Transfer of the molybdenum cofactor synthesized by Rhodobacter capsulatus MoeA to XdhC and MobA

TusA influences Fe-S cluster assembly and iron homeostasis in E. coli by reducing the translation efficiency of Fur

All sulfur transfer pathways have generally a l-cysteine desulfurase as an initial sulfur-mobilizing enzyme in common, which serves as a sulfur donor for the biosynthesis of numerous sulfur-containing biomolecules in the cell. In Escherichia coli, the housekeeping l-cysteine desulfurase IscS has several interaction partners, which bind at different sites of the protein. So far, the interaction sites of IscU, Fdx, CyaY, and IscX involved in iron-sulfur (Fe-S) cluster assembly have been mapped, in addition to TusA, which is required for molybdenum cofactor biosynthesis and mnm5s2U34 tRNA modifications, and ThiI, which is involved in thiamine biosynthesis and s4U8 tRNA modifications. Previous studies predicted that the sulfur acceptor proteins bind to IscS one at a time. E. coli TusA has, however, been suggested to be involved in Fe-S cluster assembly, as fewer Fe-S clusters were detected in a ∆tusA mutant. The basis for this reduction in Fe-S cluster content is unknown. In this work, we investigated the role of TusA in iron-sulfur cluster assembly and iron homeostasis. We show that the absence of TusA reduces the translation of fur, thereby leading to pleiotropic cellular effects, which we dissect in detail in this study.IMPORTANCEIron-sulfur clusters are evolutionarily ancient prosthetic groups. The ferric uptake regulator plays a major role in controlling the expression of iron homeostasis genes in bacteria. We show that a ∆tusA mutant is impaired in the assembly of Fe-S clusters and accumulates iron. TusA, therefore, reduces fur mRNA translation leading to pleiotropic cellular effects.

History of Maturation of Prokaryotic Molybdoenzymes-A Personal View

In prokaryotes, the role of Mo/W enzymes in physiology and bioenergetics is widely recognized. It is worth noting that the most diverse family of Mo/W enzymes is exclusive to prokaryotes, with the probable existence of several of them from the earliest forms of life on Earth. The structural organization of these enzymes, which often include additional redox centers, is as diverse as ever, as is their cellular localization. The most notable observation is the involvement of dedicated chaperones assisting with the assembly and acquisition of the metal centers, including Mo/W-bisPGD, one of the largest organic cofactors in nature. This review seeks to provide a new understanding and a unified model of Mo/W enzyme maturation.

The Role of the Nucleotides in the Insertion of the bis-Molybdopterin Guanine Dinucleotide Cofactor into apo-Molybdoenzymes

The role of the GMP nucleotides of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor of the DMSO reductase family has long been a subject of discussion. The recent characterization of the bis-molybdopterin (bis-Mo-MPT) cofactor present in the E. coli YdhV protein, which differs from bis-MGD solely by the absence of the nucleotides, now enables studying the role of the nucleotides of bis-MGD and bis-MPT cofactors in Moco insertion and the activity of molybdoenzymes in direct comparison. Using the well-known E. coli TMAO reductase TorA as a model enzyme for cofactor insertion, we were able to show that the GMP nucleotides of bis-MGD are crucial for the insertion of the bis-MGD cofactor into apo-TorA.

Sulfide and transition metals - A partnership for life

Sulfide and transition metals often came together in Biology. The variety of possible structural combinations enabled living organisms to evolve an array of highly versatile metal-sulfide centers to fulfill different physiological roles. The ubiquitous iron‑sulfur centers, with their structural, redox, and functional diversity, are certainly the best-known partners, but other metal-sulfide centers, involving copper, nickel, molybdenum or tungsten, are equally crucial for Life. This review provides a concise overview of the exclusive sulfide properties as a metal ligand, with emphasis on the structural aspects and biosynthesis. Sulfide as catalyst and as a substrate is discussed. Different enzymes are considered, including xanthine oxidase, formate dehydrogenases, nitrogenases and carbon monoxide dehydrogenases. The sulfide effect on the activity and function of iron‑sulfur, heme and zinc proteins is also addressed.

Tungstoenzymes: Occurrence, Catalytic Diversity and Cofactor Synthesis

Tungsten is the heaviest element used in biological systems. It occurs in the active sites of several bacterial or archaeal enzymes and is ligated to an organic cofactor (metallopterin or metal binding pterin; MPT) which is referred to as tungsten cofactor (Wco). Wco-containing enzymes are found in the dimethyl sulfoxide reductase (DMSOR) and the aldehyde:ferredoxin oxidoreductase (AOR) families of MPT-containing enzymes. Some depend on Wco, such as aldehyde oxidoreductases (AORs), class II benzoyl-CoA reductases (BCRs) and acetylene hydratases (AHs), whereas others may incorporate either Wco or molybdenum cofactor (Moco), such as formate dehydrogenases, formylmethanofuran dehydrogenases or nitrate reductases. The obligately tungsten-dependent enzymes catalyze rather unusual reactions such as ones with extremely low-potential electron transfers (AOR, BCR) or an unusual hydration reaction (AH). In recent years, insights into the structure and function of many tungstoenzymes have been obtained. Though specific and unspecific ABC transporter uptake systems have been described for tungstate and molybdate, only little is known about further discriminative steps in Moco and Wco biosynthesis. In bacteria producing Moco- and Wco-containing enzymes simultaneously, paralogous isoforms of the metal insertase MoeA may be specifically involved in the molybdenum- and tungsten-insertion into MPT, and in targeting Moco or Wco to their respective apo-enzymes. Wco-containing enzymes are of emerging biotechnological interest for a number of applications such as the biocatalytic reduction of CO2, carboxylic acids and aromatic compounds, or the conversion of acetylene to acetaldehyde.

The biosynthesis of the molybdenum cofactors in Escherichia coli

The biosynthesis of the molybdenum cofactor (Moco) is highly conserved among all kingdoms of life. In all molybdoenzymes containing Moco, the molybdenum atom is coordinated to a dithiolene group present in the pterin-based 6-alkyl side chain of molybdopterin (MPT). In general, the biosynthesis of Moco can be divided into four steps in in bacteria: (i) the starting point is the formation of the cyclic pyranopterin monophosphate (cPMP) from 5'-GTP, (ii) in the second step the two sulfur atoms are inserted into cPMP leading to the formation of MPT, (iii) in the third step the molybdenum atom is inserted into MPT to form Moco and (iv) in the fourth step bis-Mo-MPT is formed and an additional modification of Moco is possible with the attachment of a nucleotide (CMP or GMP) to the phosphate group of MPT, forming the dinucleotide variants of Moco. This review presents an update on the well-characterized Moco biosynthesis in the model organism Escherichia coli including novel discoveries from the recent years.

Coordinated regulation of nitrogen fixation and molybdate transport by molybdenum

Biological nitrogen fixation, the reduction of chemically inert dinitrogen to bioavailable ammonia, is a central process in the global nitrogen cycle highly relevant for life on earth. N₂ reduction to NH₃ is catalyzed by nitrogenases exclusively synthesized by diazotrophic prokaryotes. All diazotrophs have a molybdenum nitrogenase containing the unique iron-molybdenum cofactor FeMoco. In addition, some diazotrophs encode one or two alternative Mo-free nitrogenases that are less efficient at reducing N₂ than Mo-nitrogenase. To permit biogenesis of Mo-nitrogenase and other molybdoenzymes when Mo is scarce, bacteria synthesize the high-affinity molybdate transporter ModABC. Generally, Mo supports expression of Mo-nitrogenase genes, while it represses production of Mo-free nitrogenases and ModABC. Since all three nitrogenases and ModABC can reach very high levels at suitable Mo concentrations, tight Mo-mediated control saves considerable resources and energy. This review outlines the similarities and differences in Mo-responsive regulation of nitrogen fixation and molybdate transport in diverse diazotrophs.

Modulating the Molybdenum Coordination Sphere of Escherichia coli Trimethylamine N-Oxide Reductase

The well-studied enterobacterium Escherichia coli present in the human gut can reduce trimethylamine N-oxide (TMAO) to trimethylamine during anaerobic respiration. The TMAO reductase TorA is a monomeric, bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor-containing enzyme that belongs to the dimethyl sulfoxide reductase family of molybdoenzymes. We report on a system for the in vitro reconstitution of TorA with molybdenum cofactors (Moco) from different sources. Higher TMAO reductase activities for TorA were obtained when using Moco sources containing a sulfido ligand at the molybdenum atom. For the first time, we were able to isolate functional bis-MGD from Rhodobacter capsulatus formate dehydrogenase (FDH), which remained intact in its isolated state and after insertion into apo-TorA yielded a highly active enzyme. Combined characterizations of the reconstituted TorA enzymes by electron paramagnetic resonance spectroscopy and direct electrochemistry emphasize that TorA activity can be modified by changes in the Mo coordination sphere. The combination of these results together with studies of amino acid exchanges at the active site led us to propose a novel model for binding of the substrate to the molybdenum atom of TorA.

Bacterial molybdoenzymes: old enzymes for new purposes

Molybdoenzymes are widespread in eukaryotic and prokaryotic organisms where they play crucial functions in detoxification reactions in the metabolism of humans and bacteria, in nitrate assimilation in plants and in anaerobic respiration in bacteria. To be fully active, these enzymes require complex molybdenum-containing cofactors, which are inserted into the apoenzymes after folding. For almost all the bacterial molybdoenzymes, molybdenum cofactor insertion requires the involvement of specific chaperones. In this review, an overview on the molybdenum cofactor biosynthetic pathway is given together with the role of specific chaperones dedicated for molybdenum cofactor insertion and maturation. Many bacteria are involved in geochemical cycles on earth and therefore have an environmental impact. The roles of molybdoenzymes in bioremediation and for environmental applications are presented.

Biosynthesis and Insertion of the Molybdenum Cofactor

The transition element molybdenum (Mo) is of primordial importance for biological systems, because it is required by enzymes catalyzing key reactions in the global carbon, sulfur, and nitrogen metabolism. To gain biological activity, Mo has to be complexed by a special cofactor. With the exception of bacterial nitrogenase, all Mo-dependent enzymes contain a unique pyranopterin-based cofactor coordinating a Mo atom at their catalytic site. Various types of reactions are catalyzed by Mo-enzymes in prokaryotes including oxygen atom transfer, sulfur or proton transfer, hydroxylation, or even nonredox reactions. Mo-enzymes are widespread in prokaryotes and many of them were likely present in the Last Universal Common Ancestor. To date, more than 50--mostly bacterial--Mo-enzymes are described in nature. In a few eubacteria and in many archaea, Mo is replaced by tungsten bound to the same unique pyranopterin. How Mo-cofactor is synthesized in bacteria is reviewed as well as the way until its insertion into apo-Mo-enzymes.

The role of FeS clusters for molybdenum cofactor biosynthesis and molybdoenzymes in bacteria

The biosynthesis of the molybdenum cofactor (Moco) has been intensively studied, in addition to its insertion into molybdoenzymes. In particular, a link between the assembly of molybdoenzymes and the biosynthesis of FeS clusters has been identified in the recent years: 1) the synthesis of the first intermediate in Moco biosynthesis requires an FeS-cluster containing protein, 2) the sulfurtransferase for the dithiolene group in Moco is also involved in the synthesis of FeS clusters, thiamin and thiolated tRNAs, 3) the addition of a sulfido-ligand to the molybdenum atom in the active site additionally involves a sulfurtransferase, and 4) most molybdoenzymes in bacteria require FeS clusters as redox active cofactors. In this review we will focus on the biosynthesis of the molybdenum cofactor in bacteria, its modification and insertion into molybdoenzymes, with an emphasis to its link to FeS cluster biosynthesis and sulfur transfer.

The biosynthesis of the molybdenum cofactors

The biosynthesis of the molybdenum cofactors (Moco) is an ancient, ubiquitous, and highly conserved pathway leading to the biochemical activation of molybdenum. Moco is the essential component of a group of redox enzymes, which are diverse in terms of their phylogenetic distribution and their architectures, both at the overall level and in their catalytic geometry. A wide variety of transformations are catalyzed by these enzymes at carbon, sulfur and nitrogen atoms, which include the transfer of an oxo group or two electrons to or from the substrate. More than 50 molybdoenzymes were identified to date. In all molybdoenzymes except nitrogenase, molybdenum is coordinated to a dithiolene group on the 6-alkyl side chain of a pterin called molybdopterin (MPT). The biosynthesis of Moco can be divided into three general steps, with a fourth one present only in bacteria and archaea: (1) formation of the cyclic pyranopterin monophosphate, (2) formation of MPT, (3) insertion of molybdenum into molybdopterin to form Moco, and (4) additional modification of Moco in bacteria with the attachment of a nucleotide to the phosphate group of MPT, forming the dinucleotide variant of Moco. This review will focus on the biosynthesis of Moco in bacteria, humans and plants.

The Biosynthesis of the Molybdenum Cofactor in Escherichia coli and Its Connection to FeS Cluster Assembly and the Thiolation of tRNA

The thiolation of biomolecules is a complex process that involves the activation of sulfur. The L-cysteine desulfurase IscS is the main sulfur mobilizing protein in Escherichia coli that provides the sulfur from L-cysteine to several important biomolecules in the cell such as iron sulfur (FeS) clusters, molybdopterin (MPT), thiamine, and thionucleosides of tRNA. Various proteins mediate the transfer of sulfur from IscS to various biomolecules using different interaction partners. A direct connection between the sulfur-containing molecules FeS clusters, thiolated tRNA, and the molybdenum cofactor (Moco) has been identified. The first step of Moco biosynthesis involves the conversion of 5′GTP to cyclic pyranopterin monophosphate (cPMP), a reaction catalyzed by a FeS cluster containing protein. Formed cPMP is further converted to MPT by insertion of two sulfur atoms. The sulfur for this reaction is provided by the L-cysteine desulfurase IscS in addition to the involvement of the TusA protein. TusA is also involved in the sulfur transfer for the thiolation of tRNA. This review will describe the biosynthesis of Moco in E. coli in detail and dissects the sulfur transfer pathways for Moco and tRNA and their connection to FeS cluster biosynthesis.

Biochemical, stabilization and crystallization studies on a molecular chaperone (PaoD) involved in the maturation of molybdoenzymes

Molybdenum and tungsten enzymes require specific chaperones for folding and cofactor insertion. PaoD is the chaperone of the periplasmic aldehyde oxidoreductase PaoABC. It is the last gene in the paoABCD operon in Escherichia coli and its presence is crucial for obtaining mature enzyme. PaoD is an unstable, 35 kDa, protein. Our biochemical studies showed that it is a dimer in solution with a tendency to form large aggregates, especially after freezing/thawing cycles. In order to improve stability, PaoD was thawed in the presence of two ionic liquids [C4mim]Cl and [C2OHmim]PF6 and no protein precipitation was observed. This allowed protein concentration and crystallization using polyethylene glycol or ammonium sulfate as precipitating agents. Saturation transfer difference - nuclear magnetic resonance (STD-NMR) experiments have also been performed in order to investigate the effect of the ionic liquids in the stabilization process, showing a clear interaction between the acidic ring protons of the cation and, most likely, negatively charged residues at the protein surface. DLS assays also show a reduction of the overall size of the protein aggregates in presence of ionic liquids. Furthermore, cofactor binding studies on PaoD showed that the protein is able to discriminate between molybdenum and tungsten bound to the molybdenum cofactor, since only a Mo-MPT form of the cofactor remained bound to PaoD.

The chaperone FdsC for Rhodobacter capsulatus formate dehydrogenase binds the bis-molybdopterin guanine dinucleotide cofactor

Molybdoenzymes are complex enzymes in which the molybdenum cofactor (Moco) is deeply buried in the enzyme. Most molybdoenzymes contain a specific chaperone for the insertion of Moco. For the formate dehydrogenase FdsGBA from Rhodobacter capsulatus the two chaperones FdsC and FdsD were identified to be essential for enzyme activity, but are not a subunit of the mature enzyme. Here, we purified and characterized the FdsC protein after heterologous expression in Escherichia coli. We were able to copurify FdsC with the bound Moco derivate bis-molybdopterin guanine dinucleotide. This cofactor successfully was used as a source to reconstitute the activity of molybdoenzymes.

Molybdenum enzymes, their maturation and molybdenum cofactor biosynthesis in Escherichia coli

Molybdenum cofactor (Moco) biosynthesis is an ancient, ubiquitous, and highly conserved pathway leading to the biochemical activation of molybdenum. Moco is the essential component of a group of redox enzymes, which are diverse in terms of their phylogenetic distribution and their architectures, both at the overall level and in their catalytic geometry. A wide variety of transformations are catalyzed by these enzymes at carbon, sulfur and nitrogen atoms, which include the transfer of an oxo group or two electrons to or from the substrate. More than 50 molybdoenzymes were identified in bacteria to date. In molybdoenzymes Mo is coordinated to a dithiolene group on the 6-alkyl side chain of a pterin called molybdopterin (MPT). The biosynthesis of Moco can be divided into four general steps in bacteria: 1) formation of the cyclic pyranopterin monophosphate, 2) formation of MPT, 3) insertion of molybdenum into molybdopterin to form Moco, and 4) additional modification of Moco with the attachment of GMP or CMP to the phosphate group of MPT, forming the dinucleotide variant of Moco. This review will focus on molybdoenzymes, the biosynthesis of Moco, and its incorporation into specific target proteins focusing on Escherichia coli. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Prokaryotic assembly factors for the attachment of flavin to complex II

Complex II (also known as Succinate dehydrogenase or Succinate-ubiquinone oxidoreductase) is an important respiratory enzyme that participates in both the tricarboxylic acid cycle and electron transport chain. Complex II consists of four subunits including a catalytic flavoprotein (SdhA), an iron-sulphur subunit (SdhB) and two hydrophobic membrane anchors (SdhC and SdhD). Complex II also contains a number of redox cofactors including haem, Fe-S clusters and FAD, which mediate electron transfer from succinate oxidation to the reduction of the mobile electron carrier ubiquinone. The flavin cofactor FAD is an important redox cofactor found in many proteins that participate in oxidation/reduction reactions. FAD is predominantly bound non-covalently to flavoproteins, with only a small percentage of flavoproteins, such as complex II, binding FAD covalently. Aside from a few examples, the mechanisms of flavin attachment have been a relatively unexplored area. This review will discuss the FAD cofactor and the mechanisms used by flavoproteins to covalently bind FAD. Particular focus is placed on the attachment of FAD to complex II with an emphasis on SdhE (a DUF339/SDH5 protein previously termed YgfY), the first protein identified as an assembly factor for FAD attachment to flavoproteins in prokaryotes. The molecular details of SdhE-dependent flavinylation of complex II are discussed and comparisons are made to known cofactor chaperones. Furthermore, an evolutionary hypothesis is proposed to explain the distribution of SdhE homologues in bacterial and eukaryotic species. Mechanisms for regulating SdhE function and how this may be linked to complex II function in different bacterial species are also discussed. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

The History of the Discovery of the Molybdenum Cofactor and Novel Aspects of its Biosynthesis in Bacteria

Biosynthesis of the molybdenum cofactor in bacteria is described with a detailed analysis of each individual reaction leading to the formation of stable intermediates during the synthesis of molybdopterin from GTP. As a starting point, the discovery of molybdopterin and the elucidation of its structure through the study of stable degradation products are described. Subsequent to molybdopterin synthesis, the molybdenum atom is added to the molybdopterin dithiolene group to form the molybdenum cofactor. This cofactor is either inserted directly into specific molybdoenzymes or is further modified by the addition of nucleotides to the molybdopterin phosphate group or the replacement of ligands at the molybdenum center.

Homologous gene clusters of nicotine catabolism, including a new ω-amidase for α-ketoglutaramate, in species of three genera of Gram-positive bacteria

Gram-positive soil bacteria Arthrobacter nicotinovorans, Nocardioides sp. JS614 and Rhodococcus opacus were shown to contain similarly organized clusters of homologous genes for nicotine catabolism. An uncharacterized gene of a predicted nitrilase within these gene clusters was cloned from A. nicotinovorans and biochemical data unexpectedly showed that the protein exhibited ω-amidase activity toward α-ketoglutaramate. Structural modelling of the protein suggested the presence of the catalytic triad Cys-Glu-Lys, characteristic of this class of enzymes, and supported α-ketoglutaramate as substrate. A-ketoglutaramate could be generated by hydrolytic cleavage of the C-N bond of the trihydroxypyridine ring produced by nicotine catabolism in these bacteria. This ω-amidase, together with glutamate dehydrogenase, may form a physiologically relevant enzyme couple, leading to transformation of metabolically inert α-ketoglutaramate derived from trihydroxypyridine into glutamate, a central compound of nitrogen metabolism.

The role of system-specific molecular chaperones in the maturation of molybdoenzymes in bacteria

Biogenesis of prokaryotic molybdoenzymes is a complex process with the final step representing the insertion of a matured molybdenum cofactor (Moco) into a folded apoenzyme. Usually, specific chaperones of the XdhC family are required for the maturation of molybdoenzymes of the xanthine oxidase family in bacteria. Enzymes of the xanthine oxidase family are characterized to contain an equatorial sulfur ligand at the molybdenum center of Moco. This sulfur ligand is inserted into Moco while bound to the XdhC-like protein and before its insertion into the target enzyme. In addition, enzymes of the xanthine oxidase family bind either the molybdopterin (Mo-MPT) form of Moco or the modified molybdopterin cytosine dinucleotide cofactor (MCD). In both cases, only the matured cofactor is inserted by a proofreading process of XdhC. The roles of these specific XdhC-like chaperones during the biogenesis of enzymes of the xanthine oxidase family in bacteria are described.

The Tat Protein Export Pathway

Proteins that reside partially or completely outside the bacterial cytoplasm require specialized pathways to facilitate their localization. Globular proteins that function in the periplasm must be translocated across the hydrophobic barrier of the inner membrane. While the Sec pathway transports proteins in a predominantly unfolded conformation, the Tat pathway exports folded protein substrates. Protein transport by the Tat machinery is powered solely by the transmembrane proton gradient, and there is no requirement for nucleotide triphosphate hydrolysis. Proteins are targeted to the Tat machinery by N-terminal signal peptides that contain a consensus twin arginine motif. In Escherichia coli and Salmonella there are approximately thirty proteins with twin arginine signal peptides that are transported by the Tat pathway. The majority of these bind complex redox cofactors such as iron sulfur clusters or the molybdopterin cofactor. Here we describe what is known about Tat substrates in E. coli and Salmonella, the function and mechanism of Tat protein export, and how the cofactor insertion step is coordinated to ensure that only correctly assembled substrates are targeted to the Tat machinery.

Specific interactions between four molybdenum-binding proteins contribute to Mo-dependent gene regulation in Rhodobacter capsulatus

The phototrophic purple bacterium Rhodobacter capsulatus encodes two transcriptional regulators, MopA and MopB, with partially overlapping and specific functions in molybdate-dependent gene regulation. Both MopA and MopB consist of an N-terminal DNA-binding helix-turn-helix domain and a C-terminal molybdate-binding di-MOP domain. They formed homodimers as apo-proteins and in the molybdate-bound state as shown by yeast two-hybrid (Y2H) studies, glutaraldehyde cross-linking, gel filtration chromatography, and copurification experiments. Y2H studies suggested that both the DNA-binding and the molybdate-binding domains contribute to dimer formation. Analysis of molybdate binding to MopA and MopB revealed a binding stoichiometry of four molybdate oxyanions per homodimer. Specific interaction partners of MopA and MopB were the molybdate transporter ATPase ModC and the molbindin-like Mop protein, respectively. Like other molbindins, the R. capsulatus Mop protein formed hexamers, which were stabilized by binding of six molybdate oxyanions per hexamer. Heteromer formation of MopA and MopB was shown by Y2H studies and copurification experiments. Reporter gene activity of a strictly MopA-dependent mop-lacZ fusion in mutant strains defective for either mopA, mopB, or both suggested that MopB negatively modulates expression of the mop promoter. We propose that depletion of the active MopA homodimer pool by formation of MopA-MopB heteromers might represent a fine-tuning mechanism controlling mop gene expression.

Site directed mutagenesis of amino acid residues at the active site of mouse aldehyde oxidase AOX1

Mouse aldehyde oxidase (mAOX1) forms a homodimer and belongs to the xanthine oxidase family of molybdoenzymes which are characterized by an essential equatorial sulfur ligand coordinated to the molybdenum atom. In general, mammalian AOs are characterized by broad substrate specificity and an yet obscure physiological function. To define the physiological substrates and the enzymatic characteristics of mAOX1, we established a system for the heterologous expression of the enzyme in Eschericia coli. The recombinant protein showed spectral features and a range of substrate specificity similar to the native protein purified from mouse liver. The EPR data of recombinant mAOX1 were similar to those of AO from rabbit liver, but differed from the homologous xanthine oxidoreductase enzymes. Site-directed mutagenesis of amino acids Val806, Met884 and Glu1265 at the active site resulted in a drastic decrease in the oxidation of aldehydes with no increase in the oxidation of purine substrates. The double mutant V806E/M884R and the single mutant E1265Q were catalytically inactive enzymes regardless of the aldehyde or purine substrates tested. Our results show that only Glu1265 is essential for the catalytic activity by initiating the base-catalyzed mechanism of substrate oxidation. In addition, it is concluded that the substrate specificity of molybdo-flavoenzymes is more complex and not only defined by the three characterized amino acids in the active site.

The CoxD protein of Oligotropha carboxidovorans is a predicted AAA+ ATPase chaperone involved in the biogenesis of the CO dehydrogenase [CuSMoO2] cluster

CO dehydrogenase from the Gram-negative chemolithoautotrophic eubacterium Oligotropha carboxidovorans OM5 is a structurally characterized molybdenum-containing iron-sulfur flavoenzyme, which catalyzes the oxidation of CO (CO + H(2)O --> CO(2) + 2e(-) + 2H(+)). It accommodates in its active site a unique bimetallic [CuSMoO(2)] cluster, which is subject to post-translational maturation. Insertional mutagenesis of coxD has established its requirement for the assembly of the [CuSMoO(2)] cluster. Disruption of coxD led to a phenotype of the corresponding mutant OM5 D::km with the following characteristics: (i) It was impaired in the utilization of CO, whereas the utilization of H(2) plus CO(2) was not affected; (ii) Under appropriate induction conditions bacteria synthesized a fully assembled apo-CO dehydrogenase, which could not oxidize CO; (iii) Apo-CO dehydrogenase contained a [MoO(3)] site in place of the [CuSMoO(2)] cluster; and (iv) Employing sodium sulfide first and then the Cu(I)-(thiourea)(3) complex, the non-catalytic [MoO(3)] site could be reconstituted in vitro to a [CuSMoO(2)] cluster capable of oxidizing CO. Sequence information suggests that CoxD is a MoxR-like AAA+ ATPase chaperone related to the hexameric, ring-shaped BchI component of Mg(2+)-chelatases. Recombinant CoxD, which appeared in Escherichia coli in inclusion bodies, occurs exclusively in cytoplasmic membranes of O. carboxidovorans grown in the presence of CO, and its occurrence coincided with GTPase activity upon sucrose density gradient centrifugation of cell extracts. The presumed function of CoxD is the partial unfolding of apo-CO dehydrogenase to assist in the stepwise introduction of sulfur and copper in the [MoO(3)] center of the enzyme.

The mechanism of assembly and cofactor insertion into Rhodobacter capsulatus xanthine dehydrogenase

Rhodobacter capsulatus xanthine dehydrogenase (XDH) is a molybdo-flavoprotein that is highly homologous to the homodimeric mammalian xanthine oxidoreductase. However, the bacterial enzyme has an (alphabeta)(2) heterotetrameric structure, and the cofactors were identified to be located on two different polypeptides. We have analyzed the mechanism of cofactor insertion and subunit assembly of R. capsulatus XDH, using engineered subunits with appropriate substitutions in the interfaces. In an (alphabeta) heterodimeric XDH containing the XdhA and XdhB subunits, the molybdenum cofactor (Moco) was shown to be absent, indicating that dimerization of the (alphabeta) subunits has to precede Moco insertion. In an (alphabeta)(2) XDH heterotetramer variant, including only one active Moco-center, the active (alphabeta) site of the chimeric enzyme was shown to be fully active, revealing that the two subunits act independent without cooperativity. Amino acid substitutions at two cysteine residues coordinating FeSI of the two [2Fe-2S] clusters of the enzyme demonstrate that an incomplete assembly of FeSI impairs the formation of the XDH (alphabeta)(2) heterotetramer and, thus, insertion of Moco into the enzyme. The results reveal that the insertion of the different redox centers into R. capsulatus XDH takes place sequentially. Dimerization of two (alphabeta) dimers is necessary for insertion of sulfurated Moco into apo-XDH, the last step of XDH maturation.