Synthesis of adenylated molybdopterin: an essential step for molybdenum insertion

The structural principles underlying molybdenum insertase complex assembly

Within the cell, the trace element molybdenum (Mo) is only biologically active when complexed either within the nitrogenase-specific FeMo cofactor or within the molybdenum cofactor (Moco). Moco consists of an organic part, called molybdopterin (MPT) and an inorganic part, that is, the Mo-center. The enzyme which catalyzes the Mo-center formation is the molybdenum insertase (Mo-insertase). Mo-insertases consist of two functional domains called G- and E-domain. The G-domain catalyzes the formation of adenylated MPT (MPT-AMP), which is the substrate for the E-domain, that catalyzes the actual molybdate insertion reaction. Though the functions of E- and G-domain have been elucidated to great structural and mechanistic detail, their combined function is poorly characterized. In this work, we describe a structural model of the eukaryotic Mo-insertase Cnx1 complex that was generated based on cross-linking mass spectrometry combined with computational modeling. We revealed Cnx1 to form an asymmetric hexameric complex which allows the E- and G-domain active sites to align in a catalytic productive orientation toward each other.

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.

Chlamydomonas reinhardtii-A Reference Microorganism for Eukaryotic Molybdenum Metabolism

Molybdenum (Mo) is vital for the activity of a small but essential group of enzymes called molybdoenzymes. So far, specifically five molybdoenzymes have been discovered in eukaryotes: nitrate reductase, sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase, and mARC. In order to become biologically active, Mo must be chelated to a pterin, forming the so-called Mo cofactor (Moco). Deficiency or mutation in any of the genes involved in Moco biosynthesis results in the simultaneous loss of activity of all molybdoenzymes, fully or partially preventing the normal development of the affected organism. To prevent this, the different mechanisms involved in Mo homeostasis must be finely regulated. Chlamydomonas reinhardtii is a unicellular, photosynthetic, eukaryotic microalga that has produced fundamental advances in key steps of Mo homeostasis over the last 30 years, which have been extrapolated to higher organisms, both plants and animals. These advances include the identification of the first two molybdate transporters in eukaryotes (MOT1 and MOT2), the characterization of key genes in Moco biosynthesis, the identification of the first enzyme that protects and transfers Moco (MCP1), the first characterization of mARC in plants, and the discovery of the crucial role of the nitrate reductase-mARC complex in plant nitric oxide production. This review aims to provide a comprehensive summary of the progress achieved in using C. reinhardtii as a model organism in Mo homeostasis and to propose how this microalga can continue improving with the advancements in this field in the future.

Unveiling the mechanisms and biosynthesis of a novel nickel-pincer enzyme

The nickel-pincer nucleotide (NPN) coenzyme, a substituted pyridinium mononucleotide that tri-coordinates nickel, was first identified covalently attached to a lysine residue in the LarA protein of lactate racemase. Starting from nicotinic acid adenine dinucleotide, LarB carboxylates C5 of the pyridinium ring and hydrolyzes the phosphoanhydride, LarE converts the C3 and C5 carboxylates to thiocarboxylates, and LarC incorporates nickel to form a C-Ni and two S-Ni bonds, during the biosynthesis of this cofactor. LarB uses a novel carboxylation mechanism involving the transient formation of a cysteinyl-pyridinium adduct. Depending on the source of the enzyme, LarEs either catalyze a sacrificial sulfur transfer from a cysteinyl side chain resulting in the formation of dehydroalanine or they utilize a [4Fe-4S] cluster bound by three cysteine residues to accept and transfer a non-core sulfide atom. LarC is a CTP-dependent enzyme that cytidinylylates its substrate, adds nickel, then hydrolyzes the product to release NPN and CMP. Homologs of the four lar genes are widely distributed in microorganisms, with some species containing multiple copies of larA whereas others lack this gene, consistent with the cofactor serving other functions. Several LarA-like proteins were shown to catalyze racemase or epimerase activities using 2-hydroxyacid substrates other than lactic acid. Thus, lactate racemase is the founding member of a large family of NPN-containing enzymes.

Function of Molybdenum Insertases

For most organisms molybdenum is essential for life as it is found in the active site of various vitally important molybdenum dependent enzymes (Mo-enzymes). Here, molybdenum is bound to a pterin derivative called molybdopterin (MPT), thus forming the molybdenum cofactor (Moco). Synthesis of Moco involves the consecutive action of numerous enzymatic reaction steps, whereby molybdenum insertases (Mo-insertases) catalyze the final maturation step, i.e., the metal insertion reaction yielding Moco. This final maturation step is subdivided into two partial reactions, each catalyzed by a distinctive Mo-insertase domain. Initially, MPT is adenylylated by the Mo-insertase G-domain, yielding MPT-AMP which is used as substrate by the E-domain. This domain catalyzes the insertion of molybdate into the MPT dithiolene moiety, leading to the formation of Moco-AMP. Finally, the Moco-AMP phosphoanhydride bond is cleaved by the E-domain to liberate Moco from its synthesizing enzyme. Thus formed, Moco is physiologically active and may be incorporated into the different Mo-enzymes or bind to carrier proteins instead.

Genome-Wide Association Study Reveals Genomic Regions Associated With Molybdenum Accumulation in Wheat Grains

Molybdenum (Mo) is an essential micronutrient for almost all organisms. Wheat, a major staple crop worldwide, is one of the main dietary sources of Mo. However, the genetic basis for the variation of Mo content in wheat grains remains largely unknown. Here, a genome-wide association study (GWAS) was performed on the Mo concentration in the grains of 207 wheat accessions to dissect the genetic basis of Mo accumulation in wheat grains. As a result, 77 SNPs were found to be significantly associated with Mo concentration in wheat grains, among which 52 were detected in at least two sets of data and distributed on chromosome 2A, 7B, and 7D. Moreover, 48 out of the 52 common SNPs were distributed in the 726,761,412-728,132,521 bp genomic region of chromosome 2A. Three putative candidate genes, including molybdate transporter 1;2 (TraesCS2A02G496200), molybdate transporter 1;1 (TraesCS2A02G496700), and molybdopterin biosynthesis protein CNX1 (TraesCS2A02G497200), were identified in this region. These findings provide new insights into the genetic basis for Mo accumulation in wheat grains and important information for further functional characterization and breeding to improve wheat grain quality.

Characterization of the nickel-inserting cyclometallase LarC from Moorella thermoacetica and identification of a cytidinylylated reaction intermediate

LarC catalyzes the CTP-dependent insertion of nickel ion into pyridinium-3,5-bisthiocarboxylic acid mononucleotide (P2TMN), the final biosynthetic step for generating the nickel-pincer nucleotide (NPN) enzyme cofactor. In this study, we characterized a LarC homolog from Moorella thermoacetica (LarCMt) and characterized selected properties of the protein. We ruled out the hypothesis that enzyme inhibition by its product pyrophosphate accounts for its apparent single-turnover activity. Most notably, we identified a cytidinylylated-substrate intermediate that is formed during the reaction of LarCMt. Selected LarCMt variants with substitutions at the predicted CTP-binding site retained substantial amounts of activity, but exhibited greatly reduced levels of the CMP-P2TMN intermediate. In contrast, enhanced amounts of the CMP-P2TMN intermediate were generated when using LarCMt from cells grown on medium without supplemental nickel. On the basis of these results, we propose a functional role for CTP in the unprecedented nickel-insertase reaction during NPN biosynthesis.

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.

Engineering heterologous molybdenum-cofactor-biosynthesis and nitrate-assimilation pathways enables nitrate utilization by Saccharomyces cerevisiae

Metabolic capabilities of cells are not only defined by their repertoire of enzymes and metabolites, but also by availability of enzyme cofactors. The molybdenum cofactor (Moco) is widespread among eukaryotes but absent from the industrial yeast Saccharomyces cerevisiae. No less than 50 Moco-dependent enzymes covering over 30 catalytic activities have been described to date, introduction of a functional Moco synthesis pathway offers interesting options to further broaden the biocatalytic repertoire of S. cerevisiae. In this study, we identified seven Moco biosynthesis genes in the non-conventional yeast Ogataea parapolymorpha by SpyCas9-mediated mutational analysis and expressed them in S. cerevisiae. Functionality of the heterologously expressed Moco biosynthesis pathway in S. cerevisiae was assessed by co-expressing O. parapolymorpha nitrate-assimilation enzymes, including the Moco-dependent nitrate reductase. Following two-weeks of incubation, growth of the engineered S. cerevisiae strain was observed on nitrate as sole nitrogen source. Relative to the rationally engineered strain, the evolved derivatives showed increased copy numbers of the heterologous genes, increased levels of the encoded proteins and a 5-fold higher nitrate-reductase activity in cell extracts. Growth at nM molybdate concentrations was enabled by co-expression of a Chlamydomonas reinhardtii high-affinity molybdate transporter. In serial batch cultures on nitrate-containing medium, a non-engineered S. cerevisiae strain was rapidly outcompeted by the spoilage yeast Brettanomyces bruxellensis. In contrast, an engineered and evolved nitrate-assimilating S. cerevisiae strain persisted during 35 generations of co-cultivation. This result indicates that the ability of engineered strains to use nitrate may be applicable to improve competitiveness of baker's yeast in industrial processes upon contamination with spoilage yeasts.

Fosdenopterin: a First-in-class Synthetic Cyclic Pyranopterin Monophosphate for the Treatment of Molybdenum Cofactor Deficiency Type A

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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.

Tungstoenzymes: Occurrence, Catalytic Diversity and Cofactor Synthesis

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

Biological pincer complexes

At least two types of pincer complexes are known to exist in biology. A metal-pyrroloquinolone quinone (PQQ) cofactor was first identified in bacterial methanol dehydrogenase, and later also found in selected short-chain alcohol dehydrogenases of other microorganisms. The PQQ-associated metal can be calcium, magnesium, or a rare earth element depending on the enzyme sequence. Synthesis of this organic ligand requires a series of accessory proteins acting on a small peptide, PqqA. Binding of metal to PQQ yields an ONO-type pincer complex. More recently, a nickel-pincer nucleotide (NPN) cofactor was discovered in lactate racemase, LarA. This cofactor derives from nicotinic acid adenine dinucleotide via action of a carboxylase/hydrolase, sulfur transferase, and nickel insertase, resulting in an SCS-type pincer complex. The NPN cofactor likely occurs in selected other racemases and epimerases of bacteria, archaea, and a few eukaryotes.

The First Step of Neurospora crassa Molybdenum Cofactor Biosynthesis: Regulatory Aspects under N-Derepressing and Nitrate-Inducing Conditions

Molybdenum cofactor (Moco) is the active site prosthetic group found in all Moco dependent enzymes, except for nitrogenase. Mo-enzymes are crucial for viability throughout all kingdoms of life as they catalyze a diverse set of two electron transfer reactions. The highly conserved Moco biosynthesis pathway consists of four different steps in which guanosine triphosphate is converted into cyclic pyranopterin monophosphate, molybdopterin (MPT), and subsequently adenylated MPT and Moco. Although the enzymes and mechanisms involved in these steps are well characterized, the regulation of eukaryotic Moco biosynthesis is not. Within this work, we described the regulation of Moco biosynthesis in the filamentous fungus Neurospora crassa, which revealed the first step of the multi-step pathway to be under transcriptional control. We found, that upon the induction of high cellular Moco demand a single transcript variant of the nit-7 gene is increasingly formed pointing towards, that essentially the encoded enzyme NIT7-A is the key player for Moco biosynthesis activity in Neurospora.

Insights into the Cnx1E catalyzed MPT-AMP hydrolysis

Molybdenum insertases (Mo-insertases) catalyze the final step of molybdenum cofactor (Moco) biosynthesis, an evolutionary old and highly conserved multi-step pathway. In the first step of the pathway, GTP serves as substrate for the formation of cyclic pyranopterin monophosphate, which is subsequently converted into molybdopterin (MPT) in the second pathway step. In the following synthesis steps, MPT is adenylated yielding MPT-AMP that is subsequently used as substrate for enzyme catalyzed molybdate insertion. Molybdate insertion and MPT-AMP hydrolysis are catalyzed by the Mo-insertase E-domain. Earlier work reported a highly conserved aspartate residue to be essential for Mo-insertase functionality. In this work, we confirmed the mechanistic relevance of this residue for the Arabidopsis thaliana Mo-insertase Cnx1E. We found that the conservative substitution of Cnx1E residue Asp274 by Glu (D274E) leads to an arrest of MPT-AMP hydrolysis and hence to the accumulation of MPT-AMP. We further showed that the MPT-AMP accumulation goes in hand with the accumulation of molybdate. By crystallization and structure determination of the Cnx1E variant D274E, we identified the potential reason for the missing hydrolysis activity in the disorder of the region spanning amino acids 269 to 274. We reasoned that this is caused by the inability of a glutamate in position 274 to coordinate the octahedral Mg2+-water complex in the Cnx1E active site.

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.

The functional principle of eukaryotic molybdenum insertases

The molybdenum cofactor (Moco) is a redox-active prosthetic group found in the active site of Moco-dependent enzymes, which are vitally important for life. Moco biosynthesis involves several enzymes that catalyze the subsequent conversion of GTP into cyclic pyranopterin monophosphate (cPMP), molybdopterin (MPT), adenylated MPT (MPT-AMP), and finally Moco. While the underlying principles of cPMP, MPT, and MPT-AMP formation are well understood, the molybdenum insertase (Mo-insertase)-catalyzed final Moco maturation step is not. In the present study, we analyzed high-resolution X-ray datasets of the plant Mo-insertase Cnx1E that revealed two molybdate-binding sites within the active site, hence improving the current view on Cnx1E functionality. The presence of molybdate anions in either of these sites is tied to a distinctive backbone conformation, which we suggest to be essential for Mo-insertase molybdate selectivity and insertion efficiency.

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.

Iron Sulfur and Molybdenum Cofactor Enzymes Regulate the Drosophila Life Cycle by Controlling Cell Metabolism

Iron sulfur (Fe-S) clusters and the molybdenum cofactor (Moco) are present at enzyme sites, where the active metal facilitates electron transfer. Such enzyme systems are soluble in the mitochondrial matrix, cytosol and nucleus, or embedded in the inner mitochondrial membrane, but virtually absent from the cell secretory pathway. They are of ancient evolutionary origin supporting respiration, DNA replication, transcription, translation, the biosynthesis of steroids, heme, catabolism of purines, hydroxylation of xenobiotics, and cellular sulfur metabolism. Here, Fe-S cluster and Moco biosynthesis in Drosophila melanogaster is reviewed and the multiple biochemical and physiological functions of known Fe-S and Moco enzymes are described. We show that RNA interference of Mocs3 disrupts Moco biosynthesis and the circadian clock. Fe-S-dependent mitochondrial respiration is discussed in the context of germ line and somatic development, stem cell differentiation and aging. The subcellular compartmentalization of the Fe-S and Moco assembly machinery components and their connections to iron sensing mechanisms and intermediary metabolism are emphasized. A biochemically active Fe-S core complex of heterologously expressed fly Nfs1, Isd11, IscU, and human frataxin is presented. Based on the recent demonstration that copper displaces the Fe-S cluster of yeast and human ferredoxin, an explanation for why high dietary copper leads to cytoplasmic iron deficiency in flies is proposed. Another proposal that exosomes contribute to the transport of xanthine dehydrogenase from peripheral tissues to the eye pigment cells is put forward, where the Vps16a subunit of the HOPS complex may have a specialized role in concentrating this enzyme within pigment granules. Finally, we formulate a hypothesis that (i) mitochondrial superoxide mobilizes iron from the Fe-S clusters in aconitase and succinate dehydrogenase; (ii) increased iron transiently displaces manganese on superoxide dismutase, which may function as a mitochondrial iron sensor since it is inactivated by iron; (iii) with the Krebs cycle thus disrupted, citrate is exported to the cytosol for fatty acid synthesis, while succinyl-CoA and the iron are used for heme biosynthesis; (iv) as iron is used for heme biosynthesis its concentration in the matrix drops allowing for manganese to reactivate superoxide dismutase and Fe-S cluster biosynthesis to reestablish the Krebs cycle.

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.

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.

Study of Different Variants of Mo Enzyme crARC and the Interaction with Its Partners crCytb5-R and crCytb5-1

The mARC (mitochondrial Amidoxime Reducing Component) proteins are recently discovered molybdenum (Mo) Cofactor containing enzymes. They are involved in the reduction of several N-hydroxylated compounds (NHC) and nitrite. Some NHC are prodrugs containing an amidoxime structure or mutagens such as 6-hydroxylaminopurine (HAP). We have studied this protein in the green alga Chlamydomonas reinhardtii (crARC). Interestingly, all the ARC proteins need the reducing power supplied by other proteins. It is known that crARC requires a cytochrome b₅ (crCytb5-1) and a cytochrome b₅ reductase (crCytb5-R) that form an electron transport chain from NADH to the substrates. Here, we have investigated NHC reduction by crARC, the interaction with its partners and the function of important conserved amino acids. Interactions among crARC, crCytb5-1 and crCytb5-R have been studied by size-exclusion chromatography. A protein complex between crARC, crCytb5-1 and crCytb5-R was identified. Twelve conserved crARC amino acids have been substituted by alanine by in vitro mutagenesis. We have determined that the amino acids D182, F210 and R276 are essential for NHC reduction activity, R276 is important and F210 is critical for the Mo Cofactor chelation. Finally, the crARC C-termini were shown to be involved in protein aggregation or oligomerization.

Dimerization of the plant molybdenum insertase Cnx1E is required for synthesis of the molybdenum cofactor

The molybdenum cofactor (Moco) is a redox active prosthetic group, essentially required for numerous enzyme-catalyzed two electron transfer reactions. Moco is synthesized by an evolutionarily old and highly conserved multistep pathway. In the last step of Moco biosynthesis, the molybdenum center is inserted into the final Moco precursor adenylated molybdopterin (MPT-AMP). This unique and yet poorly characterized maturation reaction finally yields physiologically active Moco. In the model plant Arabidopsis, the two domain enzyme, Cnx1, is required for Moco formation. Recently, a genetic screen identified novel Arabidopsis cnx1 mutant plant lines each harboring a single amino acid exchange in the N-terminal Cnx1E domain. Biochemical characterization of the respective recombinant Cnx1E variants revealed two different amino acid exchanges (S197F and G175D) that impair Cnx1E dimerization, thus linking Cnx1E oligomerization to Cnx1 functionality. Analysis of the Cnx1E structure identified Cnx1E active site-bound molybdate and magnesium ions, which allowed to fine-map the Cnx1E MPT-AMP-binding site.

Structural Framework for Metal Incorporation during Molybdenum Cofactor Biosynthesis

The molybdenum cofactor (Moco) is essential for the catalytic activity of all molybdenum-containing enzymes with the exception of nitrogenase. Moco biosynthesis follows an evolutionarily highly conserved pathway and genetic deficiencies in the corresponding human enzymes result in Moco deficiency, which manifests itself in severe neurological symptoms and death in childhood. In humans the final steps of Moco biosynthesis are catalyzed by gephyrin, specifically the penultimate adenylation of molybdopterin (MPT) by its N-terminal G domain (GephG) and the final metal incorporation by its C-terminal E domain (GephE). To better understand the poorly defined molecular framework of this final step, we determined high-resolution crystal structures of GephE in the apo state and in complex with ADP, AMP, and molybdate. Our data provide novel insights into the catalytic steps leading to final Moco maturation, namely deadenylation as well as molybdate binding and insertion.

Biosynthesis and Insertion of the Molybdenum Cofactor

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

Investigation of molybdenum cofactor deficiency due to MOCS2 deficiency in a newborn baby

Background

Molybdenum cofactor deficiency (MOCD) is a severe autosomal recessive neonatal metabolic disease that causes seizures and death or severe brain damage. Symptoms, signs and cerebral images can resemble those attributed to intrapartum hypoxia. In humans, molybdenum cofactor (MOCO) has been found to participate in four metabolic reactions: aldehyde dehydrogenase (or oxidase), xanthine oxidoreductase (or oxidase) and sulfite oxidase, and some of the components of molybdenum cofactor synthesis participate in amidoxime reductase. A newborn girl developed refractory seizures, opisthotonus, exaggerated startle reflexes and vomiting on the second day of life. Treatment included intravenous fluid, glucose supplementation, empiric antibiotic therapy and anticonvulsant medication. Her encephalopathy progressed, and she was given palliative care and died aged 1 week. There were no dysmorphic features, including ectopia lentis but ultrasonography revealed a thin corpus callosum.

Objectives

The aim of this study is to provide etiology, prognosis and genetic counseling.

Methods

Biochemical analysis of urine, blood, Sanger sequencing of leukocyte DNA, and analysis of the effect of the mutation on protein expression.

Results

Uric acid level was low in blood, and S-sulfo-L-cysteine and xanthine were elevated in urine. Compound Z was detected in urine. Two MOCS2 gene mutations were identified: c.501 + 2delT, which disrupts a conserved splice site sequence, and c.419C > T (pS140F). Protein expression studies confirmed that the p.S140F substitution was pathogenic. The parents were shown to be heterozygous carriers.

Conclusions

Mutation analysis confirmed that the MOCD in this family could not be treated with cPMP infusion, and enabled prenatal diagnosis and termination of a subsequent affected pregnancy.

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Molybdenum cofactor deficiency is a severe autosomal recessive metabolic disease.

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In neonates it can resemble hypoxemic ischemic encephalopathy.

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An affected neonate had high urine L-sulfo-S-cysteine, xanthine, and low blood uric acid.

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Compound Z was detected in urine.

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Of 2 mutations found in the MOC2A gene, one was shown to disrupt protein expression.

Biosynthesis and Insertion of the Molybdenum Cofactor

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

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.

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.

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

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

Identification of a bis-molybdopterin intermediate in molybdenum cofactor biosynthesis in Escherichia coli

The molybdenum cofactor is an important cofactor, and its biosynthesis is essential for many organisms, including humans. Its basic form comprises a single molybdopterin (MPT) unit, which binds a molybdenum ion bearing three oxygen ligands via a dithiolene function, thus forming Mo-MPT. In bacteria, this form is modified to form the bis-MPT guanine dinucleotide cofactor with two MPT units coordinated at one molybdenum atom, which additionally contains GMPs bound to the terminal phosphate group of the MPTs (bis-MGD). The MobA protein catalyzes the nucleotide addition to MPT, but the mechanism of the biosynthesis of the bis-MGD cofactor has remained enigmatic. We have established an in vitro system for studying bis-MGD assembly using purified compounds. Quantification of the MPT/molybdenum and molybdenum/phosphorus ratios, time-dependent assays for MPT and MGD detection, and determination of the numbers and lengths of Mo-S and Mo-O bonds by X-ray absorption spectroscopy enabled identification of a novel bis-Mo-MPT intermediate on MobA prior to nucleotide attachment. The addition of Mg-GTP to MobA loaded with bis-Mo-MPT resulted in formation and release of the final bis-MGD product. This cofactor was fully functional and reconstituted the catalytic activity of apo-TMAO reductase (TorA). We propose a reaction sequence for bis-MGD formation, which involves 1) the formation of bis-Mo-MPT, 2) the addition of two GMP units to form bis-MGD on MobA, and 3) the release and transfer of the mature cofactor to the target protein TorA, in a reaction that is supported by the specific chaperone TorD, resulting in an active molybdoenzyme.

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.

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.

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

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

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.

Molybdenum metabolism in the alga Chlamydomonas stands at the crossroad of those in Arabidopsis and humans

Molybdenum (Mo) is a very scarce element whose function is fundamental in living beings within the active site of Mo-oxidoreductases, playing key roles in the metabolism of N, S, purines, hormone biosynthesis, transformation of drugs and xenobiotics, etc. In eukaryotes, each step from Mo acquisition until its incorporation into a biologically active molybdenum cofactor (Moco) together with the assembly of this Moco in Mo-enzymes is almost understood. The deficiency in function of a particular molybdoenzyme can be critical for the survival of the organism dependent on the pathway involved. However, incapacity in forming a functional Moco has a pleiotropic effect in the different processes involving this cofactor. A detailed overview of Mo metabolism: (a) specific transporters for molybdate, (b) the universal biosynthesis pathway for Moco from GTP, (c) Moco-carrier and Moco-binding proteins for Moco transfer and (d) Mo-enzymes, is analyzed in light of recent findings and three systems are compared, the unicellular microalga Chlamydomonas, the plant Arabidopsis and humans.

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

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

Crystal structures, dynamics and functional implications of molybdenum-cofactor biosynthesis protein MogA from two thermophilic organisms

Molybdenum-cofactor (Moco) biosynthesis is an evolutionarily conserved pathway in almost all kingdoms of life, including humans. Two proteins, MogA and MoeA, catalyze the last step of this pathway in bacteria, whereas a single two-domain protein carries out catalysis in eukaryotes. Here, three crystal structures of the Moco-biosynthesis protein MogA from the two thermophilic organisms Thermus thermophilus (TtMogA; 1.64 Å resolution, space group P2(1)) and Aquifex aeolicus (AaMogA; 1.70 Å resolution, space group P2(1) and 1.90 Å resolution, space group P1) have been determined. The functional roles and the residues involved in oligomerization of the protein molecules have been identified based on a comparative analysis of these structures with those of homologous proteins. Furthermore, functional roles have been proposed for the N- and C-terminal residues. In addition, a possible protein-protein complex of MogA and MoeA has been proposed and the residues involved in protein-protein interactions are discussed. Several invariant water molecules and those present at the subunit interfaces have been identified and their possible structural and/or functional roles are described in brief. In addition, molecular-dynamics and docking studies with several small molecules (including the substrate and the product) have been carried out in order to estimate their binding affinities towards AaMogA and TtMogA. The results obtained are further compared with those obtained for homologous eukaryotic proteins.

Duplicated gephyrin genes showing distinct tissue distribution and alternative splicing patterns mediate molybdenum cofactor biosynthesis, glycine receptor clustering, and escape behavior in zebrafish

Gephyrin mediates the postsynaptic clustering of glycine receptors (GlyRs) and GABA(A) receptors at inhibitory synapses and molybdenum-dependent enzyme (molybdoenzyme) activity in non-neuronal tissues. Gephyrin knock-out mice show a phenotype resembling both defective glycinergic transmission and molybdenum cofactor (Moco) deficiency and die within 1 day of birth due to starvation and dyspnea resulting from deficits in motor and respiratory networks, respectively. To address whether gephyrin function is conserved among vertebrates and whether gephyrin deficiency affects molybdoenzyme activity and motor development, we cloned and characterized zebrafish gephyrin genes. We report here that zebrafish have two gephyrin genes, gphna and gphnb. The former is expressed in all tissues and has both C3 and C4 cassette exons, and the latter is expressed predominantly in the brain and spinal cord and harbors only C4 cassette exons. We confirmed that all of the gphna and gphnb splicing isoforms have Moco synthetic activity. Antisense morpholino knockdown of either gphna or gphnb alone did not disturb synaptic clusters of GlyRs in the spinal cord and did not affect touch-evoked escape behaviors. However, on knockdown of both gphna and gphnb, embryos showed impairments in GlyR clustering in the spinal cord and, as a consequence, demonstrated touch-evoked startle response behavior by contracting antagonistic muscles simultaneously, instead of displaying early coiling and late swimming behaviors, which are executed by side-to-side muscle contractions. These data indicate that duplicated gephyrin genes mediate Moco biosynthesis and control postsynaptic clustering of GlyRs, thereby mediating key escape behaviors in zebrafish.