The tetrahydropyranopterin structure of the sulfur-free and metal-free molybdenum cofactor precursor

The molybdenum cofactor (Moco), a highly conserved pterin compound coordinating molybdenum (Mo), is required for the activity of all Mo-dependent enzymes with the exception of nitrogenase. Moco is synthesized by a unique and evolutionary old multi-step pathway with two intermediates identified so far, the sulfur-free and metal-free pterin derivative precursor Z and molybdopterin, a pterin with an enedithiolate function essential for Mo ligation. The latter pterin component is believed to form a tetrahydropyranopterin similar to the one found for Moco in the crystal structure of Mo as well as tungsten (W) enzymes. Here we report the spectroscopic characterization and structure elucidation of precursor Z purified from Escherichia coli overproducing MoaA and MoaC, two proteins essential for bacterial precursor Z synthesis. We have shown that purified precursor Z is as active as precursor Z present in E. coli cell extracts, demonstrating that no modifications during the purification procedure have occurred. High resolution electrospray ionization mass spectrometry afforded a [M + H]+ ion compatible with a molecular formula of C10H15N5O8P. Consequently 1H NMR spectroscopy not allowed structural characterization of the molecule but confirmed that this intermediate undergoes direct oxidation to the previously well characterized non-productive follow-up product compound Z. The 1H chemical shift and coupling constant data are incompatible with previous structural proposals and indicate that precursor Z already is a tetrahydropyranopterin system and carries a geminal diol function in the C1' position.     

Crystal structure of the gephyrin-related molybdenum cofactor biosynthesis protein MogA from Escherichia coli

Molybdenum cofactor (Moco) biosynthesis is an evolutionarily conserved pathway in archaea, eubacteria, and eukaryotes, including humans. Genetic deficiencies of enzymes involved in this biosynthetic pathway trigger an autosomal recessive disease with severe neurological symptoms, which usually leads to death in early childhood. The MogA protein exhibits affinity for molybdopterin, the organic component of Moco, and has been proposed to act as a molybdochelatase incorporating molybdenum into Moco. MogA is related to the protein gephyrin, which, in addition to its role in Moco biosynthesis, is also responsible for anchoring glycinergic receptors to the cytoskeleton at inhibitory synapses. The high resolution crystal structure of the Escherichia coli MogA protein has been determined, and it reveals a trimeric arrangement in which each monomer contains a central, mostly parallel beta-sheet surrounded by alpha-helices on either side. Based on structural and biochemical data, a putative active site was identified, including two residues that are essential for the catalytic mechanism.     

Structural Insights into Putative Molybdenum Cofactor Biosynthesis Protein C (MoaC2) from Mycobacterium tuberculosis H37Rv

The Molybdenum cofactor (Moco) biosynthesis pathway is an evolutionary conserved pathway seen in almost all eukaryotes including the pathogenic species Mycobacterium tuberculosis. This pathway comprises of several novel reactions which include the initial formation of precursor Z from guanosine triphosphate (GTP), catalysed by two enzymes MoaA and MoaC. Although Moco biosynthesis is well understood, the first step is still not clear. In M. tuberculosis H37Rv, three orthologous genes of MoaC have been annotated: moaC1 (Rv3111), moaC2 (Rv0864) and moaC3 (Rv3324c). Rv0864 (MoaC2) is a 17.5 kDa protein and is reported to be down-regulated by ∼3 times in the nutrient starvation model for Mycobacterium tuberculosis. The crystal structure of Moco-biosynthesis protein MoaC2 from Mycobacterium tuberculosis (2.20 Å resolution, space group P2₁3) has been determined. Based on a comparative analysis of structures of homologous proteins, conserved residues were identified and are implicated in structural and functional roles. Molecular docking studies with probable ligands carried out in order to identify its ligand, suggests that pteridinebenzomonophosphate as the most likely ligand. Sequence based interaction study identified MoaA1 to interact with MoaC2. A homology model of MoaA1 was then complexed with MoaC2 and protein–protein interactions are also discussed.     

Function and structure of the molybdenum cofactor carrier protein from Chlamydomonas reinhardtii

The molybdenum cofactor (Moco) forms the catalytic site in all eukaryotic molybdenum enzymes and is synthesized by a multistep biosynthetic pathway. The mechanism of transfer, storage, and insertion of Moco into the appropriate apo-enzyme is poorly understood. In Chlamydomonas reinhardtii, a Moco carrier protein (MCP) has been identified and characterized recently. Here we show biochemical evidence that MCP binds Moco as well as the tungstate-substituted form of the cofactor (Wco) with high affinity, whereas molybdopterin, the ultimate cofactor precursor, is not bound. This binding selectivity points to a specific metal-mediated interaction with MCP, which protects Moco and Wco from oxidation with t((1/2)) of 24 and 96 h, respectively. UV-visible spectroscopy showed defined absorption bands at 393, 470, and 570 nm pointing to ene-diothiolate and protein side-chain charge transfer bonds with molybdenum. We have determined the crystal structure of MCP at 1.6 Angstrom resolution using seleno-methionated and native protein. The monomer constitutes a Rossmann fold with two homodimers forming a symmetrical tetramer in solution. Based on conserved surface residues, charge distribution, shape, in silico docking studies, structural comparisons, and identification of an anionbinding site, a prominent surface depression was proposed as a Moco-binding site, which was confirmed by structure-guided mutagenesis coupled to substrate binding studies.     

Identification of persulfide-binding and disulfide-forming cysteine residues in the NifS-like domain of the molybdenum cofactor sulfurase ABA3 by cysteine-scanning mutagenesis

The Moco (molybdenum cofactor) sulfurase ABA3 from Arabidopsis thaliana catalyses the sulfuration of the Moco of aldehyde oxidase and xanthine oxidoreductase, which represents the final activation step of these enzymes. ABA3 consists of an N-terminal NifS-like domain that exhibits L-cysteine desulfurase activity and a C-terminal domain that binds sulfurated Moco. The strictly conserved Cys430 in the NifS-like domain binds a persulfide intermediate, which is abstracted from the substrate L-cysteine and finally needs to be transferred to the Moco of aldehyde oxidase and xanthine oxidoreductase. In addition to Cys?3?, another eight cysteine residues are located in the NifS-like domain, with two of them being highly conserved among Moco sulfurase proteins and, at the same time, being in close proximity to Cys?3?. By determination of the number of surface-exposed cysteine residues and the number of persulfide-binding cysteine residues in combination with the sequential substitution of each of the nine cysteine residues, a second persulfide-binding cysteine residue, Cys2??, was identified. Furthermore, the active-site Cys?3? was found to be located on top of a loop structure, formed by the two flanking residues Cys?2? and Cys?3?, which are likely to form an intramolecular disulfide bridge. These findings are confirmed by a structural model of the NifS-like domain, which indicates that Cys?2? and Cys?3? are within disulfide bond distance and that a persulfide transfer from Cys?3? to Cys2?? is indeed possible.     

Splice-specific functions of gephyrin in molybdenum cofactor biosynthesis

Gephyrin is a multifunctional protein involved in the clustering of inhibitory neuroreceptors. In addition, gephyrin catalyzes the last step in molybdenum cofactor (Moco) biosynthesis essential for the activities of Mo-dependent enzymes such as sulfite oxidase and xanthine oxidoreductase. Functional complexity and diversity of gephyrin is believed to be regulated by alternative splicing in a tissue-specific manner. Here, we investigated eight gephyrin variants with combinations of seven alternatively spliced exons located in the N-terminal G domain, the central domain, and the C-terminal E domain. Their activity in Moco synthesis was analyzed in vivo by reconstitution of gephyrin-deficient L929 cells, which were found to be defective in the G domain of gephyrin. Individual domain functions were assayed in addition and confirmed that variants containing either an additional C5 cassette or missing the C6 cassette are inactive in Moco synthesis. In contrast, different alterations within the central domain retained the Moco synthetic activity of gephyrin. The recombinant gephyrin G domain containing the C5 cassette forms dimers in solution, binds molybdopterin, but is unable to catalyze molybdopterin (MPT) adenylylation. Determination of Moco and MPT content in different tissues showed that besides liver and kidney, brain was capable of synthesizing Moco most efficiently. Subsequent analysis of cultured neurons and glia cells demonstrated glial Moco synthesis due to the expression of gephyrins containing the cassettes C2 and C6 with and without C3.1.     

A widespread riboswitch candidate that controls bacterial genes involved in molybdenum cofactor and tungsten cofactor metabolism

We have identified a highly conserved RNA motif located upstream of genes encoding molybdate transporters, molybdenum cofactor (Moco) biosynthesis enzymes, and proteins that utilize Moco as a coenzyme. Bioinformatics searches have identified 176 representatives in gamma-Proteobacteria, delta-Proteobacteria, Clostridia, Actinobacteria, Deinococcus-Thermus species and DNAs from environmental samples. Using genetic assays, we demonstrate that a Moco RNA in Escherichia coli associated with the Moco biosynthetic operon controls gene expression in response to Moco production. In addition, we provide evidence indicating that this conserved RNA discriminates against closely related analogues of Moco. These results, together with extensive phylogenetic conservation and typical gene control structures near some examples, indicate that representatives of this structured RNA represent a novel class of riboswitches that sense Moco. Furthermore, we identify variants of this RNA that are likely to be triggered by the related tungsten cofactor (Tuco), which carries tungsten in place of molybdenum as the metal constituent.     

Rhodobacter capsulatus XdhC is involved in molybdenum cofactor binding and insertion into xanthine dehydrogenase

Rhodobacter capsulatus xanthine dehydrogenase (XDH) is a cytoplasmic enzyme with an (alphabeta)2 heterodimeric structure that is highly identical to homodimeric eukaryotic xanthine oxidoreductases. The crystal structure revealed that the molybdenum cofactor (Moco) is deeply buried within the protein. A protein involved in Moco insertion and XDH maturation has been identified, which was designated XdhC. XdhC was shown to be essential for the production of active XDH but is not a subunit of the purified enzyme. Here we describe the purification of XdhC and the detailed characterization of its role for XDH maturation. We could show that XdhC binds Moco in stoichiometric amounts, which subsequently can be inserted into Moco-free apo-XDH. A specific interaction between XdhC and XdhB was identified. We show that XdhC is required for the stabilization of the sulfurated form of Moco present in enzymes of the xanthine oxidase family. Our findings imply that enzyme-specific proteins exist for the biogenesis of molybdoenzymes, coordinating Moco binding and insertion into their respective target proteins. So far, the requirement of such proteins for molybdoenzyme maturation has been described only for prokaryotes.     

Molybdoenzymes and molybdenum cofactor in plants

The transition element molybdenum (Mo) is essential for (nearly) all organisms and occurs in more than 40 enzymes catalysing diverse redox reactions, however, only four of them have been found in plants. (1) Nitrate reductase catalyses the key step in inorganic nitrogen assimilation, (2) aldehyde oxidase(s) have been shown to catalyse the last step in the biosynthesis of the phytohormone abscisic acid, (3) xanthine dehydrogenase is involved in purine catabolism and stress reactions, and (4) sulphite oxidase is probably involved in detoxifying excess sulphite. Among Mo-enzymes, the alignment of amino acid sequences permits domains that are well conserved to be defined. With the exception of bacterial nitrogenase, Mo-enzymes share a similar pterin compound at their catalytic sites, the molybdenum cofactor. Mo itself seems to be biologically inactive unless it is complexed by the cofactor. This molybdenum cofactor combines with diverse apoproteins where it is responsible for the correct anchoring and positioning of the Mo-centre within the holo-enzyme so that the Mo-centre can interact with other components of the enzyme's electron transport chain. A model for the three-step biosynthesis of Moco involving the complex interaction of six proteins will be described. A putative Moco-storage protein distributing Moco to the apoproteins of Mo-enzymes will be discussed. After insertion, xanthine dehydrogenase and aldehyde oxidase, but not nitrate reductase and sulphite oxidase, require the addition of a terminal sulphur ligand to their Mo-site, which is catalysed by the sulphur transferase ABA3.     

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

The molybdenum cofactor (Moco) exists in different variants in the cell and can be directly inserted into molybdoenzymes utilizing the molybdopterin (MPT) form of Moco. In bacteria such as Rhodobacter capsulatus and Escherichia coli, MPT is further modified by attachment of a GMP nucleotide, forming MPT guanine dinucleotide (MGD). In this work, we analyzed the distribution and targeting of different forms of Moco to their respective user enzymes by proteins that bind Moco and are involved in its further modification. The R. capsulatus proteins MogA, MoeA, MobA, and XdhC were purified, and their specific interactions were analyzed. Interactions between the protein pairs MogA-MoeA, MoeA-XdhC, MoeA-MobA, and XdhC-MobA were identified by surface plasmon resonance measurements. In addition, the transfer of Moco produced by the MogA-MoeA complex to XdhC was investigated. A direct competition of MobA and XdhC for Moco binding was determined. In vitro analyses showed that XdhC bound to MobA, prevented the binding of Moco to MobA, and thereby inhibited MGD biosynthesis. The data were confirmed by in vivo studies in R. capsulatus cells showing that overproduction of XdhC resulted in a 50% decrease in the activity of bis-MGD-containing Me(2)SO reductase. We propose that, in bacteria, the distribution of Moco in the cell and targeting to the respective user enzymes are accomplished by specific proteins involved in Moco binding and modification.     

Elucidation of the dual role of Mycobacterial MoeZR in molybdenum cofactor biosynthesis and cysteine biosynthesis

The pathway of molybdenum cofactor biosynthesis has been studied in detail by using proteins from Mycobacterium species, which contain several homologs associated with the first steps of Moco biosynthesis. While all Mycobacteria species contain a MoeZR, only some strains have acquired an additional homolog, MoeBR, by horizontal gene transfer. The role of MoeBR and MoeZR was studied in detail for the interaction with the two MoaD-homologs involved in Moco biosynthesis, MoaD1 and MoaD2, in addition to the CysO protein involved in cysteine biosynthesis. We show that both proteins have a role in Moco biosynthesis, while only MoeZR, but not MoeBR, has an additional role in cysteine biosynthesis. MoeZR and MoeBR were able to complement an E. coli moeB mutant strain, but only in conjunction with the Mycobacterial MoaD1 or MoaD2 proteins. Both proteins were able to sulfurate MoaD1 and MoaD2 in vivo, while only MoeZR additionally transferred the sulfur to CysO. Our in vivo studies show that Mycobacteria have acquired several homologs to maintain Moco biosynthesis. MoeZR has a dual role in Moco- and cysteine biosynthesis and is involved in the sulfuration of MoaD and CysO, whereas MoeBR only has a role in Moco biosynthesis, which is not an essential function for Mycobacteria.     

The molybdenum cofactor biosynthetic protein Cnx1 complements molybdate-repairable mutants, transfers molybdenum to the metal binding pterin, and is associated with the cytoskeleton

Molybdenum (Mo) plays an essential role in the active site of all eukaryotic Mo-containing enzymes. In plants, Mo enzymes are important for nitrate assimilation, phytohormone synthesis, and purine catabolism. Mo is bound to a unique metal binding pterin (molybdopterin [MPT]), thereby forming the active Mo cofactor (Moco), which is highly conserved in eukaryotes, eubacteria, and archaebacteria. Here, we describe the function of the two-domain protein Cnx1 from Arabidopsis in the final step of Moco biosynthesis. Cnx1 is constitutively expressed in all organs and in plants grown on different nitrogen sources. Mo-repairable cnxA mutants from Nicotiana plumbaginifolia accumulate MPT and show altered Cnx1 expression. Transformation of cnxA mutants and the corresponding Arabidopsis chl-6 mutant with cnx1 cDNA resulted in functional reconstitution of their Moco deficiency. We also identified a point mutation in the Cnx1 E domain of Arabidopsis chl-6 that causes the molybdate-repairable phenotype. Recombinant Cnx1 protein is capable of synthesizing Moco. The G domain binds and activates MPT, whereas the E domain is essential for activating Mo. In addition, Cnx1 binds to the cytoskeleton in the same way that its mammalian homolog gephyrin does in neuronal cells, which suggests a hypothetical model for anchoring the Moco-synthetic machinery by Cnx1 in plant cells.     

Crystal structure of molybdopterin synthase and its evolutionary relationship to ubiquitin activation

Molybdenum cofactor (Moco) biosynthesis is an evolutionarily conserved pathway present in eubacteria, archaea and eukaryotes, including humans. Genetic deficiencies of enzymes involved in Moco biosynthesis in humans lead to a severe and usually fatal disease. Moco contains a tricyclic pyranopterin, termed molybdopterin (MPT), that bears the cis-dithiolene group responsible for molybdenum ligation. The dithiolene group of MPT is generated by MPT synthase, which consists of a large and small subunits. The 1.45 A resolution crystal structure of MPT synthase reveals a heterotetrameric protein in which the C-terminus of each small subunit is inserted into a large subunit to form the active site. In the activated form of the enzyme this C-terminus is present as a thiocarboxylate. In the structure of a covalent complex of MPT synthase, an isopeptide bond is present between the C-terminus of the small subunit and a Lys side chain in the large subunit. The strong structural similarity between the small subunit of MPT synthase and ubiquitin provides evidence for the evolutionary antecedence of the Moco biosynthetic pathway to the ubiquitin dependent protein degradation pathway.     

The Mechanism of nucleotide-assisted molybdenum insertion into molybdopterin. A novel route toward metal cofactor assembly

The molybdenum cofactor (Moco) is synthesized by an ancient and conserved biosynthetic pathway. In plants, the two-domain protein Cnx1 catalyzes the insertion of molybdenum into molybdopterin (MPT), a metal-free phosphorylated pyranopterin carrying an ene-dithiolate. Recently, we identified a novel biosynthetic intermediate, adenylated molybdopterin (MPT-AMP), which is synthesized by the C-terminal G domain of Cnx1. Here, we show that MPT-AMP and molybdate bind in an equimolar and cooperative way to the other N-terminal E domain (Cnx1E). Tungstate and sulfate compete for molybdate, which demonstrates the presence of an anion-binding site for molybdate. Cnx1E catalyzes the Zn(2+)-/Mg(2+)-dependent hydrolysis of MPT-AMP but only when molybdate is bound as co-substrate. MPT-AMP hydrolysis resulted in stoichiometric release of Moco that was quantitatively incorporated into plant apo-sulfite oxidase. Upon Moco formation AMP is release as second product of the reaction. When comparing MPT-AMP hydrolysis with the formation of Moco and AMP a 1.5-fold difference in reaction rates were observed. Together with the strict dependence of the reaction on molybdate the formation of adenylated molybdate as reaction intermediate in the nucleotide-assisted metal transfer reaction to molybdopterin is proposed.     

Mutational analysis of Escherichia coli MoeA: two functional activities map to the active site cleft

The molybdenum cofactor is ubiquitous in nature, and the pathway for Moco biosynthesis is conserved in all three domains of life. Recent work has helped to illuminate one of the most enigmatic steps in Moco biosynthesis, ligation of metal to molybdopterin (the organic component of the cofactor) to form the active cofactor. In Escherichia coli, the MoeA protein mediates ligation of Mo to molybdopterin while the MogA protein enhances this process in an ATP-dependent manner. The X-ray crystal structures for both proteins have been previously described as well as two essential MogA residues, Asp49 and Asp82. Here we describe a detailed mutational analysis of the MoeA protein. Variants of conserved residues at the putative active site of MoeA were analyzed for a loss of function in two different, previously described assays, one employing moeA- crude extracts and the other utilizing a defined system. Oddly, no correlation was observed between the activity in the two assays. In fact, our results showed a general trend toward an inverse relationship between the activity in each assay. Moco binding studies indicated a strong correlation between a variant's ability to bind Moco and its activity in the purified component assay. Crystal structures of the functionally characterized MoeA variants revealed no major structural changes, indicating that the functional differences observed are not due to disruption of the protein structure. On the basis of these results, two different functional areas were assigned to regions at or near the MoeA active site cleft.     

Molybdenum enzymes and molybdenum cofactor in mycobacteria

When intracelluar pathogens enter the host macrophages where in addition to oxidative and antibiotic mechanisms of antimicrobial activity, nutrients are deprived. Human pathogen Mycobacterium tuberculosis is one of macrophage parasitisms, which can replicate and persist for decades in dormancy state in virulent environments. It is very successful in escaping the killing mechanisms of macrophage. Molybdenum (Mo) enzymes involve in the global carbon, sulfur, and nitrogen cycles by catalyzing important redox reactions. There are several Mo enzymes in mycobacteria and they exert several important physiological functions, such as dormancy regulation, the metabolism of energy sources, and nitrogen source. Pterin-based Mo cofactor (Moco) is the common cofactor of the Mo enzymes in mycobacteria but the cofactor biosynthesis is nearly an untapped area. The present article discusses the physiological function of Mo enzymes and the structural feature of the genes coding for Moco biosynthesis enzymes in mycobacteria.     

The crystal structure of the Escherichia coli MobA protein provides insight into molybdopterin guanine dinucleotide biosynthesis

The molybdenum cofactor (Moco) is found in a variety of enzymes present in all phyla and comprises a family of related molecules containing molybdopterin (MPT), a tricyclic pyranopterin with a cis-dithiolene group, as the invariant essential moiety. MPT biosynthesis involves a conserved pathway, but some organisms perform additional reactions that modify MPT. In eubacteria, the cofactor is often present in a dinucleotide form combining MPT and a purine or pyrimidine nucleotide via a pyrophosphate linkage. In Escherichia coli, the MobA protein links a guanosine 5'-phosphate to MPT forming molybdopterin guanine dinucleotide. This reaction requires GTP, MgCl(2), and the MPT form of the cofactor and can efficiently reconstitute Rhodobacter sphaeroides apo-DMSOR, an enzyme that requires molybdopterin guanine dinucleotide for activity. In this paper, we present the crystal structure of MobA, a protein containing 194 amino acids. The MobA monomer has an alpha/beta architecture in which the N-terminal half of the molecule adopts a Rossman fold. The structure of MobA has striking similarity to Bacillus subtilis SpsA, a nucleotide-diphospho-sugar transferase involved in sporulation. The cocrystal structure of MobA and GTP reveals that the GTP-binding site is located in the N-terminal half of the molecule. Conserved residues located primarily in three signature sequence motifs form crucial interactions with the bound nucleotide. The binding site for MPT is located adjacent to the GTP-binding site in the C-terminal half of the molecule, which contains another set of conserved residues presumably involved in MPT binding.     

Identification of a Rhodobacter capsulatus L-cysteine desulfurase that sulfurates the molybdenum cofactor when bound to XdhC and before its insertion into xanthine dehydrogenase

The molybdenum cofactor (Moco) containing enzymes aldehyde oxidase and xanthine dehydrogenase (XDH) require for activity a sulfuration step that inserts a terminal sulfur ligand into Moco. XdhC was shown to be essential for the production of active XDH in Rhodobacter capsulatus but is itself not a subunit of the purified enzyme. XdhC binds stoichiometric amounts of Moco and is further able to transfer its bound Moco to XDH. Previous work suggested that XdhC particularly stabilizes the sulfurated form of Moco before the insertion into XDH. In this work, we identify an R. capsulatus l-cysteine desulfurase, NifS4, which is involved in the formation of the Mo=S ligand of Moco. We show that NifS4 interacts with XdhC and not with XDH. NifS4 mobilizes sulfur from l-cysteine by formation of a protein-bound persulfide intermediate and transfers this sulfur further to Moco. This reaction was shown to be more effective than the chemical sulfuration of Moco using sulfide as sulfur source. Further studies clearly showed that Moco is sulfurated before the insertion into XDH, while it is bound to XdhC. Conclusively, XdhC has a versatile role in R. capsulatus: binding of Moco, interaction with NifS4 for the sulfuration of Moco, protection of sulfurated Moco from oxidation, and further transfer to XDH.     

The active site of the molybdenum cofactor biosynthetic protein domain Cnx1G

The final step of molybdenum cofactor biosynthesis in plants is catalyzed by the two-domain protein Cnx1. The G domain of Cnx1 (Cnx1G) binds molybdopterin with high affinity and transfers molybdenum to molybdopterin. Here, we describe the functional and structural characterization of structure-based Cnx1G mutants. For molybdopterin binding residues Thr542 and Ser573 were found to be important because different mutations of those residues resulted in 7- to 26-fold higher k(D) values for molybdopterin binding. Furthermore, we showed that the terminal phosphate of molybdopterin is directly involved in protein-pterin interactions as dephosphorylated molybdopterin binds with one magnitude of order lower affinity to the wild-type protein. Molybdopterin binding was not affected in mutants defective in Ser476, Asp486, or Asp515. However, molybdenum insertion was completely abolished, indicating their important role for catalysis. Based on these results we propose the binding of molybdopterin to a large depression in the structure of Cnx1G formed by beta5, alpha5, beta6, and alpha6, whereas the negatively charged depression formed by the loop between beta3 and alpha4, the N-terminal end of alpha2, the 3(10) helix, and the region between beta6 and alpha6 is involved in catalysis.     

The crystal structure of methylglyoxal synthase from Escherichia coli.

BACKGROUND: The reaction mechanism of methylglyoxal synthase (MGS) is believed to be similar to that of triosephosphate isomerase (TIM). Both enzymes utilise dihydroxyacetone phosphate (DHAP) to form an enediol(ate) phosphate intermediate as the first step of their reaction pathways. However, the second catalytic step in the MGS reaction pathway is characterized by the elimination of phosphate and collapse of the enediol(ate) to form methylglyoxal instead of reprotonation to form the isomer glyceraldehyde 3-phosphate. RESULTS: The crystal structure of MGS bound to formate and substoichiometric amounts of phosphate in the space group P6522 has been determined at 1.9 A resolution. This structure shows that the enzyme is a homohexamer composed of interacting five-stranded beta/alpha proteins, rather than the hallmark alpha/beta barrel structure of TIM. The conserved residues His19, Asp71, and His98 in each of the three monomers in the asymmetric unit bind to a formate ion that is present in the crystallization conditions. Differences in the three monomers in the asymmetric unit are localized at the mouth of the active site and can be ascribed to the presence or absence of a bound phosphate ion. CONCLUSIONS: In agreement with site-directed mutagenesis and mechanistic enzymology, the structure suggests that Asp71 acts as the catalytic base. Further, Asp20 and Asp101 are involved in intersubunit salt bridges. These salt bridges may provide a pathway for transmitting allosteric information.