J Biol Inorg Chem (2011) 16:443–460 DOI 10.1007/s00775-010-0741-z ORIGINAL PAPER Nitrite reduction by xanthine oxidase family enzymes: a new class of nitrite reductases Luisa B. Maia • Jose´ J. G. Moura Received: 9 August 2010 / Accepted: 19 November 2010 / Published online: 19 December 2010 Ó SBIC 2010 Abstract Mammalian xanthine oxidase (XO) and Des- Keywords Nitrite reduction Á Nitric oxide formation Á ulfovibrio gigas aldehyde oxidoreductase (AOR) are Molybdenum Á Xanthine oxidase Á Aldehyde members of the XO family of mononuclear molybdoen- oxidoreductase zymes that catalyse the oxidative hydroxylation of a wide range of aldehydes and heterocyclic compounds. Much less Abbreviations known is the XO ability to catalyse the nitrite reduction to AOR Aldehyde oxidoreductase nitric oxide radical (NO). To assess the competence of other DMSOR Dimethylsulfoxide reductase XO family enzymes to catalyse the nitrite reduction and to EPR Electron paramagnetic resonance shed some light onto the molecular mechanism of this Fe/S Iron–sulfur centre reaction, we characterised the anaerobic XO- and AOR- Fe/S–NO Dinitrosyl–iron–sulfur complex catalysed nitrite reduction. The identification of NO as the (MGD) –Fe Ferrous complex of di(N-methyl-D- reaction product was done with a NO-selective electrode 2 glucamine dithiocarbamate) and by electron paramagnetic resonance (EPR) spectros- (MGD) –Fe–NO Mononitrosyl–iron complex copy. The steady-state kinetic characterisation corroborated 2 Mo-enzymes Pterin–molybdenum-containing the XO-catalysed nitrite reduction and demonstrated, for the enzymes first time, that the prokaryotic AOR does catalyse the nitrite NaR Nitrate reductases reduction to NO, in the presence of any electron donor to the NO Nitric oxide radical enzyme, substrate (aldehyde) or not (dithionite). Nitrite SO Sulfite oxidase binding and reduction was shown by EPR spectroscopy to XO Xanthine oxidase occur on a reduced molybdenum centre. A molecular mechanism of AOR- and XO-catalysed nitrite reduction is discussed, in which the higher oxidation states of molyb- denum seem to be involved in oxygen-atom insertion, whereas the lower oxidation states would favour oxygen- Introduction atom abstraction. Our results define a new catalytic per- formance for AOR—the nitrite reduction—and propose a Molybdenum is present in a wide variety of enzymes, in new class of molybdenum-containing nitrite reductases. both prokaryotes and eukaryotes, where it performs cata- lytic roles in important redox reactions of the carbon, nitrogen and sulfur cycles [1–3]. Additionally, heterome- L. B. Maia Á J. J. G. Moura (&) REQUIMTE, Centro de Quı´mica Fina e Biotecnologia, tallic clusters of molybdenum are also found in other Departamento de Quı´mica, proteins whose physiological function is not yet known Faculdade de Cieˆncias e Tecnologia, [4, 5]. Universidade Nova de Lisboa, With the exception of the unique iron–molybdenum Campus da Caparica, 2829-516 Caparica, Portugal cofactor of the nitrogenase, molybdenum is found in e-mail: [email protected] enzyme catalytic sites coordinated by the cis-dithiolene 123 444 J Biol Inorg Chem (2011) 16:443–460 a b Scheme 1 The pterin cofactor structure (a) and the proposed group. For simplicity, the apical oxo group and the pterin sulfur atoms mechanism of XO-catalysed hydroxylation (b). a The pterin cofactor were omitted from the reduced molybdenum coordination sphere (ii, is a pyranopterin-dithiolate moiety, which forms a five-membered iii). The catalysis of hydroxylation is initiated by the activation of the ene-1,2-dithiolate chelate ring with the molybdenum atom. In molybdenum-labile –OH/OH2 group by a neighbouring unprotonated eukaryotes, the pterin–molybdenum cofactor is found in the simplest glutamate residue. The Mo6?–O- formed (i) makes a nucleophilic monophosphate form (R is one H atom). In prokaryotes, however, the attack on the substrate carbon atom (R–CH), with simultaneous cofactor is found esterificated with several nucleotides: R could be hydride transfer to Mo=S (ii). Subsequently, the product (R–CO-)is one cytidine monophosphate, one guanosine monophosphate or one displaced by a water molecule and a new –OH2 moiety reoccupies the adenosine monophosphate. b The oxidised molybdenum (i) is coor- initial position (iii). The two electrons transferred from the substrate dinated by one apical oxo group, two sulfur atoms of the pterin to the molybdenum are then rapidly transferred to the FAD site, where cofactor (not represented), one hydroxide/water and one sulfido the reduction of molecular oxygen takes place group of one or two pterin cofactor1 molecules (Scheme 1a) pterin cofactor and X represents terminal =O, =S, or =Se [1–3]. The coordination sphere of the molybdenum atom is groups [7, 8]. This family comprises many enzymes with completed with oxygen, sulfur or selenium atoms in a diverse functions, such as aldehyde oxidoreductase (AOR) diversity of arrangements (see below). In addition, these from Desulfovibrio species, human XO and Eubacterium pterin–molybdenum-containing enzymes (Mo-enzymes) barkeri nicotinate dehydrogenase. The CO dehydrogenase maycontainotherredoxcentres,suchasflavins,haems from Oligotropha carboxidovorans, with its unique binu- and iron–sulfur centres (Fe/S), which are involved in clear copper–molybdenum cofactor LMo–S–Cu–S–cys- the intramolecular and intermolecular electron transfer teine residue (–OH/OH2) (=O), is also included in the XO processes. family. The sulfite oxidase (SO) family, with an LMo–S– According to the proposed structures of the molybde- cysteine residue (–OH/OH2) (=O) core, includes well- num centres, the Mo-enzymes have been divided into three known enzymes such as human SO and plant assimilatory families [6]. The xanthine oxidase (XO) family has an nitrate reductases (NaR, enzymes that catalyse the first and LMo=X (–OH/OH2) (=O) core, where L stands for the rate-limiting step of nitrate assimilation in plants, algae and fungi), but also the prokaryotic sulfite dehydrogenases [9]. The dimethylsulfoxide reductase (DMSOR) family has an 1 The pterin–molybdenum cofactor is still commonly referred to as L MoXY core, where X and Y represent terminal =O, ‘‘molybdopterin’’, because, historically, the cofactor was first iden- 2 tified in molybdenum-containing enzymes. However, the same –OH, =S, and –SH groups and/or oxygen, sulfur or sele- cofactor molecule is used to coordinate tungsten in some organisms nium atoms from cysteine, selenocysteine, serine or that are able to utilise that element. To avoid confusion between aspartate residue side chains [10]. This is a larger and more molybdenum and tungsten-containing enzymes, the denomination diverse family, constituted by only prokaryotic enzymes ‘‘pterin–molybdenum cofactor’’, proposed by the Nomenclature Committee of the International Union of Biochemistry (http://www. such as the DMSOR and formate dehydrogenase, as well as chem.qmul.ac.uk/iubmb/etp/) was chosen. dissimilatory NaR (periplasmic or membrane-associated 123 J Biol Inorg Chem (2011) 16:443–460 445 enzymes that function as respiratory oxidases) and assim- of the cis-dithiolene (–S–C=C–S–) group of the pterin ilatory NaR (cytoplasmatic enzymes), arsenite oxidase and cofactor molecule, one essential =S group and one labile – arsenate reductase. A fourth family was recently proposed OH/–OH2 group [13]. Physiologically, XO is a key enzyme to include the pterin–tungsten cofactor-containing alde- in purine catabolism, where it catalyses the hydroxylation hyde:ferredoxin oxidoreductases (EC 1.2.7.5), found only of both hypoxanthine and xanthine to the terminal in archaea [11]. metabolite urate. However, it also catalyses the In general, the Mo-enzymes catalyse the transfer of an hydroxylation of a wide variety of other nitrogen- oxygen atom from water to the product or from the substrate containing heterocyclic compounds and aldehydes, with to water, in reactions that imply a net exchange of two the simultaneous reduction of O2 [14]. electrons and in which the molybdenum cycles between The hydroxylation catalysis [15, 16] is initiated with the 6? 4? Mo and Mo [1–3]. It is based on this catalytic feature activation of the molybdenum-labile –OH/OH2 group by a that these Mo-enzymes are frequently referred to as ox- neighbouring unprotonated glutamate residue (Glu-1261 in otransferases (although there is at least one important the bovine enzyme) to form Mo6?–O- (base-assisted exception, the formate oxidation catalysed by the formate catalysis) (Scheme 1b, i). The hydride transfer to the dehydrogenase, Eq. 1). In particular, the enzymes of the XO essential =S group follows, with the simultaneous nucleo- family catalyse the hydrolysis of a C–H bond with formation philic attack of the Mo–O- on the carbocation formed. of a C–O bond, in reactions of oxidative hydroxylation (e.g. This concerted attack results in the formation of a covalent xanthine hydroxylation catalysed by XO, Eq. 2)[7, 8]. The intermediate, Mo4?–O–C–R (–SH) (Scheme 1b, ii). Sub- hydroxybenzoyl-CoA reductase is an important exception sequently, the product (e.g. urate) is displaced by a water within the XO family, as it catalyses the irreversible dehy- molecule (Scheme 1b, ii ? iii) and a new –OH2 moiety 4? droxylation (a reduction) of the phenol ring. The CO reoccupies the initial position to give Mo –OH2 (–SH) dehydrogenase is another exception, since the CO2 forma- (Scheme 1b, iii). The two electrons thus transferred from tion from CO does not involve the hydrolysis of a C–H the substrate to the molybdenum (Mo6? ? Mo4?, reduc- bond. The members of the SO family, in contrast, catalyse tive half-reaction) are then rapidly distributed throughout the simple transfer of an oxygen atom from or to a lone the Fe/S and FAD Scheme 1b, iii ? i), according to their electron pair of the substrate (e.g. nitrate reduction catalysed redox potentials. At the FAD site, the electrons are finally •- by NaR, Eq. 3, or sulfite oxidation catalysed by SO, Eq. 4, transferred to O2 (oxidative half-reaction), to give O2 and respectively) [9].