catalyzed by a key enzyme of this pathway, pyruvate synthase/pyruvate ferredoxin oxidoreductase. The reduction potentials of Fd I and Fd II were determined by cyclic voltammetry to be -514 and -584 mV, respectively, which are more electronegative than any previously studied Fds in which two [4Fe-4S] clusters display a single transition. Further spectroscopic studies indicated that the CD spectrum of oxidized Fd I closely resembled that of Fd II; however, both spectra appeared to be unique relative to ferredoxins studied previously. Double integration of the EPR signal of the two Fds yielded approximately approximately 2.0 spins per molecule, compatible with the idea that C. tepidum Fd I and Fd II accept 2 electrons upon reduction. These results suggest that the C. tepidum Fd I and Fd II polypeptides each contain two bound [4Fe-4S] clusters. C. tepidum Fd I and Fd II are novel 2[4Fe-4S] Fds, which were shown previously to function as biological electron donors or acceptors for C. tepidum pyruvate synthase/pyruvate ferredoxin oxidoreductase (Yoon, K.-S., Hille, R., Hemann, C. F., and Tabita, F. R. (1999) J. Biol. Chem. 274, 29772-29778). Kinetic measurements indicated that Fd I had approximately 2.3-fold higher affinity than Fd II. The results of amino acid sequence alignments, molecular modeling, oxidation-reduction potentials, and spectral properties strongly indicate that the C. tepidum Fds are chimeras of both clostridial-type and chromatium-type Fds, suggesting that the two Fds are likely intermediates in the evolutional development of 2[4Fe-4S] clusters compared with the well described clostridial and chromatium types.

A conserved tryptophan in nitric oxide synthase regulates heme-dioxy reduction by tetrahydrobiopterin

In nitric oxide synthase (NOS), (6R)-tetrahydrobiopterin (H(4)B) binds near the heme and can reduce a heme-dioxygen intermediate (Fe(II)O(2)) during Arg hydroxylation [Wei, C.-C., Wang, Z.-Q., Wang, Q., Meade, A. L., Hemann, C., Hille, R., and Stuehr, D. J. (2001) J. Biol. Chem. 276, 315-319]. A conserved Trp engages in aromatic stacking with H(4)B, and its mutation inhibits NO synthesis. To examine how this W457 impacts H(4)B redox function, we performed single turnover reactions with the mouse inducible NOS oxygenase domain (iNOSoxy) mutants W457F and W457A. Ferrous mutants containing Arg and H(4)B were mixed with O(2)-containing buffer, and then heme spectral transitions, H(4)B radical formation, and Arg hydroxylation were followed versus time. A heme Fe(II)O(2) intermediate was observed in W457A and W457F and had normal spectral characteristics. However, its disappearance rate (6.5 s(-1) in W457F and 3.0 s(-1) in W457A) was slower than in wild-type (12.5 s(-1)). Rates of H(4)B radical formation (7.1 s(-1) in W457F and 2.7 s(-1) in W457A) matched their rates of Fe(II)O(2) disappearance, but were slower than radical formation in wild-type (13 s(-1)). The extent of H(4)B radical formation in the mutants was similar to wild-type, but their radical decayed 2-4 times faster. These kinetic changes correlated with slower and less extensive Arg hydroxylation by the mutants (wild-type > W457F > W457A). We conclude that W457 ensures a correct tempo of electron transfer from H(4)B to heme Fe(II)O(2), possibly by stabilizing the H(4)B radical. Proper control of these parameters may help maximize Arg hydroxylation and minimize uncoupled O(2) activation at the heme.

Apoptosis: live or die--hard work either way!

This review presents a brief overview of the cell's apoptotic machinery, including specific and indirect death signals. Specific death signals are transferred via death ligands, death receptors, and their intracellular signalling pathways. Indirect death signals cumulate a wide range of stimuli that potentially harm survival of cells. These include intercalating drugs, irradiation or altered intracellular signalling. Herein, a focal point is the mitochondrial control of specific death enzymes--so called caspases--by members of the pro-apoptotic Bax and BH3 subfamily or the anti-apoptotic Bcl-2 subfamily. While the initiation of cell death happens through a variety of signalling systems, the activation of caspases plays a pivotal role in the progression towards the final morphologic findings in cells undergoing apoptosis. Caspases appear to directly cleave and inactivate substrates that are clinical for the maintenance of cell structure and function but also regulate the activity of other enzymes that induce the apoptotic phenotype within the cell. The insulin-like growth factors (IGFs) are potent proliferation factors and potently inhibit apoptosis acting via the ubiquitously expressed IGF-I receptor. Within IGF-I receptor signalling, key to the inhibition of apoptosis are the RAS/RAF/mitogen-activated protein (MAP)-kinase pathway and the PI 3'-kinase pathway. To give an example of high clinical relevance of apoptosis within endocrine disorders, apoptotic death of pancreatic beta cells in type 1 diabetes disease and the involvement of IGF-II in beta cell survival and beta cell function is discussed in detail. Finally, further understanding of signalling systems that are involved in proliferation or in apoptosis might provide novel tools to treat or even heal disorders like type I diabetes.

Coupled electron/proton transfer in complex flavoproteins: solvent kinetic isotope effect studies of electron transfer in xanthine oxidase and trimethylamine dehydrogenase

A solvent kinetic isotope effect study of electron transfer in two complex flavoproteins, xanthine oxidase and trimethylamine dehydrogenase, has been undertaken. With xanthine oxidase, electron transfer from the molybdenum center to the proximal iron-sulfur center of the enzyme occurs with a modest solvent kinetic isotope effect of 2.2, indicating that electron transfer out of the molybdenum center is at least partially coupled to deprotonation of the Mo(V) donor. A Marcus-type analysis yields a decay factor, beta, of 1.4 A(-1), indicating that, although the pyranopterin cofactor of the molybdenum center forms a nearly contiguous covalent bridge from the molybdenum atom to the proximal iron-sulfur center of the enzyme, it affords no exceptionally effective mode of electron transfer between the two centers. For trimethylamine dehydrogenase, rates of electron equilibration between the flavin and iron-sulfur center of the one-electron reduced enzyme have been determined, complementing previous studies of electron transfer in the two-electron reduced form. The results indicate a substantial solvent kinetic isotope effect of 10 +/- 4, consistent with a model for electron transfer that involves discrete protonation/deprotonation and electron transfer steps. This contrasts to the behavior seen with xanthine oxidase, and the basis for this difference is discussed in the context of the structures for the two proteins and the ionization properties of their flavin sites. With xanthine oxidase, a rationale is presented as to why it is desirable in certain cases that the physical layout of redox-active sites not be uniformly increasing in reduction potential in the direction of physiological electron transfer.

Inactivation of C30A trimethylamine dehydrogenase by N-cyclopropyl-alpha-methylbenzylamine, 1-phenylcyclopropylamine, and phenylhydrazine

Trimethylamine dehydrogenase (TMADH) from the bacterium Methylophilus methylotrophus (sp. W(3)A(1)) and its C30A mutant were inactivated with three known inactivators of monoamine oxidase, namely, phenylhydrazine, N-cyclopropyl-alpha-methylbenzylamine, and 1-phenylcyclopropylamine. All three compounds irreversibly inactivated both the wild-type and C30A mutant enzymes, although phenylhydrazine was 10 times more potent than N-cyclopropyl-alpha-methylbenzylamine, which was much more potent than 1-phenylcyclopropylamine. The change in the UV--visible absorption spectra upon modification indicated that the flavin was modified. In the case of the C30A mutant, the absence of a covalent attachment of the flavin to the polypeptide has permitted LC-electrospray mass spectrometry of the reaction product to be undertaken, demonstrating new mass peaks corresponding to various chemically modified forms of the flavin cofactor. In the case of N-cyclopropyl-alpha-methylbenzylamine, masses corresponding to hydroxy-FMN and hydroxyriboflavin were detected. 1-Phenylcyclopropylamine inactivation of the C30A mutant produced three modified flavins, as evidenced by the electrospray mass spectrum: hydroxy-FMN, FMN plus C(6)H(5)COCH(2)CH(2), and hydroxy-FMN plus C(6)H(5)COCH(2)CH(2). Phenylhydrazine inactivation of the C30A mutant gave at least seven different modified flavins: hydroxyriboflavin, hydroxy-FMN, two apparently isomeric compounds corresponding to hydroxy-FMN plus one phenyl group, two apparently isomeric compounds corresponding to FMN plus one phenyl group, and FMN plus two phenyl groups. Covalent flavin adduct formation appears to be the only modification because dialysis of the inactive enzyme followed by reconstitution with FMN restores the enzyme activity to that of a noninactivated control.

35 GHz ENDOR characterization of the "very rapid" signal of xanthine oxidase reacted with 2-hydroxy-6-methylpurine (13C8): evidence against direct Mo-C8 interaction

Xanthine oxidase is a molybdenum-containing enzyme that catalyzes the hydroxylation of xanthine and a wide variety of other aromatic heterocycles. In the course of the reaction with xanthine and substrates such as 2-hydroxy-6-methylpurine (HMP), the enzyme gives rise to a Mo(V) EPR signal, denoted "very rapid", that arises from an authentic catalytic intermediate. The two alternative catalytic mechanisms proposed for this enzyme differ critically in whether the distance between Mo and C8 of the purine nucleus in this intermediate is short enough to admit a direct bonding interaction. To examine this distance, we have performed 13C ENDOR measurements of the "very rapid" EPR signal generated by xanthine oxidase during reaction with 13C8-HMP. The resulting (13)C8 hyperfine tensor, A = [10.2(1), 7.0(1), 6.5(1)] MHz, is discussed in the framework of a detailed consideration of factors involved in extracting metrical parameters from an anisotropic hyperfine interaction composed of contributions from multiple sources, in particular, the effect of the local contributions from spin density on (13)C8. The analysis presented here gives a Mo...C distance whose value is expected to be ca. 2.7-2.9 A in the "very rapid" intermediates formed with both xanthine and HMP, consistent with plausible bond lengths for a Mo-O-C8 fragment where C8 is a trigonal-planar aromatic carbon. The difference from earlier conclusions is explained. The data thus do not support the existence of a direct Mo-C bond in the signal-giving species. This conclusion supports a mechanism that does not involve such an interaction and which begins with base-assisted nucleophilic attack of the Mo(VI)-OH group on the C-8 of substrate, with concomitant hydride transfer to the Mo=S group to give Mo(IV)-SH; the EPR-active "very rapid" species then forms by one-electron oxidation and deprotonation to yield the EPR-detectable Mo(V)OS(OR) species. We further discuss the complexities and limitations of the semiempirical method used to arrive at these conclusions.

Crystal structure of the 100 kDa arsenite oxidase from Alcaligenes faecalis in two crystal forms at 1.64 A and 2.03 A

BACKGROUND

Arsenite oxidase from Alcaligenes faecalis NCIB 8687 is a molybdenum/iron protein involved in the detoxification of arsenic. It is induced by the presence of AsO(2-) (arsenite) and functions to oxidize As(III)O(2-), which binds to essential sulfhydryl groups of proteins and dithiols, to the relatively less toxic As(V)O(4)(3-) (arsenate) prior to methylation.

RESULTS

Using a combination of multiple isomorphous replacement with anomalous scattering (MIRAS) and multiple-wavelength anomalous dispersion (MAD) methods, the crystal structure of arsenite oxidase was determined to 2.03 A in a P2(1) crystal form with two molecules in the asymmetric unit and to 1.64 A in a P1 crystal form with four molecules in the asymmetric unit. Arsenite oxidase consists of a large subunit of 825 residues and a small subunit of approximately 134 residues. The large subunit contains a Mo site, consisting of a Mo atom bound to two pterin cofactors, and a [3Fe-4S] cluster. The small subunit contains a Rieske-type [2Fe-2S] site.

CONCLUSIONS

The large subunit of arsenite oxidase is similar to other members of the dimethylsulfoxide (DMSO) reductase family of molybdenum enzymes, particularly the dissimilatory periplasmic nitrate reductase from Desulfovibrio desulfuricans, but is unique in having no covalent bond between the polypeptide and the Mo atom. The small subunit has no counterpart among known Mo protein structures but is homologous to the Rieske [2Fe-2S] protein domain of the cytochrome bc(1) and cytochrome b(6)f complexes and to the Rieske domain of naphthalene 1,2-dioxygenase.