The fourth mammalian molybdenum enzyme mARC: current state of research

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.

The mitochondrial amidoxime reducing component-from prodrug-activation mechanism to drug-metabolizing enzyme and onward to drug target

The mitochondrial amidoxime-reducing component (mARC) is one of five known molybdenum enzymes in eukaryotes. mARC belongs to the MOSC domain superfamily, a large group of so far poorly studied molybdoenzymes. mARC was initially discovered as the enzyme activating N-hydroxylated prodrugs of basic amidines but has since been shown to also reduce a variety of other N-oxygenated compounds, for example, toxic nucleobase analogs. Under certain circumstances, mARC might also be involved in reductive nitric oxide synthesis through reduction of nitrite. Recently, mARC enzymes have received a lot of attention due to their apparent involvement in lipid metabolism and, in particular, because many genome-wide association studies have shown a common variant of human mARC1 to have a protective effect against liver disease. The mechanism linking mARC enzymes with lipid metabolism remains unknown. Here, we give a comprehensive overview of what is currently known about mARC enzymes, their substrates, structure, and apparent involvement in human disease.

Hepatocyte mARC1 promotes fatty liver disease

Background & Aims

Non-alcoholic fatty liver disease (NAFLD) has a prevalence of ∼25% worldwide, with significant public health consequences yet few effective treatments. Human genetics can help elucidate novel biology and identify targets for new therapeutics. Genetic variants in mitochondrial amidoxime-reducing component 1 (MTARC1) have been associated with NAFLD and liver-related mortality; however, its pathophysiological role and the cell type(s) mediating these effects remain unclear. We aimed to investigate how MTARC1 exerts its effects on NAFLD by integrating human genetics with in vitro and in vivo studies of mARC1 knockdown.

Methods

Analyses including multi-trait colocalisation and Mendelian randomisation were used to assess the genetic associations of MTARC1. In addition, we established an in vitro long-term primary human hepatocyte model with metabolic readouts and used the Gubra Amylin NASH (GAN)-diet non-alcoholic steatohepatitis mouse model treated with hepatocyte-specific N-acetylgalactosamine (GalNAc)-siRNA to understand the in vivo impacts of MTARC1.

Results

We showed that genetic variants within the MTARC1 locus are associated with liver enzymes, liver fat, plasma lipids, and body composition, and these associations are attributable to the same causal variant (p.A165T, rs2642438 G>A), suggesting a shared mechanism. We demonstrated that increased MTARC1 mRNA had an adverse effect on these traits using Mendelian randomisation, implying therapeutic inhibition of mARC1 could be beneficial. In vitro mARC1 knockdown decreased lipid accumulation and increased triglyceride secretion, and in vivo GalNAc-siRNA-mediated knockdown of mARC1 lowered hepatic but increased plasma triglycerides. We found alterations in pathways regulating lipid metabolism and decreased secretion of 3-hydroxybutyrate upon mARC1 knockdown in vitro and in vivo.

Conclusions

Collectively, our findings from human genetics, and in vitro and in vivo hepatocyte-specific mARC1 knockdown support the potential efficacy of hepatocyte-specific targeting of mARC1 for treatment of NAFLD.

Impact and implications

We report that genetically predicted increases in MTARC1 mRNA associate with poor liver health. Furthermore, knockdown of mARC1 reduces hepatic steatosis in primary human hepatocytes and a murine NASH model. Together, these findings further underscore the therapeutic potential of targeting hepatocyte MTARC1 for NAFLD.

Active Site Structures of the Escherichia coli N-Hydroxylaminopurine Resistance Molybdoenzyme YcbX

X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data have been used to characterize the coordination environment for the catalytic Mo site of Escherichia coli YcbX in two different oxidation states. In the oxidized state, the Mo(VI) ion is coordinated by two terminal oxo ligands, a thiolate S atom from cysteine, and two S donors from the bidentate pyranopterin ene-1,2-dithiolate (pyranopterin dithiolene). Upon reduction, it is the more basic equatorial oxo ligand that is protonated, with a Mo-Oeq bond distance that is best described as either a short Mo4+-OH2 bond or a long Mo4+-OH bond. Mechanistic implications for substrate reduction are discussed in light of these structural details.

Unusual Biotransformation Reactions of Drugs and Drug Candidates

Detailed assessment of the fate of drugs in nonclinical test species and humans is essential to ensure the safety and efficacy of medicines in patients. In this context, biotransformation of drugs and drug candidates has been an area of keen interest over many decades in the pharmaceutical industry as well as academia. Although many of the enzymes and biotransformation pathways involved in the metabolism of xenobiotics and more specifically drugs have been well characterized, each drug molecule is unique and constitutes specific challenges for the biotransformation scientist. In this mini-review written for the special issue on the occasion of the 50th Anniversary celebration of Drug Metabolism and Disposition and to celebrate contributions of F. Peter Guengerich, one of the pioneers of the drug metabolism field, recently reported "unusual" biotransformation reactions are presented. Scientific and technological advances in the "toolbox" of the biotransformation scientists are summarized. As the pharmaceutical industry continues to explore therapeutic modalities different from the traditional small molecule drugs, the new challenges confronting the biotransformation scientist as well as future opportunities are discussed. SIGNIFICANCE STATEMENT: For the biotransformation scientists, it is essential to share and be aware of unexpected biotransformation reactions so that they can increase their confidence in predicting metabolites of drugs in humans to ensure the safety and efficacy of these metabolites before the medicines reach large numbers of patients. The purpose of this review is to highlight recent observations of "unusual" metabolites so that the scientists working in the area of drug metabolism can strengthen their readiness in expecting the unexpected.

Enzyme Electrode Biosensors for N-Hydroxylated Prodrugs Incorporating the Mitochondrial Amidoxime Reducing Component

Human mitochondrial amidoxime reducing component 1 and 2 (mARC1 and mARC2) were immobilised on glassy carbon electrodes using the crosslinker glutaraldehyde. Voltammetry was performed in the presence of the artificial electron transfer mediator methyl viologen, whose redox potential lies negative of the enzymes' Mo^VI/V and Mo^V/IV redox potentials which were determined from optical spectroelectrochemical and EPR measurements. Apparent Michaelis constants obtained from catalytic limiting currents at various substrate concentrations were comparable to those previously reported in the literature from enzymatic assays. Kinetic parameters for benzamidoxime reduction were determined from cyclic voltammograms simulated using Digisim. pH dependence and stability of the enzyme electrode with time were also determined from limiting catalytic currents in saturating concentrations of benzamidoxime. The same electrode remained active after at least 9 days. Fabrication of this versatile and cost-effective biosensor is effective in screening new pharmaceutically important substrates and mARC inhibitors.

Electrochemically driven catalysis of the bacterial molybdenum enzyme YiiM

The Mo-dependent enzyme YiiM enzyme from Escherichia coli is a member of the sulfite oxidase family and shares many similarities with the well-studied human mitochondrial amidoxime reducing component (mARC). We have investigated YiiM catalysis using electrochemical and spectroscopic methods. EPR monitored redox potentiometry found the active site redox potentials to be Mo^VI/V -0.02 V and Mo^V/IV -0.12 V vs NHE at pH 7.2. In the presence of methyl viologen as an electrochemically reduced electron donor, YiiM catalysis was studied with a range of potential substrates. YiiM preferentially reduces N-hydroxylated compounds such as hydroxylamines, amidoximes, N-hydroxypurines and N-hydroxyureas but shows little or no activity against amine-oxides or sulfoxides. The pH optimum for catalysis was 7.1 and a bell-shaped pH profile was found with pK_a values of 6.2 and 8.1 either side of this optimum that are associated with protonation/deprotonations that modulate activity. Simulation of the experimental voltammetry elucidated kinetic parameters associated with YiiM catalysis with the substrates 6-hydroxyaminopurine and benzamidoxime.

Interindividual Variability and Differential Tissue Abundance of Mitochondrial Amidoxime Reducing Component Enzymes in Humans

Mitochondrial amidoxime-reducing component (mARC) enzymes are molybdenum-containing proteins that metabolize a number of endobiotics and xenobiotics. The interindividual variability and differential tissue abundance of mARC1 and mARC2 were quantified using targeted proteomics in three types of tissue fractions: 1) pediatric liver tissue homogenates, 2) total membrane fraction of the paired liver and kidney samples from pediatric and adult donors, and 3) pooled S9 fractions of the liver, intestine, kidney, lung, and heart. The absolute levels of mARC1 and mARC2 in the pediatric liver homogenate were 40.08 ± 4.26 and 24.58 ± 4.02 pmol/mg homogenate protein, respectively, and were independent of age and sex. In the total membrane fraction of the paired liver and kidney samples, the abundance of hepatic mARC1 and mARC2 was comparable, whereas mARC2 abundance in the kidney was approximately 9-fold higher in comparison with mARC1. The analysis of the third set of samples (i.e., S9 fraction) revealed that mARC1 abundance in the kidney, intestine, and lung was 5- to 13-fold lower than the liver S9 abundance, whereas mARC2 abundance was approximately 3- and 16-fold lower in the intestine and lung than the liver S9, respectively. In contrast, the kidney mARC2 abundance in the S9 fraction was approximately 2.5-fold higher as compared with the hepatic mARC2 abundance. The abundance of mARC enzymes in the heart was below the limit of quantification (∼0.6 pmol/mg protein). The mARC enzyme abundance data presented here can be used to develop physiologically based pharmacokinetic models for the prediction of in vivo pharmacokinetics of mARC substrates. SIGNIFICANCE STATEMENT: A precise targeted quantitative proteomics method was developed and applied to quantify newly discovered drug-metabolizing enzymes, mARC1 and mARC2, in pediatric and adult tissue samples. The data suggest that mARC enzymes are ubiquitously expressed in an isoform-specific manner in the human liver, kidney, intestine, and lung, and the enzyme abundance is not associated with age and sex. These data are important for developing physiologically based pharmacokinetic models for the prediction of in vivo pharmacokinetics of mARC substrates.

Downregulation of MARC2 Promotes Immune Escape and Is Associated With Immunosuppression of Hepatocellular Carcinoma

The N-reductive enzyme system (NRES), composed of MARC1, MARC2, CYB5, and CYB5R, is responsible for the reduction of N-oxygenated compounds and participates in several physiological processes. For example, MARC2 serves as an important prognostic indicator and is downregulated in hepatocellular carcinoma, and the downregulation of MARC2 is critical to the regulation of lipid metabolism and cell cycle progression. However, the role of MARC2 in tumor immune microenvironment modification had not previously been investigated. In this study, we found that downregulation of MARC2 was associated with the differentiation of CD4+T cells into regulatory T cells (Tregs). Furthermore, restoring the expression of MARC2 could increase the expression of HLA-C and B2M via PPARA-related lipid metabolism signaling pathways, which could facilitate tumor antigen presentation to the tumor-infiltrating T cells. Additionally, MARC2 expression negatively correlated with several immune checkpoints. The immune checkpoint burden was generated based on 28 MARC2-related immune checkpoints. Patients with a higher immune checkpoint burden were predicted to have a poorer prognosis and a lower level of activated CD8⁺ T cells. The results showed that expression of the NRES is a prognostic indicator of hepatocellular carcinoma and MARC2 contributes significantly to predict the prognosis. Finally, loss of MARC2 in HCC patients was found to facilitate immune escape and was associated with immunosuppression.

A Novel Prodrug of a nNOS Inhibitor with Improved Pharmacokinetic Potential

Under different pathological conditions, aberrant induction of neuronal nitric oxide synthase (nNOS) generates overproduction of NO that can cause irreversible cell damage. The aim of this study was to develop an amidoxime prodrug of a potent nNOS inhibitor, the benzhydryl acetamidine. We synthesized the benzhydryl acetamidoxime, which was evaluated in vitro to ascertain the potential NOS inhibitory activity, as well as conducting bioconversion into the parent acetamidine. The prodrug was also profiled for in vitro physicochemical properties, by determining the lipophilicity, passive permeation through the human gastrointestinal tract and across the blood-brain barrier by PAMPA, and chemical, enzymatic, and plasma stability. The obtained data demonstrate that the amidoxime prodrug shows an improved pharmacokinetic profile with respect to the acetamidine nNOS inhibitor, thus suggesting that it could be a promising lead compound to treat all those pathological conditions in which nNOS activity is dysregulated.

Molybdenum cofactor biology, evolution and deficiency

The molybdenum cofactor (Moco) represents an ancient metal‑sulfur cofactor, which participates as catalyst in carbon, nitrogen and sulfur cycles, both on individual and global scale. Given the diversity of biological processes dependent on Moco and their evolutionary age, Moco is traced back to the last universal common ancestor (LUCA), while Moco biosynthetic genes underwent significant changes through evolution and acquired additional functions. In this review, focused on eukaryotic Moco biology, we elucidate the benefits of gene fusions on Moco biosynthesis and beyond. While originally the gene fusions were driven by biosynthetic advantages such as coordinated expression of functionally related proteins and product/substrate channeling, they also served as origin for the development of novel functions. Today, Moco biosynthetic genes are involved in a multitude of cellular processes and loss of the according gene products result in severe disorders, both related to Moco biosynthesis and secondary enzyme functions.

A drug-biomarker interaction model to predict the key targets of Scutellaria barbata D. Don in adverse-risk acute myeloid leukaemia

A poor prognosis, relapse and resistance are burning issues during adverse-risk acute myeloid leukaemia (AML) treatment. As a natural medicine, Scutellaria barbata D. Don (SBD) has shown impressive antitumour activity in various cancers. Thus, SBD may become a potential drug in adverse-risk AML treatment. This study aimed to screen the key targets of SBD in adverse-risk AML using the drug-biomarker interaction model through bioinformatics and network pharmacology methods. First, the adverse-risk AML-related critical biomarkers and targets of SBD active ingredient were obtained from The Cancer Genome Atlas database and several pharmacophore matching databases. Next, the protein-protein interaction network was constructed, and topological analysis and pathway enrichment were used to screen key targets and main pathways of intervention of SBD in adverse-risk AML. Finally, molecular docking was implemented for key target verification. The results suggest that luteolin and quercetin are the main active components of SBD against adverse-risk AML, and affected drug resistance, apoptosis, immune regulation and angiogenesis through the core targets AKT1, MAPK1, IL6, EGFR, SRC, VEGFA and TP53. We hope the proposed drug-biomarker interaction model provides an effective strategy for the research and development of antitumour drugs.

Mitochondrial amidoxime-reducing component 2 (MARC2) has a significant role in N-reductive activity and energy metabolism

The mitochondrial amidoxime-reducing component (MARC) is a mammalian molybdenum-containing enzyme. All annotated mammalian genomes harbor two MARC genes, MARC1 and MARC2, which share a high degree of sequence similarity. Both molybdoenzymes reduce a variety of N-hydroxylated compounds. Besides their role in N-reductive drug metabolism, only little is known about their physiological functions. In this study, we characterized an existing KO mouse model lacking the functional MARC2 gene and fed a high-fat diet and also performed in vivo and in vitro experiments to characterize reductase activity toward known MARC substrates. MARC2 KO significantly decreased reductase activity toward several N-oxygenated substrates, and for typical MARC substrates, only small residual reductive activity was still detectable in MARC2 KO mice. The residual detected reductase activity in MARC2 KO mice could be explained by MARC1 expression that was hardly unaffected by KO, and we found no evidence of significant activity of other reductase enzymes. These results clearly indicate that MARC2 is mainly responsible for N-reductive biotransformation in mice. Striking phenotypical features of MARC2 KO mice were lower body weight, increased body temperature, decreased levels of total cholesterol, and increased glucose levels, supporting previous findings that MARC2 affects energy pathways. Of note, the MARC2 KO mice were resistant to high-fat diet-induced obesity. We propose that the MARC2 KO mouse model could be a powerful tool for predicting MARC-mediated drug metabolism and further investigating MARC's roles in energy homeostasis.

Evaluation of amidoxime derivatives as prodrug candidates of potent bis-cationic antimalarials

Plasmodium falciparum is responsible for most of the cases of malaria and its resistance to established antimalarial drugs is a major issue. Thus, new chemotherapies are needed to fight the emerging multi-drug resistance of P. falciparum malaria, like choline analogues targeting plasmodial phospholipidic metabolism. Here we describe the synthesis of amidoxime derivatives as prodrug candidates of reverse-benzamidines and hybrid compounds able to mimic choline, as well as the design of a new series of asymmetrical bis-cationic compounds. Bioconversion studies were conducted on amidoximes in asymmetrical series and showed that amidoxime prodrug strategy could be applied on C-alkylamidine moieties, like benzamidines and that N-substituents did not alter the bioconversion of amidoximes. The antimalarial activity of the three series of compounds was evaluated in vitro against P. falciparum and in vivo against P. vinckei petteri in mice.

From the Eukaryotic Molybdenum Cofactor Biosynthesis to the Moonlighting Enzyme mARC

All eukaryotic molybdenum (Mo) enzymes contain in their active site a Mo Cofactor (Moco), which is formed by a tricyclic pyranopterin with a dithiolene chelating the Mo atom. Here, the eukaryotic Moco biosynthetic pathway and the eukaryotic Moco enzymes are overviewed, including nitrate reductase (NR), sulfite oxidase, xanthine oxidoreductase, aldehyde oxidase, and the last one discovered, the moonlighting enzyme mitochondrial Amidoxime Reducing Component (mARC). The mARC enzymes catalyze the reduction of hydroxylated compounds, mostly N-hydroxylated (NHC), but as well of nitrite to nitric oxide, a second messenger. mARC shows a broad spectrum of NHC as substrates, some are prodrugs containing an amidoxime structure, some are mutagens, such as 6-hydroxylaminepurine and some others, which most probably will be discovered soon. Interestingly, all known mARC need the reducing power supplied by different partners. For the NHC reduction, mARC uses cytochrome b5 and cytochrome b5 reductase, however for the nitrite reduction, plant mARC uses NR. Despite the functional importance of mARC enzymatic reactions, the structural mechanism of its Moco-mediated catalysis is starting to be revealed. We propose and compare the mARC catalytic mechanism of nitrite versus NHC reduction. By using the recently resolved structure of a prokaryotic MOSC enzyme, from the mARC protein family, we have modeled an in silico three-dimensional structure of a eukaryotic homologue.

Crystal structure of human mARC1 reveals its exceptional position among eukaryotic molybdenum enzymes

Biotransformation enzymes ensure a viable homeostasis by regulating reversible cycles of oxidative and reductive reactions. The metabolism of nitrogen-containing compounds is of high pharmaceutical and toxicological relevance because N-oxygenated metabolites derived from reactions mediated by cytochrome P450 enzymes or flavin-dependent monooxygenases are in some cases highly toxic or mutagenic. The molybdenum-dependent mitochondrial amidoxime-reducing component (mARC) was found to be an extremely efficient counterpart, which is able to reduce the full range of N-oxygenated compounds and thereby mediates detoxification reactions. However, the 3D structure of this enzyme was unknown. Here we present the high-resolution crystal structure of human mARC. We give detailed insight into the coordination of its molybdenum cofactor (Moco), the catalytic mechanism, and its ability to reduce a wide range of N-oxygenated compounds. The identification of two key residues will allow future discrimination between mARC paralogs and ensure correct annotation. Since our structural findings contradict in silico predictions that are currently made by online databases, we propose domain definitions for members of the superfamily of Moco sulfurase C-terminal (MOSC) domain-containing proteins. Furthermore, we present evidence for an evolutionary role of mARC for the emergence of the xanthine oxidase protein superfamily. We anticipate the hereby presented crystal structure to be a starting point for future descriptions of MOSC proteins, which are currently poorly structurally characterized.

Detoxification of Trimethylamine N-Oxide by the Mitochondrial Amidoxime Reducing Component mARC

Although known for years, the toxic effects of trimethylamine N-oxide (TMAO), a physiological metabolite, were just recently discovered and are currently under investigation. It is known that elevated TMAO plasma levels correlate with an elevated risk for cardiovascular disease (CVD). Even though there is a general consensus about the existence of a causal relationship between TMAO and CVD, the underlying mechanisms are not fully understood. TMAO is an oxidation product of the hepatic flavin-containing monooxygenases (FMO), mainly of isoform 3, and it is conceivable that humans also have an enzyme reversing this toxification by reducing TMAO to its precursor trimethylamine (TMA). All prokaryotic enzymes that use TMAO as a substrate have molybdenum-containing cofactors in common. Such molybdenum-containing enzymes also exist in mammals, with the so-called mitochondrial amidoxime reducing component (mARC) representing the most recently discovered mammalian molybdenum enzyme. The enzyme has been found to exist in two isoforms, mARC1 and mARC2, both being capable of reducing a variety of N-oxygenated compounds, including nonphysiological N-oxides. To investigate whether the two isoforms of this enzyme are able to reduce and detoxify TMAO, we developed a suitable analytical method and tested TMAO reduction with a recombinant enzyme system. We found that one of the two recombinant human mARC proteins, namely, hmARC1, reduces TMAO to TMA. The N-reductive activity is relatively low and identified via the kinetic parameters with K_m = (30.4 ± 9.8) mM and V_max = (100.5 ± 12.2) nmol/(mg protein·min). Nevertheless, the ubiquitous tissue expression of hmARC1 allows a continuous reduction of TMAO whereas the counter-reaction, the production of TMAO through FMO3, can take place only in the liver where FMO3 is expressed. TMAO reduction in porcine liver subfractions showed the characteristic enrichment of N-reductive activity in the outer mitochondrial membrane. TMAO reduction was also found in human cell cultures. These findings indicate the role of hmARC1 in the metabolomic pathway of TMAO, which might contribute to the prevention of CVD. This also hints at a physiological function of the molybdenum enzyme, which remains mainly unknown to date.

Crystal structure of the hydroxylaminopurine resistance protein, YiiM, and its putative molybdenum cofactor-binding catalytic site

The molybdenum cofactor (Moco) is a molybdenum-conjugated prosthetic group that is ubiquitously found in plants, animals, and bacteria. Moco is required for the nitrogen-reducing reaction of the Moco sulfurase C-terminal domain (MOSC) family. Despite the biological significance of MOSC proteins in the conversion of prodrugs and resistance against mutagens, their structural features and Moco-mediated catalysis mechanism have not been described in detail. YiiM is a MOSC protein that is involved in reducing mutagenic 6-N-hydroxylaminopurine to nontoxic adenine in bacteria. Here, we report two crystal structures of YiiM: one from Gram-positive Geobacillus stearothermophilus (gsYiiM) and the other from Gram-negative Escherichia coli (ecYiiM). Although gsYiiM and ecYiiM differ in oligomerization state and protein stability, both consist of three structural modules (a β-barrel and two α-helix bundles) and feature a cavity surrounded by the three modules. The cavity is characterized by positive electrostatic potentials and high sequence conservation. Moreover, the ecYiiM cavity houses a phosphate group, which emulates a part of Moco, and contains a highly reactive invariant cysteine residue. We thus propose that the cavity is the catalytic site where Moco binds and the substrate is reduced. Moreover, our comparative structural analysis highlights the common but distinct structural features of MOSC proteins.

The molybdenum cofactor enzyme mARC: Moonlighting or promiscuous enzyme?

Molybdenum (Mo) is present in the active center of eukaryotic enzymes as a tricyclic pyranopterin chelate compound forming the Mo Cofactor (Moco). Four Moco containing enzymes are known in eukaryotes, nitrate reductase (NR), sulfite oxidase (SO), xanthine oxidoreductase (XOR), and aldehyde oxidase (AO). A fifth Moco enzyme has been recently identified. Because of the ability of this enzyme to convert by reduction several amidoximes prodrugs into their active amino forms, it was named mARC (mitochondrial Amidoxime Reducing Component). This enzyme is also able to catalyze the reduction of a broad range of N-hydroxylated compounds (NHC) as the base analogue 6-hydroxylaminopurine (HAP), as well as nitrite to nitric oxide (NO). All the mARC proteins need reducing power that is supplied by other proteins. The human and plants mARC proteins require a Cytochrome b5 (Cytb5) and a Cytochrome b5 reductase (Cytb5-R) to form an electron transfer chain from NADH to the NHC. Recently, plant mARC proteins were shown to be implicated in the reduction of nitrite to NO, and it was proposed that the electrons required for the reaction were supplied by NR instead of Cytochrome b5 components. This newly characterized mARC activity was termed NO Forming Nitrite Reductase (NOFNiR). Moonlighting proteins form a special class of multifunctional enzymes that can perform more than one function; if the extra function is not physiologically relevant, they are called promiscuous enzymes. In this review, we summarize the current knowledge on the mARC protein, and we propose that mARC is a new moonlighting enzyme. © 2017 BioFactors, 43(4):486-494, 2017.

Uptake, transport and distribution of molybdenum in two oilseed rape (Brassica napus L.) cultivars under different nitrate/ammonium ratios

OBJECTIVES

To investigate the effects of different nitrate sources on the uptake, transport, and distribution of molybdenum (Mo) between two oilseed rape (Brassica napus L.) cultivars, L0917 and ZS11.

METHODS

A hydroponic culture experiment was conducted with four nitrate/ammonium (NO₃^-:NH₄⁺) ratios (14:1, 9:6, 7.5:7.5, and 1:14) at a constant nitrogen concentration of 15 mmol/L. We examined Mo concentrations in roots, shoots, xylem and phloem sap, and subcellular fractions of leaves to contrast Mo uptake, transport, and subcellular distribution between ZS11 and L0917.

RESULTS

Both the cultivars showed maximum biomass and Mo accumulation at the 7.5:7.5 ratio of NO₃^-:NH₄⁺ while those were decreased by the 14:1 and 1:14 treatments. However, the percentages of root Mo (14.8% and 15.0% for L0917 and ZS11, respectively) were low under the 7.5:7.5 treatment, suggesting that the equal NO₃^-:NH₄⁺ ratio promoted Mo transportation from root to shoot. The xylem sap Mo concentration and phloem sap Mo accumulation of L0917 were lower than those of ZS11 under the 1:14 treatment, which suggests that higher NO₃^-:NH₄⁺ ratio was more beneficial for L0917. On the contrary, a lower NO₃^-:NH₄⁺ ratio was more beneficial for ZS11 to transport and remobilize Mo. Furthermore, the Mo concentrations of both the cultivars' leaf organelles were increased but the Mo accumulations of the cell wall and soluble fraction were reduced significantly under the 14:1 treatment, meaning that more Mo was accumulated in organelles under the highest NO₃^-:NH₄⁺ ratio.

CONCLUSIONS

This investigation demonstrated that the capacities of Mo absorption, transportation and subcellular distribution play an important role in genotype-dependent differences in Mo accumulation under low or high NO₃^-:NH₄⁺ ratio conditions.

Molybdenum-containing nitrite reductases: Spectroscopic characterization and redox mechanism

OBJECTIVES

This review summarizes the spectroscopic results, which will provide useful suggestions for future research. In addition, the fields that urgently need more information are also advised.

BACKGROUND

Nitrite-NO-cGMP has been considered as an important signaling pathway of NO in human cells. To date, all the four known human molybdenum-containing enzymes, xanthine oxidase, aldehyde oxidase, sulfite oxidase, and mitochondrial amidoxime-reducing component, have been shown to function as nitrite reductases under hypoxia by biochemical, cellular, or animal studies. Various spectroscopic techniques have been applied to investigate the structure and catalytic mechanism of these enzymes for more than 20 years.

METHODS

We summarize the published data on the applications of UV-vis and EPR spectroscopies, and X-ray crystallography in studying nitrite reductase activity of the four human molybdenum-containing enzymes.

RESULTS

UV-vis has provided useful information on the redox active centers of these enzymes. The utilization of EPR spectroscopy has been critical in determining the coordination and redox status of the Mo center during catalysis. Despite the lack of substrate-bound crystal structures of these nitrite reductases, valuable structural information has been obtained by X-ray crystallography.

CONCLUSIONS

To fully understand the catalytic mechanisms of these physiologically/pathologically important nitrite reductases, structural studies on substrate-redox center interaction are needed.

Defining the Role of the NADH-Cytochrome-b5 Reductase 3 in the Mitochondrial Amidoxime Reducing Component Enzyme System

The importance of the mitochondrial amidoxime reducing component (mARC)-containing enzyme system in N-reductive metabolism has been studied extensively. It catalyzes the reduction of various N-hydroxylated compounds and therefore acts as the counterpart of cytochrome P450- and flavin-containing monooxygenase-catalyzed oxidations at nitrogen centers. This enzyme system was found to be responsible for the activation of amidoxime and N-hydroxyguanidine prodrugs in drug metabolism. The synergy of three components (mARC, cytochrome b5, and the appropriate reductase) is crucial to exert the N-reductive catalytic effect. Previous studies have demonstrated the involvement of the specific isoforms of the molybdoenzyme mARC and the electron transport protein cytochrome b5 in N-reductive metabolism. To date, the corresponding reductase involved in N-reductive metabolism has yet to be defined because previous investigations have presented ambiguous results. Using small interfering RNA-mediated knockdown in human cells and assessing the stoichiometry of the enzyme system reconstituted in vitro, we provide evidence that NADH-cytochrome-b5 reductase 3 is the principal reductase involved in the mARC enzyme system and is an essential component of N-reductive metabolism in human cells. In addition, only minimal levels of cytochrome-b5 reductase 3 protein are sufficient for catalysis, which impeded previous attempts to identify the reductase.

The pivotal role of the mitochondrial amidoxime reducing component 2 in protecting human cells against apoptotic effects of the base analog N6-hydroxylaminopurine

N-Hydroxylated nucleobases and nucleosides as N-hydroxylaminopurine (HAP) or N-hydroxyadenosine (HAPR) may be generated endogenously in the course of cell metabolism by cytochrome P450, by oxidative stress or by a deviating nucleotide biosynthesis. These compounds have shown to be toxic and mutagenic for procaryotic and eucaryotic cells. For DNA replication fidelity it is therefore of great importance that organisms exhibit effective mechanisms to remove such non-canonical base analogs from DNA precursor pools. In vitro, the molybdoenzymes mitochondrial amidoxime reducing component 1 and 2 (mARC1 and mARC2) have shown to be capable of reducing N-hydroxylated base analogs and nucleoside analogs to the corresponding canonical nucleobases and nucleosides upon reconstitution with the electron transport proteins cytochrome b5 and NADH-cytochrome b5 reductase. By RNAi-mediated down-regulation of mARC in human cell lines the mARC-dependent N-reductive detoxication of HAP in cell metabolism could be demonstrated. For HAPR, on the other hand, the reduction to adenosine seems to be of less significance in the detoxication pathway of human cells as HAPR is primarily metabolized to inosine by direct dehydroxylamination catalyzed by adenosine deaminase. Furthermore, the effect of mARC knockdown on sensitivity of human cells to HAP was examined by flow cytometric quantification of apoptotic cell death and detection of poly (ADP-ribose) polymerase (PARP) cleavage. mARC2 was shown to protect HeLa cells against the apoptotic effects of the base analog, whereas the involvement of mARC1 in reductive detoxication of HAP does not seem to be pivotal.

Comparison of minipig, dog, monkey and human drug metabolism and disposition

INTRODUCTION

This article gives an overview of the drug metabolism and disposition (ADME) characteristics of the most common non-rodent species used in toxicity testing of drugs (minipigs, dogs, and monkeys) and compares these to human characteristics with regard to enzymes mediating the metabolism of drugs and the transport proteins which contribute to the absorption, distribution and excretion of drugs.

METHODS

Literature on ADME and regulatory guidelines of relevance in drug development of small molecules has been gathered.

RESULTS

Non-human primates (monkeys) are the species that is closest to humans in terms of genetic homology. Dogs have an advantage due to the ready availability of comprehensive background data for toxicological safety assessment and dogs are easy to handle. Pigs have been used less than dogs and monkeys as a model in safety assessment of drug candidates. However, when a drug candidate is metabolised by aldehyde oxidase (AOX1), N-acetyltransferases (NAT1 and NAT2) or cytochrome (CYP2C9-like) enzymes which are not expressed in dogs, but are present in pigs, this species may be a better choice than dogs, provided that adequate exposure can be obtained in pigs. Conversely, pigs might not be the right choice if sulfation, involving 3-phospho-adenosyl-5-phosphosulphate sulphotransferase (PAPS) is an important pathway in the human metabolism of a drug candidate.

DISCUSSION

In general, the species selection should be based on comparison between in vitro studies with human cell-based systems and animal-cell-based systems. Results from pharmacokinetic studies are also important for decision-making by establishing the obtainable exposure level in the species. Access to genetically humanized mouse models and highly sensitive analytical methods (accelerator mass spectrometry) makes it possible to improve the chance of finding all metabolites relevant for humans before clinical trials have been initiated and, if necessary, to include another animal species before long term toxicity studies are initiated. In conclusion, safety testing can be optimized by applying knowledge about species ADME differences and utilising advanced analytical techniques.

Electrochemical and mARC-catalyzed enzymatic reduction of para-substituted benzamidoximes: consequences for the prodrug concept "amidoximes instead of amidines"

The mitochondrial amidoxime reducing component (mARC) activates amidoxime prodrugs by reduction to the corresponding amidine drugs. This study analyzes relationships between the chemical structure of the prodrug and its metabolic activation and compares its enzyme-mediated vs. electrochemical reduction. The enzyme kinetic parameters KM and Vmax for the N-reduction of ten para-substituted derivatives of the model compound benzamidoxime were determined by incubation with recombinant proteins and subcellular fractions from pig liver followed by quantification of the metabolites by HPLC. A clear influence of the substituents at position 4 on the chemical properties of the amidoxime function was confirmed by correlation analyses of (1) H NMR chemical shifts and the redox potentials of the 4-substituted benzamidoximes with Hammett's σ. However, no clear relationship between the kinetic parameters for the enzymatic reduction and Hammett's σ or the lipophilicity could be found. It is thus concluded that these properties as well as the redox potential of the amidoxime can be largely ignored during the development of new amidoxime prodrugs, at least regarding prodrug activation.

The mammalian molybdenum enzymes of mARC

The "mitochondrial amidoxime reducing component" (mARC) is the most recently discovered molybdenum-containing enzyme in mammals. All mammalian genomes studied to date contain two mARC genes: MARC1 and MARC2. The proteins encoded by these genes are mARC-1 and mARC-2 and represent the simplest form of eukaryotic molybdenum enzymes, only binding the molybdenum cofactor. In the presence of NADH, mARC proteins exert N-reductive activity together with the two electron transport proteins cytochrome b5 type B and NADH cytochrome b5 reductase. This enzyme system is capable of reducing a great variety of N-hydroxylated substrates. It plays a decisive role in the activation of prodrugs containing an amidoxime structure, and in detoxification pathways, e.g., of N-hydroxylated purine and pyrimidine bases. It belongs to a group of drug metabolism enzymes, in particular as a counterpart of P450 formed N-oxygenated metabolites. Its physiological relevance, on the other hand, is largely unknown. The aim of this article is to summarize our current knowledge of these proteins with a special focus on the mammalian enzymes and their N-reductive activity.

Molybdenum site structure of MOSC family proteins

Mo K-edge X-ray absorption spectroscopy has been used to probe as-isolated structures of the MOSC family proteins pmARC-1 and HMCS-CT. The Mo K-edge near-edge spectrum of HMCS-CT is shifted ~2.5 eV to lower energy compared to the pmARC-1 spectrum, which indicates that as-isolated HMCS-CT is in a more reduced state than pmARC-1. Extended X-ray absorption fine structure analysis indicates significant structural differences between pmARC-1 and HMCS-CT, with the former being a dioxo site and the latter possessing only a single terminal oxo ligand. The number of terminal oxo donors is consistent with pmARC-1 being in the Mo(VI) oxidation state and HMCS-CT in the Mo(IV) state. These structures are in accord with oxygen-atom-transfer reactivity for pmARC-1 and persulfide bond cleavage chemistry for HMCS-CT.

Molybdenum and Tungsten

Molybdenum (Mo) is an essential micronutrient for the majority of organisms ranging from bacteria to animals. To fulfil its biological role, it is incorporated into a pterin-based Mo-cofactor (Moco) and can be found in the active centre of more than 50 enzymes that are involved in key reactions of carbon, nitrogen and sulfur metabolism. Five of the Mo-enzymes are present in eukaryotes: nitrate reductase (NR), sulfite oxidase (SO), aldehyde oxidase (AO), xanthine oxidase (XO) and the amidoxime-reducing component (mARC). Cells acquire Mo in form of the oxyanion molybdate using specific molybdate transporters. In bacteria, molybdate transport is an extensively studied process and is mediated mainly by the ATP-binding cassette system ModABC. In contrast, in eukaryotes, molybdate transport is poorly understood since specific molybdate transporters remained unknown until recently. Two rather distantly related families of proteins, MOT1 and MOT2, are involved in eukaryotic molybdate transport. They each feature high-affinity molybdate transporters that regulate the intracellular concentration of Mo and thus control activity of Mo-enzymes. The present chapter presents an overview of the biological functions of Mo with special focus on recent data related to its uptake, binding and storage.

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.

Functional characterization of protein variants encoded by nonsynonymous single nucleotide polymorphisms in MARC1 and MARC2 in healthy Caucasians

Human molybdenum-containing enzyme mitochondrial amidoxime reducing component (mARC), cytochrome b5 type B, and NADH cytochrome b5 reductase form an N-reductive enzyme system that is capable of reducing N-hydroxylated compounds. Genetic variations are known, but their functional relevance is unclear. Our study aimed to investigate the incidence of nonsynonymous single nucleotide polymorphisms (SNPs) in the mARC genes in healthy Caucasian volunteers, to determine saturation of the protein variants with molybdenum cofactor (Moco), and to characterize the kinetic behavior of the protein variants by in vitro biotransformation studies. Genotype frequencies of six SNPs in the mARC genes (c.493A>G, c.560T>A, c.736T>A, and c.739G>C in MARC1; c.730G>A and c.735T>G in MARC2) were determined by pyrosequencing in a cohort of 340 healthy Caucasians. Protein variants were expressed in Escherichia coli. Saturation with Moco was determined by measurement of molybdenum by inductively coupled mass spectrometry. Steady state assays were performed with benzamidoxime. The six variants were of low frequency in this Caucasian population. Only one homozygous variant (c.493A; MARC1) was detected. All protein variants were able to bind Moco. Steady state assays showed statistically significant decreases of catalytic efficiency values for the mARC-2 wild type compared with the mARC-1 wild type (P < 0.05) and for two mARC-2 variants compared with the mARC-2 wild type (G244S, P < 0.05; C245W, P < 0.05). After simultaneous substitution of more than two amino acids in the mARC-1 protein, N-reductive activity was decreased 5-fold. One homozygous variant of MARC1 was detected in our sample. The encoded protein variant (A165T) showed no different kinetic parameters in the N-reduction of benzamidoxime.

Molybdenum metabolism in plants and crosstalk to iron

In the form of molybdate the transition metal molybdenum is essential for plants as it is required by a number of enzymes that catalyze key reactions in nitrogen assimilation, purine degradation, phytohormone synthesis, and sulfite detoxification. However, molybdate itself is biologically inactive and needs to be complexed by a specific organic pterin in order to serve as a permanently bound prosthetic group, the molybdenum cofactor, for the socalled molybdo-enyzmes. While the synthesis of molybdenum cofactor has been intensively studied, only little is known about the uptake of molybdate by the roots, its transport to the shoot and its allocation and storage within the cell. Yet, recent evidence indicates that intracellular molybdate levels are tightly controlled by molybdate transporters, in particular during plant development. Moreover, a tight connection between molybdenum and iron metabolisms is presumed because (i) uptake mechanisms for molybdate and iron affect each other, (ii) most molybdo-enzymes do also require iron-containing redox groups such as iron-sulfur clusters or heme, (iii) molybdenum metabolism has recruited mechanisms typical for iron-sulfur cluster synthesis, and (iv) both molybdenum cofactor synthesis and extramitochondrial iron-sulfur proteins involve the function of a specific mitochondrial ABC-type transporter.

Mass spectrometry strategies for clinical metabolomics and lipidomics in psychiatry, neurology, and neuro-oncology

Metabolomics research has the potential to provide biomarkers for the detection of disease, for subtyping complex disease populations, for monitoring disease progression and therapy, and for defining new molecular targets for therapeutic intervention. These potentials are far from being realized because of a number of technical, conceptual, financial, and bioinformatics issues. Mass spectrometry provides analytical platforms that address the technical barriers to success in metabolomics research; however, the limited commercial availability of analytical and stable isotope standards has created a bottleneck for the absolute quantitation of a number of metabolites. Conceptual and financial factors contribute to the generation of statistically under-powered clinical studies, whereas bioinformatics issues result in the publication of a large number of unidentified metabolites. The path forward in this field involves targeted metabolomics analyses of large control and patient populations to define both the normal range of a defined metabolite and the potential heterogeneity (eg, bimodal) in complex patient populations. This approach requires that metabolomics research groups, in addition to developing a number of analytical platforms, build sufficient chemistry resources to supply the analytical standards required for absolute metabolite quantitation. Examples of metabolomics evaluations of sulfur amino-acid metabolism in psychiatry, neurology, and neuro-oncology and of lipidomics in neurology will be reviewed.

Design and synthesis of N¹,N⁵-bis[4-(5-alkyl-1,2,4-oxadiazol-3-yl)phenyl]glutaramides as potential antifungal prodrugs

A facile three step synthesis of a group of N1,N5-bis[4-(5-alkyl-1,2,4-oxadiazol-3-yl)phenyl]glutaramides, N1,N5-bis[4-(1,2,4-oxadiazol-3-yl)phenyl]glutaramide and N1,N5-bis[4-(5-oxo-4,5-dihydro-1,2,4-oxadiazol-3-yl)phenyl]glutaramide is described. These products are designed to function as masked bis-amidine prodrugs of a promising N1,N5-bis[4-(N'-(carbamimidoyl)phenyl]glutaramide antifungal lead.

The involvement of mitochondrial amidoxime reducing components 1 and 2 and mitochondrial cytochrome b5 in N-reductive metabolism in human cells

The mitochondrial amidoxime reducing component mARC is a recently discovered molybdenum enzyme in mammals. mARC is not active as a standalone protein, but together with the electron transport proteins NADH-cytochrome b5 reductase (CYB5R) and cytochrome b5 (CYB5), it catalyzes the reduction of N-hydroxylated compounds such as amidoximes. The mARC-containing enzyme system is therefore considered to be responsible for the activation of amidoxime prodrugs. All hitherto analyzed mammalian genomes code for two mARC genes (also referred to as MOSC1 and MOSC2), which share high sequence similarities. By RNAi experiments in two different human cell lines, we demonstrate for the first time that both mARC proteins are capable of reducing N-hydroxylated substrates in cell metabolism. The extent of involvement is highly dependent on the expression level of the particular mARC protein. Furthermore, the mitochondrial isoform of CYB5 (CYB5B) is clearly identified as an essential component of the mARC-containing N-reductase system in human cells. The participation of the microsomal isoform (CYB5A) in N-reduction could be excluded by siRNA-mediated down-regulation in HEK-293 cells and knock-out in mice. Using heme-free apo-CYB5, the contribution of mitochondrial CYB5 to N-reductive catalysis was proven to strictly depend on heme. Finally, we created recombinant CYB5B variants corresponding to four nonsynonymous single nucleotide polymorphisms (SNPs). Investigated mutations of the heme protein seemed to have no significant impact on N-reductive activity of the reconstituted enzyme system.

The molybdenum oxotransferases and related enzymes

A perspective is provided of recent advances in our understanding of molybdenum-containing enzymes other than nitrogenase, a large and diverse group of enzymes that usually (but not always) catalyze oxygen atom transfer to or from a substrate, utilizing a Mo=O group as donor or acceptor. An emphasis is placed on the diversity of protein structure and reaction catalyzed by each of the three major families of these enzymes.

Molybdenum cofactor deficiency: review of 12 cases (MoCD and review)

Molybdenum cofactor deficiency is a rare inborn error of metabolism. The major clinical symptoms are intractable neonatal seizures, progressive encephalopathy, facial dysmorphic features and feeding difficulties. Most of the patients are misdiagnosed as hypoxic ischemic encephalopathy. The majority of patients have mutations in the MOCS1 and MOCS2 genes. Although the therapeutic treatment strategies have not been improved, genetic analysis is essential to elucidate the disease. Here, we report a review of 12 patients with Molybdenum cofactor deficiency reported from Turkey.

The mitochondrial Amidoxime Reducing Component (mARC) is involved in detoxification of N-hydroxylated base analogues

The "mitochondrial Amidoxime Reducing Component" (mARC) is the newly discovered fourth molybdenum enzyme in mammals. All hitherto analyzed mammals express two mARC proteins, referred to as mARC1 and mARC2. Together with their electron transport proteins cytochrome b(5) and NADH cytochrome b(5) reductase, they form a three-component enzyme system and catalyze the reduction of N-hydroxylated prodrugs. Here, we demonstrate the reductive detoxification of toxic and mutagenic N-hydroxylated nucleobases and their corresponding nucleosides by the mammalian mARC-containing enzyme system. The N-reductive activity was found in all tested tissues with the highest detectable conversion rates in liver, kidney, thyroid, and pancreas. According to the presumed localization, the N-reductive activity is most pronounced in enriched mitochondrial fractions. In vitro assays with the respective recombinant three-component enzyme system show that both mARC isoforms are able to reduce N-hydroxylated base analogues, with mARC1 representing the more efficient isoform. On the basis of the high specific activities with N-hydroxylated base analogues relative to other N-hydroxylated substrates, our data suggest that mARC proteins might be involved in protecting cellular DNA from misincorporation of toxic N-hydroxylated base analogues during replication by converting them to the correct purine or pyrimidine bases, respectively.

Cell biology of molybdenum in plants and humans

The transition element molybdenum (Mo) needs to be complexed by a special cofactor in order to gain catalytic activity. With the exception of bacterial Mo-nitrogenase, where Mo is a constituent of the FeMo-cofactor, Mo is bound to a pterin, thus forming the molybdenum cofactor Moco, which in different variants is the active compound at the catalytic site of all other Mo-containing enzymes. In eukaryotes, the most prominent Mo-enzymes are nitrate reductase, sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase, and the mitochondrial amidoxime reductase. The biosynthesis of Moco involves the complex interaction of six proteins and is a process of four steps, which also requires iron, ATP and copper. After its synthesis, Moco is distributed to the apoproteins of Mo-enzymes by Moco-carrier/binding proteins. A deficiency in the biosynthesis of Moco has lethal consequences for the respective organisms. In humans, Moco deficiency is a severe inherited inborn error in metabolism resulting in severe neurodegeneration in newborns and causing early childhood death. This article is part of a Special Issue entitled: Cell Biology of Metals.