Molybdenum cofactor biosynthesis in plants and humans

Viperin-like proteins interfere with RNA viruses in plants

Plant viruses cause substantial losses in crop yield and quality; therefore, devising new, robust strategies to counter viral infections has important implications for agriculture. Virus inhibitory protein endoplasmic reticulum-associated interferon-inducible (Viperin) proteins are conserved antiviral proteins. Here, we identified a set of Viperin and Viperin-like proteins from multiple species and tested whether they could interfere with RNA viruses in planta. Our data from transient and stable overexpression of these proteins in Nicotiana benthamiana reveal varying levels of interference against the RNA viruses tobacco mosaic virus (TMV), turnip mosaic virus (TuMV), and potato virus x (PVX). Harnessing the potential of these proteins represents a novel avenue in plant antiviral approaches, offering a broader and more effective spectrum for application in plant biotechnology and agriculture. Identifying these proteins opens new avenues for engineering a broad range of resistance to protect crop plants against viral pathogens.

Investigation of multidirectional toxicity induced by high-dose molybdenum exposure with Allium test

In this study, the multifaceted toxicity induced by high doses of the essential trace element molybdenum in Allium cepa L. was investigated. Germination, root elongation, weight gain, mitotic index (MI), micronucleus (MN), chromosomal abnormalities (CAs), Comet assay, malondialdehyde (MDA), proline, superoxide dismutase (SOD), catalase (CAT) and anatomical parameters were used as biomarkers of toxicity. In addition, detailed correlation and PCA analyzes were performed for all parameters discussed. On the other hand, this study focused on the development of a two hidden layer deep neural network (DNN) using Matlab. Four experimental groups were designed: control group bulbs were germinated in tap water and application group bulbs were germinated with 1000, 2000 and 4000 mg/L doses of molybdenum for 72 h. After germination, root tips were collected and prepared for analysis. As a result, molybdenum exposure caused a dose-dependent decrease (p < 0.05) in the investigated physiological parameter values, and an increase (p < 0.05) in the cytogenetic (except MI) and biochemical parameter values. Molybdenum exposure induced different types of CAs and various anatomical damages in root meristem cells. Comet assay results showed that the severity of DNA damage increased depending on the increasing molybdenum dose. Detailed correlation and PCA analysis results determined significant positive and negative interactions between the investigated parameters and confirmed the relationships of these parameters with molybdenum doses. It has been found that the DNN model is in close agreement with the actual data showing the accuracy of the predictions. MAE, MAPE, RMSE and R2 were used to evaluate the effectiveness of the DNN model. Collective analysis of these metrics showed that the DNN model performed well. As a result, it has been determined once again that high doses of molybdenum cause multiple toxicity in A. cepa and the Allium test is a reliable universal test for determining this toxicity. Therefore, periodic measurement of molybdenum levels in agricultural soils should be the first priority in preventing molybdenum toxicity.

The Mechanisms of Molybdate Distribution and Homeostasis with Special Focus on the Model Plant Arabidopsis thaliana

This review article deals with the pathways of cellular and global molybdate distribution in plants, especially with a full overview for the model plant Arabidopsis thaliana. In its oxidized state as bioavailable molybdate, molybdenum can be absorbed from the environment. Especially in higher plants, molybdenum is indispensable as part of the molybdenum cofactor (Moco), which is responsible for functionality as a prosthetic group in a variety of essential enzymes like nitrate reductase and sulfite oxidase. Therefore, plants need mechanisms for molybdate import and transport within the organism, which are accomplished via high-affinity molybdate transporter (MOT) localized in different cells and membranes. Two different MOT families were identified. Legumes like Glycine max or Medicago truncatula have an especially increased number of MOT1 family members for supplying their symbionts with molybdate for nitrogenase activity. In Arabidopsis thaliana especially, the complete pathway followed by molybdate through the plant is traceable. Not only the uptake from soil by MOT1.1 and its distribution to leaves, flowers, and seeds by MOT2-family members was identified, but also that inside the cell. the transport trough the cytoplasm and the vacuolar storage mechanisms depending on glutathione were described. Finally, supplying the Moco biosynthesis complex by MOT1.2 and MOT2.1 was demonstrated.

Advancing Our Understanding of Pyranopterin-Dithiolene Contributions to Moco Enzyme Catalysis

The pyranopterin dithiolene ligand is remarkable in terms of its geometric and electronic structure and is uniquely found in mononuclear molybdenum and tungsten enzymes. The pyranopterin dithiolene is found coordinated to the metal ion, deeply buried within the protein, and non-covalently attached to the protein via an extensive hydrogen bonding network that is enzyme-specific. However, the function of pyranopterin dithiolene in enzymatic catalysis has been difficult to determine. This focused account aims to provide an overview of what has been learned from the study of pyranopterin dithiolene model complexes of molybdenum and how these results relate to the enzyme systems. This work begins with a summary of what is known about the pyranopterin dithiolene ligand in the enzymes. We then introduce the development of inorganic small molecule complexes that model aspects of a coordinated pyranopterin dithiolene and discuss the results of detailed physical studies of the models by electronic absorption, resonance Raman, X-ray absorption and NMR spectroscopies, cyclic voltammetry, X-ray crystallography, and chemical reactivity.

Does the Micronutrient Molybdenum Have a Role in Gestational Complications and Placental Health?

Molybdenum is an essential trace element for human health and survival, with molybdenum-containing enzymes catalysing multiple reactions in the metabolism of purines, aldehydes, and sulfur-containing amino acids. Recommended daily intakes vary globally, with molybdenum primarily sourced through the diet, and supplementation is not common. Although the benefits of molybdenum as an anti-diabetic and antioxidant inducer have been reported in the literature, there are conflicting data on the benefits of molybdenum for chronic diseases. Overexposure and deficiency can result in adverse health outcomes and mortality, although physiological doses remain largely unexplored in relation to human health. The lack of knowledge surrounding molybdenum intake and the role it plays in physiology is compounded during pregnancy. As pregnancy progresses, micronutrient demand increases, and diet is an established factor in programming gestational outcomes and maternal health. This review summarises the current literature concerning varied recommendations on molybdenum intake, the role of molybdenum and molybdoenzymes in physiology, and the contribution these play in gestational outcomes.

Protonation and Non-Innocent Ligand Behavior in Pyranopterin Dithiolene Molybdenum Complexes

The complex [TEA][Tp*MoIV(O)(S2BMOPP)] (1) [TEA = tetraethylammonium, Tp* = tris(3,5-dimethylpyrazolyl)hydroborate, and BMOPP = 6-(3-butynyl-2-methyl-2-ol)-2-pivaloyl pterin] is a structural analogue of the molybdenum cofactor common to all pyranopterin molybdenum enzymes because it possesses a pyranopterin-ene-1,2-dithiolate ligand (S2BMOPP) that exists primarily in the ring-closed pyrano structure as a resonance hybrid of ene-dithiolate and thione-thiolate forms. Compound 1, the protonated [Tp*MoIV(O)(S2BMOPP-H)] (1-H) and one-electron-oxidized [Tp*MoV(O)(S2BMOPP)] [1-Mo(5+)] species have been studied using a combination of electrochemistry, electronic absorption, and electron paramagnetic resonance (EPR) spectroscopy. Additional insight into the nature of these molecules has been derived from electronic structure computations. Differences in dithiolene C-S bond lengths correlate with relative contributions from both ene-dithiolate and thione-thiolate resonance structures. Upon protonation of 1 to form 1-H, large spectroscopic changes are observed with transitions assigned as Mo(xy) → pyranopterin metal-to-ligand charge transfer and dithiolene → pyranopterin intraligand charge transfer, respectively, and this underscores a dramatic change in electronic structure between 1 and 1-H. The changes in electronic structure that occur upon protonation of 1 are also reflected in a large >300 mV increase in the Mo(V/IV) redox potential for 1-H, resulting from the greater thione-thiolate resonance contribution and decreased charge donation that stabilize the Mo(IV) state in 1-H with respect to one-electron oxidation. EPR spin Hamiltonian parameters for one-electron-oxidized 1-Mo(5+) and uncyclized [Tp*MoV(O)(S2BDMPP)] [3-Mo(5+)] [BDMPP = 6-(3-butynyl-2,2-dimethyl)-2-pivaloyl pterin] are very similar to each other and to those of [Tp*MoVO(bdt)] (bdt = 1,2-ene-dithiolate). This indicates that the dithiolate form of the ligand dominates at the Mo(V) level, consistent with the demand for greater S → Mo charge donation and a corresponding increase in Mo-S covalency as the oxidation state of the metal is increased. Protonation of 1 represents a simple reaction that models how the transfer of a proton from neighboring acidic amino acid residues to the Mo cofactor at a nitrogen atom within the pyranopterin dithiolene (PDT) ligand in pyranopterin molybdenum enzymes can impact the electronic structure of the Mo-PDT unit. This work also illustrates how pyran ring-chain tautomerization drives changes in resonance contributions to the dithiolene chelate and may adjust the reduction potential of the Mo ion.

The History of the Molybdenum Cofactor—A Personal View

The transition element molybdenum (Mo) is an essential micronutrient for plants, animals, and microorganisms, where it forms part of the active center of Mo enzymes. To gain biological activity in the cell, Mo has to be complexed by a pterin scaffold to form the molybdenum cofactor (Moco). Mo enzymes and Moco are found in all kingdoms of life, where they perform vital transformations in the metabolism of nitrogen, sulfur, and carbon compounds. In this review, I recall the history of Moco in a personal view, starting with the genetics of Moco in the 1960s and 1970s, followed by Moco biochemistry and the description of its chemical structure in the 1980s. When I review the elucidation of Moco biosynthesis in the 1990s and the early 2000s, I do it mainly for eukaryotes, as I worked with plants, human cells, and filamentous fungi. Finally, I briefly touch upon human Moco deficiency and whether there is life without Moco.

Resolving the Multidecade-Long Mystery in MoaA Radical SAM Enzyme Reveals New Opportunities to Tackle Human Health Problems

MoaA is one of the most conserved radical S-adenosyl-l-methionine (SAM) enzymes, and is found in most organisms in all three kingdoms of life. MoaA contributes to the biosynthesis of molybdenum cofactor (Moco), a redox enzyme cofactor used in various enzymes such as purine and sulfur catabolism in humans and anaerobic respiration in bacteria. Unlike many other cofactors, in most organisms, Moco cannot be taken up as a nutrient and requires de novo biosynthesis. Consequently, Moco biosynthesis has been linked to several human health problems, such as human Moco deficiency disease and bacterial infections. Despite the medical and biological significance, the biosynthetic mechanism of Moco’s characteristic pyranopterin structure remained elusive for more than two decades. This transformation requires the actions of the MoaA radical SAM enzyme and another protein, MoaC. Recently, MoaA and MoaC functions were elucidated as a radical SAM GTP 3′,8-cyclase and cyclic pyranopterin monophosphate (cPMP) synthase, respectively. This finding resolved the key mystery in the field and revealed new opportunities in studying the enzymology and chemical biology of MoaA and MoaC to elucidate novel mechanisms in enzyme catalysis or to address unsolved questions in Moco-related human health problems. Here, we summarize the recent progress in the functional and mechanistic studies of MoaA and MoaC and discuss the field’s future directions.

Epigenetic modelling of former, current and never smokers

BACKGROUND

DNA methylation (DNAm) performs excellently in the discrimination of current and former smokers from never smokers, where AUCs > 0.9 are regularly reported using a single CpG site (cg05575921; AHRR). However, there is a paucity of DNAm models which attempt to distinguish current, former and never smokers as individual classes. Derivation of a robust DNAm model that accurately distinguishes between current, former and never smokers would be particularly valuable to epidemiological research (as a more accurate smoking definition vs. self-report) and could potentially translate to clinical settings. Therefore, we appraise 4 DNAm models of ternary smoking status (that is, current, former and never smokers): methylation at cg05575921 (AHRR model), weighted scores from 13 CpGs created by Maas et al. (Maas model), weighted scores from a LASSO model of candidate smoking CpGs from the literature (candidate CpG LASSO model), and weighted scores from a LASSO model supplied with genome-wide 450K data (agnostic LASSO model). Discrimination is assessed by AUC, whilst classification accuracy is assessed by accuracy and kappa, derived from confusion matrices.

RESULTS

We find that DNAm can classify ternary smoking status with reasonable accuracy, including when applied to external data. Ternary classification using only DNAm far exceeds the classification accuracy of simply assigning all classes as the most prevalent class (63.7% vs. 36.4%). Further, we develop a DNAm classifier which performs well in discriminating current from former smokers (agnostic LASSO model AUC in external validation data: 0.744). Finally, across our DNAm models, we show evidence of enrichment for biological pathways and human phenotype ontologies relevant to smoking, such as haemostasis, molybdenum cofactor synthesis, body fatness and social behaviours, providing evidence of the generalisability of our classifiers.

CONCLUSIONS

Our findings suggest that DNAm can classify ternary smoking status with close to 65% accuracy. Both the ternary smoking status classifiers and current versus former smoking status classifiers address the present lack of former smoker classification in epigenetic literature; essential if DNAm classifiers are to adequately relate to real-world populations. To improve performance further, additional focus on improving discrimination of current from former smokers is necessary.

Mechanism of molybdate insertion into pterin-based molybdenum cofactors

The molybdenum cofactor (Moco) is found in the active site of numerous important enzymes that are critical to biological processes. The bidentate ligand that chelates molybdenum in Moco is the pyranopterin dithiolene (molybdopterin, MPT). However, neither the mechanism of molybdate insertion into MPT nor the structure of Moco prior to its insertion into pyranopterin molybdenum enzymes is known. Here, we report this final maturation step, where adenylated MPT (MPT-AMP) and molybdate are the substrates. X-ray crystallography of the Arabidopsis thaliana Mo-insertase variant Cnx1E S269D D274S identified adenylated Moco (Moco-AMP) as an unexpected intermediate in this reaction sequence. X-ray absorption spectroscopy revealed the first coordination sphere geometry of Moco trapped in the Cnx1E active site. We have used this structural information to deduce a mechanism for molybdate insertion into MPT-AMP. Given their high degree of structural and sequence similarity, we suggest that this mechanism is employed by all eukaryotic Mo-insertases.

Differential protein expression during growth on model and commercial mixtures of naphthenic acids in Pseudomonas fluorescens Pf-5

Naphthenic acids (NAs) are carboxylic acids with the formula (Cn H2n+Z O2 ) and are among the most toxic, persistent constituents of oil sands process-affected waters (OSPW), produced during oil sands extraction. Currently, the proteins and mechanisms involved in NA biodegradation are unknown. Using LC-MS/MS shotgun proteomics, we identified proteins overexpressed during the growth of Pseudomonas fluorescens Pf-5 on a model NA (4'-n-butylphenyl)-4-butanoic acid (n-BPBA) and commercial NA mixture (Acros). By day 11, >95% of n-BPBA was degraded. With Acros, a 17% reduction in intensity occurred with 10-18 carbon compounds of the Z family -2 to -14 (major NA species in this mixture). A total of 554 proteins (n-BPBA) and 631 proteins (Acros) were overexpressed during growth on NAs, including several transporters (e.g., ABC transporters), suggesting a cellular protective response from NA toxicity. Several proteins associated with fatty acid, lipid, and amino acid metabolism were also overexpressed, including acyl-CoA dehydrogenase and acyl-CoA thioesterase II, which catalyze part of the fatty acid beta-oxidation pathway. Indeed, multiple enzymes involved in the fatty acid oxidation pathway were upregulated. Given the presumed structural similarity between alkyl-carboxylic acid side chains and fatty acids, we postulate that P. fluorescens Pf-5 was using existing fatty acid catabolic pathways (among others) during NA degradation.

Integrated Analysis of Molybdenum Nutrition and Nitrate Metabolism in Strawberry

Molybdenum (Mo) is a component of the Mo cofactor (Moco) of nitrate reductase (NR) and is therefore essential for nitrate metabolism. However, little is known about Mo deficiency phenotypes or about how physiological and molecular mechanisms of Mo uptake and transport influence nitrate uptake and utilization in strawberry. Here, we used physiological and cytological techniques to identify Mo deficiency phenotypes in strawberry. Seedlings cultured with MoO₄ ^2- grew well and exhibited normal microstructure and ultrastructure of leaves and roots. By contrast, seedlings cultivated under Mo-deficient conditions showed yellow leaf blades and ultrastructural changes such as irregular chloroplasts and unclear membrane structures that were similar to the symptoms of nitrogen deficiency. We cloned and analyzed a putative molybdate transporter, FaMOT1, which may encode a molybdate transporter involved in the uptake and translocation of molybdate. Interestingly, the addition of the molybdate analog tungstate led to lower tissue Mo concentrations, reduced the translocation of Mo from roots to shoots, and increased the plants' sensitivity to Mo deficiency. Seedlings cultivated with MoO₄ ^2- altered expression of genes in Moco biosynthesis. As expected, NR activity was higher under sufficient MoO₄ ^2- levels. Furthermore, seedlings grown on Mo-deficient medium exhibited decreased ¹⁵NO₃ ^- translocation and lower ¹⁵NO₃ ^- use efficiency. These findings represent an important step towards understanding how molybdate transport, concentration, and deficiency symptoms influence nitrate uptake and utilization in strawberry.

The Requirement of Inorganic Fe-S Clusters for the Biosynthesis of the Organometallic Molybdenum Cofactor

Iron-sulfur (Fe-S) clusters are essential protein cofactors. In enzymes, they are present either in the rhombic [2Fe-2S] or the cubic [4Fe-4S] form, where they are involved in catalysis and electron transfer and in the biosynthesis of metal-containing prosthetic groups like the molybdenum cofactor (Moco). Here, we give an overview of the assembly of Fe-S clusters in bacteria and humans and present their connection to the Moco biosynthesis pathway. In all organisms, Fe-S cluster assembly starts with the abstraction of sulfur from l-cysteine and its transfer to a scaffold protein. After formation, Fe-S clusters are transferred to carrier proteins that insert them into recipient apo-proteins. In eukaryotes like humans and plants, Fe-S cluster assembly takes place both in mitochondria and in the cytosol. Both Moco biosynthesis and Fe-S cluster assembly are highly conserved among all kingdoms of life. Moco is a tricyclic pterin compound with molybdenum coordinated through its unique dithiolene group. Moco biosynthesis begins in the mitochondria in a Fe-S cluster dependent step involving radical/S-adenosylmethionine (SAM) chemistry. An intermediate is transferred to the cytosol where the dithiolene group is formed, to which molybdenum is finally added. Further connections between Fe-S cluster assembly and Moco biosynthesis are discussed in detail.

Oxygen and nitrite reduction by heme-deficient sulphite oxidase in a patient with mild sulphite oxidase deficiency

Isolated sulphite oxidase deficiency (iSOD) is an autosomal recessive inborn error in metabolism characterised by accumulation of sulphite, which leads to death in early infancy. Sulphite oxidase (SO) is encoded by the SUOX gene and forms a heme- and molybdenum-cofactor-dependent enzyme localised in the intermembrane space of mitochondria. Within SO, both cofactors are embedded in two separated domains, which are linked via a flexible 11 residue tether. The two-electron oxidation of sulphite to sulphate occurs at the molybdenum active site. From there, electrons are transferred via two intramolecular electron transfer steps (IETs) via the heme cofactor and to the physiologic electron acceptor cytochrome c. Previously, we reported nitrite and oxygen to serve as alternative electron acceptors at the Moco active site, thereby overcoming IET within SO. Here, we present evidence for these reactions to occur in an iSOD patient with an unusual mild disease representation. In the patient, a homozygous c.427C>A mutation within the SUOX gene leads to replacement of the highly conserved His143 to Asn. The affected His143 is one of two heme-iron-coordinating residues within SO. We demonstrate, that the H143N SO variant fails to bind heme in vivo leading to the elimination of SO-dependent cytochrome c reduction in mitochondria. We show, that sulphite oxidation at the Moco domain is unaffected in His143Asn SO variant and demonstrate that nitrite and oxygen are able to serve as electron acceptors for sulphite-derived electrons in cellulo. As result, the patient H143N SO variant retains residual sulphite oxidising activity thus ameliorating iSOD progression.

Mechanism of Rate Acceleration of Radical C-C Bond Formation Reaction by a Radical SAM GTP 3',8-Cyclase

While the number of characterized radical S-adenosyl-l-methionine (SAM) enzymes is increasing, the roles of these enzymes in radical catalysis remain largely ambiguous. In radical SAM enzymes, the slow radical initiation step kinetically masks the subsequent steps, making it impossible to study the kinetics of radical chemistry. Due to this kinetic masking, it is unknown whether the subsequent radical reactions require rate acceleration by the enzyme active site. Here, we report the first evidence that a radical SAM enzyme MoaA accelerates the radical-mediated C-C bond formation. MoaA catalyzes an unprecedented 3',8-cyclization of GTP into 3',8-cyclo-7,8-dihydro-GTP (3',8-cH₂GTP) during the molybdenum cofactor (Moco) biosynthesis. Through a series of EPR and biochemical characterizations, we found that MoaA catalyzes a shunt pathway in which an on-pathway intermediate, GTP C-3' radical, abstracts H-4' atom from (4'R)-5'-deoxyadenosine (5'-dA) to transiently generate 5'-deoxyadenos-4'-yl radical (5'-dA-C4'^•) that is subsequently reduced stereospecifically to yield (4'S)-5'-dA. Detailed kinetic characterization of the shunt and the main pathways provided the comprehensive view of MoaA kinetics and determined the rate of the on-pathway 3',8-cyclization step as 2.7 ± 0.7 s^-1. Together with DFT calculations, this observation suggested that the 3',8-cyclization by MoaA is accelerated by 6-9 orders of magnitude. Further experimental and theoretical characterizations suggested that the rate acceleration is achieved mainly by constraining the triphosphate and guanine base positions while leaving the ribose flexible, and a transition state stabilization through H-bond and electrostatic interactions with the positively charged R17 residue. This is the first evidence for rate acceleration of radical reactions by a radical SAM enzyme and provides insights into the mechanism by which radical SAM enzymes accelerate radical chemistry.

Molybdenum-Induced Effects on Nitrogen Metabolism Enzymes and Elemental Profile of Winter Wheat (Triticum aestivum L.) Under Different Nitrogen Sources

Different nitrogen (N) sources have been reported to significantly affect the activities and expressions of N metabolism enzymes and mineral elements concentrations in crop plants. However, molybdenum-induced effects in winter wheat cultivars have still not been investigated under different N sources. Here, a hydroponic study was carried out to investigate these effects on two winter wheat cultivars (‘97003’ and ‘97014’) as Mo-efficient and Mo-inefficient, respectively, under different N sources (NO₃⁻, NH₄NO₃, and NH₄⁺). The results revealed that the activities of nitrate reductase (NR) and nitrite reductase (NiR) followed the order of NH₄NO₃ > NO₃⁻ > NH₄⁺ sources, while glutamine synthetase (GS) and glutamate synthase (GOGAT) followed the order of NH₄⁺ > NH₄NO₃ > NO₃⁻ in both the wheat cultivars. However, Mo-induced effects in the activities and expressions of N metabolism enzymes under different N sources followed the order of NH₄NO₃ > NO₃⁻ > NH₄⁺ sources, indicating that Mo has more complementary effects towards nitrate nutrition than the sole ammonium source in winter wheat. Interestingly, under −Mo-deprived conditions, cultivar ‘97003’ recorded more pronounced alterations in Mo-dependent parameters than ‘97014’ cultivar. Moreover, Mo application increased the proteins, amino acids, ammonium, and nitrite contents while concomitantly decreasing the nitrate contents in the same order of NH₄NO₃ > NO₃⁻ > NH₄⁺ sources that coincides with the Mo-induced N enzymes activities and expressions. The findings of the present study indicated that Mo plays a key role in regulating the N metabolism enzymes and assimilatory products under all the three N sources; however, the extent of complementation exists in the order of NH₄NO₃ > NO₃⁻ > NH₄⁺ sources in winter wheat. In addition, it was revealed that mineral elements profiles were mainly affected by different N sources, while Mo application generally had no significant effects on the mineral elements contents in the winter wheat leaves under different N sources.

C-C bond forming radical SAM enzymes involved in the construction of carbon skeletons of cofactors and natural products

Covering: up to the end of 2017 C-C bond formations are frequently the key steps in cofactor and natural product biosynthesis. Historically, C-C bond formations were thought to proceed by two electron mechanisms, represented by Claisen condensation in fatty acids and polyketide biosynthesis. These types of mechanisms require activated substrates to create a nucleophile and an electrophile. More recently, increasing number of C-C bond formations catalyzed by radical SAM enzymes are being identified. These free radical mediated reactions can proceed between almost any sp3 and sp2 carbon centers, allowing introduction of C-C bonds at unconventional positions in metabolites. Therefore, free radical mediated C-C bond formations are frequently found in the construction of structurally unique and complex metabolites. This review discusses our current understanding of the functions and mechanisms of C-C bond forming radical SAM enzymes and highlights their important roles in the biosynthesis of structurally complex, naturally occurring organic molecules. Mechanistic consideration of C-C bond formation by radical SAM enzymes identifies the significance of three key mechanistic factors: radical initiation, acceptor substrate activation and radical quenching. Understanding the functions and mechanisms of these characteristic enzymes will be important not only in promoting our understanding of radical SAM enzymes, but also for understanding natural product and cofactor biosynthesis.

Lessons From the Studies of a CC Bond Forming Radical SAM Enzyme in Molybdenum Cofactor Biosynthesis

MoaA is one of the founding members of the radical S-adenosyl-_L-methionine (SAM) superfamily, and together with the second enzyme, MoaC, catalyzes the construction of the pyranopterin backbone structure of the molybdenum cofactor (Moco). However, the exact functions of both MoaA and MoaC had remained ambiguous for more than 2 decades. Recently, their functions were finally elucidated through successful characterization of the MoaA product as 3',8-cyclo-7,8-dihydro-GTP (3',8-cH₂GTP), which was shown to be converted to cyclic pyranopterin monophosphate (cPMP) by MoaC. 3',8-cH₂GTP was produced in a small quantity and was highly oxygen sensitive, which explains why this compound had previously eluded characterization. This chapter describes the methodologies for the characterization of MoaA, MoaC, and 3',8-cH₂GTP, which together significantly altered the view of the mechanism of the pyranopterin backbone construction during the Moco biosynthesis. Through this chapter, we hope to share not only the protocols to study the first step of Moco biosynthesis but also the lessons we learned from the characterization of the chemically labile biosynthetic intermediate, which would be informative for the study of many other metabolic pathways and enzymes.

A concise treatment of pterins: some recent synthetic and methodology aspects and their applications in molecular sensors

A concise account of pterins in chemistry and biology and their applications in molecular sensors including their optical spectroscopic properties are described. Different natural, synthetic, biological and photophysical aspects are also discussed. Synthetic access to direct functionalised pterins and a recently reported new thiophene annulation technique are described for the synthesis of Form B of molybdenum cofactor. The receptor properties of fluorescent pterin molecules including selenopyrimidines which are rarely reported for their binding of anions and neutral molecules are also of major importance in this review. For such an old and still so young, unexplored pterin system on its power to be sensitive for physical studies especially the interaction with cations, anions and neutral molecules are fascinating and research in this area is relatively new and expected to increase fast. Pterin based receptors are for the first time put into a useful review for the advantage of those who want to explore pterin and modified pterin as chromogenic and fluorogenic sensors.

Radical Breakthroughs in Natural Product and Cofactor Biosynthesis

The radical SAM (S-adenosyl-l-methionine) superfamily is one of the largest group of enzymes with >113000 annotated sequences [Landgraf, B. J., et al. (2016) Annu. Rev. Biochem. 85, 485-514]. Members of this superfamily catalyze the reductive cleavage of SAM using an oxygen sensitive 4Fe-4S cluster to transiently generate 5'-deoxyadenosyl radical that is subsequently used to initiate diverse free radical-mediated reactions. Because of the unique reactivity of free radicals, radical SAM enzymes frequently catalyze chemically challenging reactions critical for the biosynthesis of unique structures of cofactors and natural products. In this Perspective, I will discuss the impact of characterizing novel functions in radical SAM enzymes on our understanding of biosynthetic pathways and use two recent examples from my own group with a particular emphasis on two radical SAM enzymes that are responsible for carbon skeleton formation during the biosynthesis of a cofactor and natural products.

Shared function and moonlighting proteins in molybdenum cofactor biosynthesis

The biosynthesis of the molybdenum cofactor (Moco) is a highly conserved pathway in bacteria, archaea and eukaryotes. The molybdenum atom in Moco-containing enzymes is coordinated to the dithiolene group of a tricyclic pyranopterin monophosphate cofactor. The biosynthesis of Moco can be divided into three conserved steps, with a fourth present only in bacteria and archaea: (1) formation of cyclic pyranopterin monophosphate, (2) formation of molybdopterin (MPT), (3) insertion of molybdenum into MPT to form Mo-MPT, and (4) additional modification of Mo-MPT in bacteria with the attachment of a GMP or CMP nucleotide, forming the dinucleotide variants of Moco. While the proteins involved in the catalytic reaction of each step of Moco biosynthesis are highly conserved among the Phyla, a surprising link to other cellular pathways has been identified by recent discoveries. In particular, the pathways for FeS cluster assembly and thio-modifications of tRNA are connected to Moco biosynthesis by sharing the same protein components. Further, proteins involved in Moco biosynthesis are not only shared with other pathways, but additionally have moonlighting roles. This review gives an overview of Moco biosynthesis in bacteria and humans and highlights the shared function and moonlighting roles of the participating proteins.

Effect of exogenously applied molybdenum on its absorption and nitrate metabolism in strawberry seedlings

Molybdenum (Mo)-an essential element of plants-is involved in nitrogen (N) metabolism. Plants tend to accumulate more nitrate and show lower nitrogen use efficiency (NUE) under Mo-deficient conditions. Improving NUE in fruits reduces the negative effect of large applications of chemical fertilizer, but the mechanisms underlying how Mo enhances NUE remain unclear. We cultivated strawberry seedlings sprayed with 0, 67.5, 135, 168.75, or 202.5 g Mo·ha^-1 in a non-soil culture system. The Mo concentration in every plant tissue analyzed increased gradually as Mo application level rose. Mo application affected iron, copper, and selenium adsorption in roots. Seedlings sprayed with 135 g Mo·ha^-1 had a higher [¹⁵N] shoot:root (S:R) ratio, and ¹⁵NUE, and produced higher molybdate transporter type 1 (MOT1) expression levels in the roots and leaves. Seedlings sprayed with 135 g Mo·ha^-1 also had relatively high nitrogen metabolic enzyme activities and up-regulated transcript levels of nitrate uptake genes (NRT1.1; NRT2.1) and nitrate-responsive genes. Furthermore, there was a significantly lower NO₃^- concentration in the leaves and roots, a higher NH₄⁺ concentration in leaves, and a higher glutamine/glutamate (Gln/Glu) concentration at 135 g Mo·ha^-1. Seedlings sprayed with 202.5 g Mo·ha^-1 showed the opposite trend. Taken together, these results suggest that a 135 g Mo·ha^-1 application was optimal because it enhanced NO₃^- transport from the roots to the shoots and increased NUE by mediating nitrogen metabolic enzyme activities, nitrate transport, and nitrate assimilation gene activities.

On the Role of Additional [4Fe-4S] Clusters with a Free Coordination Site in Radical-SAM Enzymes

The canonical CysXXXCysXXCys motif is the hallmark of the Radical-SAM superfamily. This motif is responsible for the ligation of a [4Fe-4S] cluster containing a free coordination site available for SAM binding. The five enzymes MoaA, TYW1, MiaB, RimO and LipA contain in addition a second [4Fe-4S] cluster itself bound to three other cysteines and thus also displaying a potentially free coordination site. This review article summarizes recent important achievements obtained on these five enzymes with the main focus to delineate the role of this additional [4Fe-4S] cluster in catalysis.

The Molybdenum Cofactor Biosynthesis Network: In vivo Protein-Protein Interactions of an Actin Associated Multi-Protein Complex

Survival of plants and nearly all organisms depends on the pterin based molybdenum cofactor (Moco) as well as its effective biosynthesis and insertion into apo-enzymes. To this end, both the central Moco biosynthesis enzymes are characterized and the conserved four-step reaction pathway for Moco biosynthesis is well-understood. However, protection mechanisms to prevent degradation during biosynthesis as well as transfer of the highly oxygen sensitive Moco and its intermediates are not fully enlightened. The formation of protein complexes involving transient protein-protein interactions is an efficient strategy for protected metabolic channelling of sensitive molecules. In this review, Moco biosynthesis and allocation network is presented and discussed. This network was intensively studied based on two in vivo interaction methods: bimolecular fluorescence complementation (BiFC) and split-luciferase. Whereas BiFC allows localisation of interacting partners, split-luciferase assay determines interaction strengths in vivo. Results demonstrate (i) interaction of Cnx2 and Cnx3 within the mitochondria and (ii) assembly of a biosynthesis complex including the cytosolic enzymes Cnx5, Cnx6, Cnx7, and Cnx1, which enables a protected transfer of intermediates. The whole complex is associated with actin filaments via Cnx1 as anchor protein. After biosynthesis, Moco needs to be handed over to the specific apo-enzymes. A potential pathway was discovered. Molybdenum-containing enzymes of the sulphite oxidase family interact directly with Cnx1. In contrast, the xanthine oxidoreductase family acquires Moco indirectly via a Moco binding protein (MoBP2) and Moco sulphurase ABA3. In summary, the uncovered interaction matrix enables an efficient transfer for intermediate and product protection via micro-compartmentation.

A Model for the Active-Site Formation Process in DMSO Reductase Family Molybdenum Enzymes Involving Oxido-Alcoholato and Oxido-Thiolato Molybdenum(VI) Core Structures

New bis(ene-1,2-dithiolato)-oxido-alcoholato molybdenum(VI) and -oxido-thiolato molybdenum(VI) anionic complexes, denoted as [Mo(VI)O(ER)L2](-) (E = O, S; L = dimethoxycarboxylate-1,2-ethylenedithiolate), were obtained from the reaction of the corresponding dioxido-molybdenum(VI) precursor complex with either an alcohol or a thiol in the presence of an organic acid (e.g., 10-camphorsulfonic acid) at low temperature. The [Mo(VI)O(ER)L2](-) complexes were isolated and characterized, and the structure of [Mo(VI)O(OEt)L2](-) was determined by X-ray crystallography. The Mo(VI) center in [Mo(VI)O(OEt)L2](-) exhibits a distorted octahedral geometry with the two ene-1,2-dithiolate ligands being symmetry inequivalent. The computed structure of [Mo(VI)O(SR)L2](-) is essentially identical to that of [Mo(VI)O(OR)L2](-). The electronic structures of the resulting molybdenum(VI) complexes were evaluated using electronic absorption spectroscopy and bonding calculations. The nature of the distorted O(h) geometry in these [Mo(VI)O(EEt)L2](-) complexes results in a lowest unoccupied molecular orbital wave function that possesses strong π* interactions between the Mo(d(xy)) orbital and the cis S(p(z)) orbital localized on one sulfur donor from a single ene-1,2-dithiolate ligand. The presence of a covalent Mo-S(dithiolene) bonding interaction in these monooxido Mo(VI) compounds contributes to their low-energy ligand-to-metal charge transfer transitions. A second important d-p π bonding interaction derives from the ∼180° O(oxo)-Mo-E-C dihedral angle involving the alcoholate and thiolate donors, and this contributes to ancillary ligand contributions to the electronic structure of these species. The formation of [Mo(VI)O(OEt)L2](-) and [Mo(VI)O(SEt)L2](-) from the dioxidomolybdenum(VI) precursor may be regarded as a model for the active-site formation process that occurs in the dimethyl sulfoxide reductase family of pyranopterin molybdenum enzymes.

Mechanistic Investigation of cPMP Synthase in Molybdenum Cofactor Biosynthesis Using an Uncleavable Substrate Analogue

Molybdenum cofactor (Moco) is essential for all kingdoms of life, plays central roles in various biological processes, and must be biosynthesized de novo. During its biosynthesis, the characteristic pyranopterin ring is constructed by a complex rearrangement of guanosine 5'-triphosphate (GTP) into cyclic pyranopterin monophosphate (cPMP) through the action of two enzymes, MoaA and MoaC. Recent studies revealed that MoaC catalyzes the majority of the transformation and produces cPMP from a unique cyclic nucleotide, 3',8-cyclo-7,8-dihydro-GTP (3',8-cH2GTP). However, the mechanism by which MoaC catalyzes this complex rearrangement is largely unexplored. Here, we report the mechanistic characterization of MoaC using an uncleavable substrate analogue, 3',8-cH2GMP[CH2]PP, as a probe to investigate the timing of cyclic phosphate formation. Using partially active MoaC variants, 3',8-cH2GMP[CH2]PP was found to be accepted by MoaC as a substrate and was converted to an analogue of the previously described MoaC reaction intermediate, suggesting that the early stage of catalysis proceeds without cyclic phosphate formation. In contrast, when it was incubated with wt-MoaC, 3',8-cH2GMP[CH2]PP caused mechanism-based inhibition. Detailed characterization of the inhibited MoaC suggested that 3',8-cH2GMP[CH2]PP is mainly converted to a molecule (compound Y) with an acid-labile triaminopyrimidinone base without an established pyranopterin structure. MS analysis of MoaC treated with 3',8-cH2GMP[CH2]PP provided strong evidence that compound Y forms a tight complex with MoaC likely through a covalent linkage. These observations are consistent with a mechanism in which cyclic phosphate ring formation proceeds in concert with the pterin ring formation. This mechanism would provide a thermodynamic driving force to complete the formation of the unique tetracyclic structure of cPMP.

Oxygen reactivity of mammalian sulfite oxidase provides a concept for the treatment of sulfite oxidase deficiency

Mammalian sulfite oxidase (SO) is a dimeric enzyme consisting of a molybdenum cofactor- (Moco) and haem-containing domain and catalyses the oxidation of toxic sulfite to sulfate. Following sulfite oxidation, electrons are passed from Moco via the haem cofactor to cytochrome c, the terminal electron acceptor. In contrast, plant SO (PSO) lacks the haem domain and electrons shuttle from Moco to molecular oxygen. Given the high similarity between plant and mammalian SO Moco domains, factors that determine the reactivity of PSO towards oxygen, remained unknown. In the present study, we generated mammalian haem-deficient and truncated SO variants and demonstrated their oxygen reactivity by hydrogen peroxide formation and oxygen-consumption studies. We found that intramolecular electron transfer between Moco and haem showed an inverse correlation to SO oxygen reactivity. Haem-deficient SO variants exhibited oxygen-dependent sulfite oxidation similar to PSO, which was confirmed further using haem-deficient human SO in a cell-based assay. This finding suggests the possibility to use oxygen-reactive SO variants in sulfite detoxification, as the loss of SO activity is causing severe neurodegeneration. Therefore we evaluated the potential use of PEG attachment (PEGylation) as a modification method for future enzyme substitution therapies using oxygen-reactive SO variants, which might use blood-dissolved oxygen as the electron acceptor. PEGylation has been shown to increase the half-life of other therapeutic proteins. PEGylation resulted in the modification of up to eight surface-exposed lysine residues of SO, an increased conformational stability and similar kinetic properties compared with wild-type SO.

Mechanism of pyranopterin ring formation in molybdenum cofactor biosynthesis

The molybdenum cofactor (Moco) is essential for all kingdoms of life, plays central roles in various biological processes, and must be biosynthesized de novo. During Moco biosynthesis, the characteristic pyranopterin ring is constructed by a complex rearrangement of guanosine 5'-triphosphate (GTP) into cyclic pyranopterin (cPMP) through the action of two enzymes, MoaA and MoaC (molybdenum cofactor biosynthesis protein A and C, respectively). Conventionally, MoaA was considered to catalyze the majority of this transformation, with MoaC playing little or no role in the pyranopterin formation. Recently, this view was challenged by the isolation of 3',8-cyclo-7,8-dihydro-guanosine 5'-triphosphate (3',8-cH2GTP) as the product of in vitro MoaA reactions. To elucidate the mechanism of formation of Moco pyranopterin backbone, we performed biochemical characterization of 3',8-cH2GTP and functional and X-ray crystallographic characterizations of MoaC. These studies revealed that 3',8-cH2GTP is the only product of MoaA that can be converted to cPMP by MoaC. Our structural studies captured the specific binding of 3',8-cH2GTP in the active site of MoaC. These observations provided strong evidence that the physiological function of MoaA is the conversion of GTP to 3',8-cH2GTP (GTP 3',8-cyclase), and that of MoaC is to catalyze the rearrangement of 3',8-cH2GTP into cPMP (cPMP synthase). Furthermore, our structure-guided studies suggest that MoaC catalysis involves the dynamic motions of enzyme active-site loops as a way to control the timing of interaction between the reaction intermediates and catalytically essential amino acid residues. Thus, these results reveal the previously unidentified mechanism behind Moco biosynthesis and provide mechanistic and structural insights into how enzymes catalyze complex rearrangement reactions.

Overexpression of a GmCnx1 gene enhanced activity of nitrate reductase and aldehyde oxidase, and boosted mosaic virus resistance in soybean

Molybdenum cofactor (Moco) is required for the activities of Moco-dependant enzymes. Cofactor for nitrate reductase and xanthine dehydrogenase (Cnx1) is known to be involved in the biosynthesis of Moco in plants. In this work, a soybean (Glycine max L.) Cnx1 gene (GmCnx1) was transferred into soybean using Agrobacterium tumefaciens-mediated transformation method. Twenty seven positive transgenic soybean plants were identified by coating leaves with phosphinothricin, bar protein quick dip stick and PCR analysis. Moreover, Southern blot analysis was carried out to confirm the insertion of GmCnx1 gene. Furthermore, expression of GmCnx1 gene in leaf and root of all transgenic lines increased 1.04-2.12 and 1.55-3.89 folds, respectively, as compared to wild type with GmCnx1 gene and in line 10 , 22 showing the highest expression. The activities of Moco-related enzymes viz nitrate reductase (NR) and aldehydeoxidase (AO) of T1 generation plants revealed that the best line among the GmCnx1 transgenic plants accumulated 4.25 μg g(-1) h(-1) and 30 pmol L(-1), respectively (approximately 2.6-fold and 3.9-fold higher than non-transgenic control plants).In addition, overexpression ofGmCnx1boosted the resistance to various strains of soybean mosaic virus (SMV). DAS-ELISA analysis further revealed that infection rate of GmCnx1 transgenic plants were generally lower than those of non-transgenic plants among two different virus strains tested. Taken together, this study showed that overexpression of a GmCnx1 gene enhanced NR and AO activities and SMV resistance, suggesting its important role in soybean genetic improvement.

C-Terminal glycine-gated radical initiation by GTP 3',8-cyclase in the molybdenum cofactor biosynthesis

The molybdenum cofactor (Moco) is an essential redox cofactor found in all kingdoms of life. Genetic mutations in the human Moco biosynthetic enzymes lead to a fatal metabolic disorder, Moco deficiency (MoCD). Greater than 50% of all human MoCD patients have mutations in MOCS1A, a radical S-adenosyl-l-methionine (SAM) enzyme involved in the conversion of guanosine 5'-triphosphate (GTP) into cyclic pyranopterin monophosphate. In MOCS1A, one of the frequently affected locations is the GG motif constituted of two consecutive Gly at the C-terminus. The GG motif is conserved among all MOCS1A homologues, but its role in catalysis or the mechanism by which its mutation causes MoCD was unknown. Here, we report the functional characterization of the GG motif using MoaA, a bacterial homologue of MOCS1A, as a model. Our study elucidated that the GG motif is essential for the activity of MoaA to produce 3',8-cH2GTP from GTP (GTP 3',8-cyclase), and that synthetic peptides corresponding to the C-terminal region of wt-MoaA rescue the GTP 3',8-cyclase activity of the GG-motif mutants. Further biochemical characterization suggested that the C-terminal tail containing the GG motif interacts with the SAM-binding pocket of MoaA, and is essential for the binding of SAM and subsequent radical initiation. In sum, these observations suggest that the C-terminal tail of MoaA provides an essential mechanism to trigger the free radical reaction, impairment of which results in the complete loss of catalytic function of the enzyme, and causes MoCD.

Too much is bad--an appraisal of phytotoxicity of elevated plant-beneficial heavy metal ions

Heavy metal ions such as cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn) are considered essential/beneficial for optimal plant growth, development, and productivity. However, these ions readily impact functions of many enzymes and proteins, halt metabolism, and exhibit phytotoxicity at supra-optimum supply. Nevertheless, the concentrations of these heavy metal ions are increasing in agricultural soils worldwide via both natural and anthropogenic sources that need immediate attention. Considering recent breakthroughs on Co, Cu, Fe, Mn, Mo, Ni, and Zn in soil-plant system, the present paper: (a) overviews the status in soils and their uptake, transport, and significance in plants; (b) critically discusses their elevated level-mediated toxicity to both plant growth/development and cell/genome; (c) briefly cross talks on the significance of potential interactions between previous plant-beneficial heavy metal ions in plants; and (d) highlights so far unexplored aspects in the current context.

Differential expression of molybdenum transport and assimilation genes between two winter wheat cultivars (Triticum aestivum)

Molybdenum (Mo) is an essential trace element for higher plants. Winter wheat cultivar 97003 has a higher Mo efficiency than 97014 under Mo-deficiency stress. Mo efficiency is related to Mo uptake, transfer and assimilation in plants. Several genes are involved in regulating Mo uptake, transfer and assimilation in plants. To obtain a better understanding of the aforementioned difference in Mo uptake, we have conducted a hydroponic trail to investigate the expression of genes related to Mo uptake, transfer and assimilation in the above two cultivars. The results indicate a closed relationship between Mo uptake and TaSultr5.1, TaSultr5.2 and TaCnx1 expression, according to a stepwise regression analysis of the time course of Mo uptake in the two cultivars. Meanwhile, expression of TaSultr5.2 in roots also showed a positive relationship with Mo uptake rates. 97003 had stronger Mo uptake than 97014 at low Mo-application rates (less than 1 μmol Mo L(-1)) due to the higher expression of TaSultr5.2, TaSultr5.1 and TaCnx1 in roots. On the contrary, Mo uptake of 97003 was weaker than 97014 at high Mo application rates (ranging from 5 to 20 μmol Mo L(-1)), which was related to significant down-regulation of TaSultr5.2 and TaCnx1 genes in roots of 97003 compared to 97014. Therefore, we speculated that the differential-expression intensities of TaSultr5.2, TaSultr5.1 and TaCnx1 could be the cause of the difference in Mo uptake between the two winter wheat cultivars at low and high Mo application levels.

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.

Molybdenum in human health and disease

Molybdenum is an essential trace element and crucial for the survival of animals. Four mammalian Mo-dependent enzymes are known, all of them harboring a pterin-based molybdenum cofactor (Moco) in their active site. In these enzymes, molybdenum catalyzes oxygen transfer reactions from or to substrates using water as oxygen donor or acceptor. Molybdenum shuttles between two oxidation states, Mo(IV) and Mo(VI). Following substrate reduction or oxidation, electrons are subsequently shuttled by either inter- or intra-molecular electron transfer chains involving prosthetic groups such as heme or iron-sulfur clusters. In all organisms studied so far, Moco is synthesized by a highly conserved multi-step biosynthetic pathway. A deficiency in the biosynthesis of Moco results in a pleitropic loss of all four human Mo-enzyme activities and in most cases in early childhood death. In this review we first introduce general aspects of molybdenum biochemistry before we focus on the functions and deficiencies of two Mo-enzymes, xanthine dehydrogenase and sulfite oxidase, caused either by deficiency of the apo-protein or a pleiotropic loss of Moco due to a genetic defect in its biosynthesis. The underlying molecular basis of Moco deficiency, possible treatment options and links to other diseases, such as neuropsychiatric disorders, will be discussed.

Nitrite reductase and nitric-oxide synthase activity of the mitochondrial molybdopterin enzymes mARC1 and mARC2

Mitochondrial amidoxime reducing component (mARC) proteins are molybdopterin-containing enzymes of unclear physiological function. Both human isoforms mARC-1 and mARC-2 are able to catalyze the reduction of nitrite when they are in the reduced form. Moreover, our results indicate that mARC can generate nitric oxide (NO) from nitrite when forming an electron transfer chain with NADH, cytochrome b5, and NADH-dependent cytochrome b5 reductase. The rate of NO formation increases almost 3-fold when pH was lowered from 7.5 to 6.5. To determine if nitrite reduction is catalyzed by molybdenum in the active site of mARC-1, we mutated the putative active site cysteine residue (Cys-273), known to coordinate molybdenum binding. NO formation was abolished by the C273A mutation in mARC-1. Supplementation of transformed Escherichia coli with tungsten facilitated the replacement of molybdenum in recombinant mARC-1 and abolished NO formation. Therefore, we conclude that human mARC-1 and mARC-2 are capable of catalyzing reduction of nitrite to NO through reaction with its molybdenum cofactor. Finally, expression of mARC-1 in HEK cells using a lentivirus vector was used to confirm cellular nitrite reduction to NO. A comparison of NO formation profiles between mARC and xanthine oxidase reveals similar Kcat and Vmax values but more sustained NO formation from mARC, possibly because it is not vulnerable to autoinhibition via molybdenum desulfuration. The reduction of nitrite by mARC in the mitochondria may represent a new signaling pathway for NADH-dependent hypoxic NO production.

Identification of a cyclic nucleotide as a cryptic intermediate in molybdenum cofactor biosynthesis

The molybdenum cofactor (Moco) is a redox cofactor found in all kingdoms of life, and its biosynthesis is essential for survival of many organisms, including humans. The first step of Moco biosynthesis is a unique transformation of guanosine 5'-triphosphate (GTP) into cyclic pyranopterin monophosphate (cPMP). In bacteria, MoaA and MoaC catalyze this transformation, although the specific functions of these enzymes were not fully understood. Here, we report the first isolation and structural characterization of a product of MoaA. This molecule was isolated under anaerobic conditions from a solution of MoaA incubated with GTP, S-adenosyl-L-methionine, and sodium dithionite in the absence of MoaC. Structural characterization by chemical derivatization, MS, and NMR spectroscopy suggested the structure of this molecule to be (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate (3',8-cH2GTP). The isolated 3',8-cH2GTP was converted to cPMP by MoaC or its human homologue, MOCS1B, with high specificities (Km < 0.060 μM and 0.79 ± 0.24 μM for MoaC and MOCS1B, respectively), suggesting the physiological relevance of 3',8-cH2GTP. These observations, in combination with some mechanistic studies of MoaA, unambiguously demonstrate that MoaA catalyzes a unique radical C-C bond formation reaction and that, in contrast to previous proposals, MoaC plays a major role in the complex rearrangement to generate the pyranopterin ring.

Visualization and quantification of protein interactions in the biosynthetic pathway of molybdenum cofactor in Arabidopsis thaliana

The molybdenum cofactor (Moco) is the active compound at the catalytic site of molybdenum enzymes. Moco is synthesized by a conserved four-step pathway involving six proteins in Arabidopsis thaliana. Bimolecular fluorescence complementation was used to study the subcellular localization and interaction of those proteins catalysing Moco biosynthesis. In addition, the independent split-luciferase approach permitted quantification of the strength of these protein-protein interactions in vivo. Moco biosynthesis starts in mitochondria where two proteins undergo tight interaction. All subsequent steps were found to proceed in the cytosol. Here, the heterotetrameric enzyme molybdopterin synthase (catalysing step two of Moco biosynthesis) and the enzyme molybdenum insertase, which finalizes Moco formation, were found to undergo tight protein interaction as well. This cytosolic multimeric protein complex is dynamic as the small subunits of molybdopterin synthase are known to go on and off in order to become recharged with sulphur. These small subunits undergo a tighter protein contact within the enzyme molybdopterin synthase as compared with their interaction with the sulphurating enzyme. The forces of each of these protein contacts were quantified and provided interaction factors. To confirm the results, in vitro experiments using a technique combining cross-linking and label transfer were conducted. The data presented allowed the outline of the first draft of an interaction matrix for proteins within the pathway of Moco biosynthesis where product-substrate flow is facilitated through micro-compartmentalization in a cytosolic protein complex. The protected sequestering of fragile intermediates and formation of the final product are achieved through a series of direct protein interactions of variable strength.

The molybdenum cofactor

The transition element molybdenum needs to be complexed by a special cofactor to gain catalytic activity. Molybdenum is bound to a unique pterin, thus forming the molybdenum cofactor (Moco), which, in different variants, is the active compound at the catalytic site of all molybdenum-containing enzymes in nature, except bacterial molybdenum nitrogenase. The biosynthesis of Moco involves the complex interaction of six proteins and is a process of four steps, which also require iron, ATP, and copper. After its synthesis, Moco is distributed, involving Moco-binding proteins. A deficiency in the biosynthesis of Moco has lethal consequences for the respective organisms.

Metal insertion into the molybdenum cofactor: product–substrate channelling demonstrates the functional origin of domain fusion in gephyrin

The complexity of eukaryotic multicellular organisms relies on evolutionary developments that include compartmentalization, alternative splicing, protein domain fusion and post-translational modification. Mammalian gephyrin uniquely exemplifies these processes by combining two enzymatic functions within the biosynthesis of the Moco (molybdenum cofactor) in a multidomain protein. It also undergoes extensive alternative splicing, especially in neurons, where it also functions as a scaffold protein at inhibitory synapses. Two out of three gephyrin domains are homologous to bacterial Moco-synthetic proteins (G and E domain) while being fused by a third gephyrin-specific central C domain. In the present paper, we have established the in vitro Moco synthesis using purified components and demonstrated an over 300-fold increase in Moco synthesis for gephyrin compared with the isolated G domain, which synthesizes adenylylated molybdopterin, and E domain, which catalyses the metal insertion at physiological molybdate concentrations in an ATP-dependent manner. We show that the C domain impacts the catalytic efficacy of gephyrin, suggesting an important structural role in product–substrate channelling as depicted by a structural model that is in line with a face-to-face orientation of both active sites. Our functional studies demonstrate the evolutionary advantage of domain fusion in metabolic proteins, which can lead to the development of novel functions in higher eukaryotes.

Structure and reversible pyran formation in molybdenum pyranopterin dithiolene models of the molybdenum cofactor

The syntheses and X-ray structures of two molybdenum pyranopterin dithiolene complexes in biologically relevant Mo(4+) and Mo(5+) states are reported. Crystallography reveals that these complexes possess a pyran ring formed through a spontaneous cyclization reaction of a dithiolene side-chain hydroxyl group at a C═N bond of the pterin. NMR data on the Mo(4+) complex suggest that a reversible pyran ring cyclization occurs in solution. These results provide experimental evidence that the pyranopterin dithiolene ligand in molybdenum and tungsten enzymes could participate in catalysis through dynamic processes modulated by the protein.