Diphthamide biosynthesis requires an organic radical generated by an iron-sulphur enzyme

Phosphomethylpyrimidine Synthase (ThiC): Trapping of Five Intermediates Provides Mechanistic Insights on a Complex Radical Cascade Reaction in Thiamin Biosynthesis

Phosphomethylpyrimidine synthase (ThiC) catalyzes the conversion of AIR to the thiamin pyrimidine HMP-P. This reaction is the most complex enzyme-catalyzed radical cascade identified to date, and the detailed mechanism has remained elusive. In this paper, we describe the trapping of five new intermediates that provide snapshots of the ThiC reaction coordinate and enable the formulation of a revised mechanism for the ThiC-catalyzed reaction.

Functional Integrity of Radical SAM Enzyme Dph1•Dph2 Requires Non-Canonical Cofactor Motifs with Tandem Cysteines

The Dph1•Dph2 heterodimer from yeast is a radical SAM (RS) enzyme that generates the 3-amino-3-carboxy-propyl (ACP) precursor for diphthamide, a clinically relevant modification on eukaryotic elongation factor 2 (eEF2). ACP formation requires SAM cleavage and atypical Cys-bound Fe-S clusters in each Dph1 and Dph2 subunit. Intriguingly, the first Cys residue in each motif is found next to another ill-defined cysteine that we show is conserved across eukaryotes. As judged from structural modeling, the orientation of these tandem cysteine motifs (TCMs) suggests a candidate Fe-S cluster ligand role. Hence, we generated, by site-directed DPH1 and DPH2 mutagenesis, Dph1•Dph2 variants with cysteines from each TCM replaced individually or in combination by serines. Assays diagnostic for diphthamide formation in vivo reveal that while single substitutions in the TCM of Dph2 cause mild defects, double mutations almost entirely inactivate the RS enzyme. Based on enhanced Dph1 and Dph2 subunit instability in response to cycloheximide chases, the variants with Cys substitutions in their cofactor motifs are particularly prone to protein degradation. In sum, we identify a fourth functionally cooperative Cys residue within the Fe-S motif of Dph2 and show that the Cys-based cofactor binding motifs in Dph1 and Dph2 are critical for the structural integrity of the dimeric RS enzyme in vivo.

Diphthamide - a conserved modification of eEF2 with clinical relevance

Diphthamide, a complex modification on eukaryotic translation elongation factor 2 (eEF2), assures reading-frame fidelity during translation. Diphthamide and enzymes for its synthesis are conserved in eukaryotes and archaea. Originally identified as target for diphtheria toxin (DT) in humans, its clinical relevance now proves to be broader than the link to pathogenic bacteria. Diphthamide synthesis enzymes (DPH1 and DPH3) are associated with cancer, and DPH gene mutations can cause diphthamide deficiency syndrome (DDS). Finally, new analyses provide evidence that diphthamide may restrict propagation of viruses including SARS-CoV-2 and HIV-1, and that DPH enzymes are targeted by viruses for degradation to overcome this restriction. This review describes how diphthamide is synthesized and functions in translation, and covers its clinical relevance in human development, cancer, and infectious diseases.

DPH1 Gene Mutations Identify a Candidate SAM Pocket in Radical Enzyme Dph1•Dph2 for Diphthamide Synthesis on EF2

In eukaryotes, the Dph1•Dph2 dimer is a non-canonical radical SAM enzyme. Using iron-sulfur (FeS) clusters, it cleaves the cosubstrate S-adenosyl-methionine (SAM) to form a 3-amino-3-carboxy-propyl (ACP) radical for the synthesis of diphthamide. The latter decorates a histidine residue on elongation factor 2 (EF2) conserved from archaea to yeast and humans and is important for accurate mRNA translation and protein synthesis. Guided by evidence from archaeal orthologues, we searched for a putative SAM-binding pocket in Dph1•Dph2 from Saccharomyces cerevisiae. We predict an SAM-binding pocket near the FeS cluster domain that is conserved across eukaryotes in Dph1 but not Dph2. Site-directed DPH1 mutagenesis and functional characterization through assay diagnostics for the loss of diphthamide reveal that the SAM pocket is essential for synthesis of the décor on EF2 in vivo. Further evidence from structural modeling suggests particularly critical residues close to the methionine moiety of SAM. Presumably, they facilitate a geometry specific for SAM cleavage and ACP radical formation that distinguishes Dph1•Dph2 from classical radical SAM enzymes, which generate canonical 5'-deoxyadenosyl (dAdo) radicals.

Yeast gene KTI13 (alias DPH8) operates in the initiation step of diphthamide synthesis on elongation factor 2

In yeast, Elongator-dependent tRNA modifications are regulated by the Kti11•Kti13 dimer and hijacked for cell killing by zymocin, a tRNase ribotoxin. Kti11 (alias Dph3) also controls modification of elongation factor 2 (EF2) with diphthamide, the target for lethal ADP-ribosylation by diphtheria toxin (DT). Diphthamide formation on EF2 involves four biosynthetic steps encoded by the DPH1-DPH7 network and an ill-defined KTI13 function. On further examining the latter gene in yeast, we found that kti13Δ null-mutants maintain unmodified EF2 able to escape ADP-ribosylation by DT and to survive EF2 inhibition by sordarin, a diphthamide-dependent antifungal. Consistently, mass spectrometry shows kti13Δ cells are blocked in proper formation of amino-carboxyl-propyl-EF2, the first diphthamide pathway intermediate. Thus, apart from their common function in tRNA modification, both Kti11/Dph3 and Kti13 share roles in the initiation step of EF2 modification. We suggest an alias KTI13/DPH8 nomenclature indicating dual-functionality analogous to KTI11/DPH3.

DPH1 and DPH2 variants that confer susceptibility to diphthamide deficiency syndrome in human cells and yeast models

The autosomal-recessive diphthamide deficiency syndrome presents as intellectual disability with developmental abnormalities, seizures, craniofacial and additional morphological phenotypes. It is caused by reduced activity of proteins that synthesize diphthamide on human translation elongation factor 2. Diphthamide synthesis requires seven proteins (DPH1-DPH7), with clinical deficiency described for DPH1, DPH2 and DPH5. A limited set of variant alleles from syndromic patients has been functionally analyzed, but databases (gnomAD) list additional so far uncharacterized variants in human DPH1 and DPH2. Because DPH enzymes are conserved among eukaryotes, their functionality can be assessed in yeast and mammalian cells. Our experimental assessment of known and uncharacterized DPH1 and DPH2 missense alleles showed that six variants are tolerated despite inter-species conservation. Ten additional human DPH1 (G113R, A114T, H132P, H132R, S136R, C137F, L138P, Y152C, S221P, H240R) and two DPH2 (H105P, C341Y) variants showed reduced functionality and hence are deficiency-susceptibility alleles. Some variants locate close to the active enzyme center and may affect catalysis, while others may impact on enzyme activation. In sum, our study has identified functionally compromised alleles of DPH1 and DPH2 genes that likely cause diphthamide deficiency syndrome.

Oxygen toxicity causes cyclic damage by destabilizing specific Fe-S cluster-containing protein complexes

Oxygen is toxic across all three domains of life. Yet, the underlying molecular mechanisms remain largely unknown. Here, we systematically investigate the major cellular pathways affected by excess molecular oxygen. We find that hyperoxia destabilizes a specific subset of Fe-S cluster (ISC)-containing proteins, resulting in impaired diphthamide synthesis, purine metabolism, nucleotide excision repair, and electron transport chain (ETC) function. Our findings translate to primary human lung cells and a mouse model of pulmonary oxygen toxicity. We demonstrate that the ETC is the most vulnerable to damage, resulting in decreased mitochondrial oxygen consumption. This leads to further tissue hyperoxia and cyclic damage of the additional ISC-containing pathways. In support of this model, primary ETC dysfunction in the Ndufs4 KO mouse model causes lung tissue hyperoxia and dramatically increases sensitivity to hyperoxia-mediated ISC damage. This work has important implications for hyperoxia pathologies, including bronchopulmonary dysplasia, ischemia-reperfusion injury, aging, and mitochondrial disorders.

Monitoring Fe-S cluster occupancy across the E. coli proteome using chemoproteomics

Iron-sulfur (Fe-S) clusters are ubiquitous metallocofactors involved in redox chemistry, radical generation and gene regulation. Common methods to monitor Fe-S clusters include spectroscopic analysis of purified proteins and autoradiographic visualization of radiolabeled iron distribution in proteomes. Here, we report a chemoproteomic strategy that monitors changes in the reactivity of Fe-S cysteine ligands to inform on Fe-S cluster occupancy. We highlight the utility of this platform in Escherichia coli by (1) demonstrating global disruptions in Fe-S incorporation in cells cultured under iron-depleted conditions, (2) determining Fe-S client proteins reliant on five scaffold, carrier and chaperone proteins within the Isc Fe-S biogenesis pathway and (3) identifying two previously unannotated Fe-S proteins, TrhP and DppD. In summary, the chemoproteomic strategy described herein is a powerful tool that reports on Fe-S cluster incorporation directly within a native proteome, enabling the interrogation of Fe-S biogenesis pathways and the identification of previously uncharacterized Fe-S proteins.

[Advances in applications of activity-based chemical probes in the characterization of amino acid reactivities]

The discovery of novel drug targets enhances the development of novel drugs, and the discovery of novel target proteins depends on highly accurate high-throughput methods of analyzing drug-protein interactions. Protein expression levels, spatial localization, and structural differences directly affect pharmacodynamics. To date, >20000 proteins have been discovered in the human proteome by the genome and proteome projects via gene and protein sequencing. Understanding the biological functions of proteins is critical in identifying and regulating biological processes, with most remaining unidentified. Until recently, >85% of proteins were considered undruggable, mainly because of the lack of binding pockets and active sites targeted by small molecules. Therefore, characterization of the reactive sites of amino acids based on proteomic hierarchy is the key to novel drug design. Recently, with the rapid development of mass spectrometry (MS), the study of drug-target protein interactions based on proteomics technology has been considerably promoted. Activity-based protein profiling (ABPP) is an active chemical probe-based method of detecting functional enzymes and drug targets in complex samples. Compared with classical proteomics strategies, ABPP is based mainly on protein activity. It has been successfully utilized to characterize the activities of numerous protease families with crucial biological functions, such as serine hydrolases, protein kinases, glycosidases, and metalloenzymes. It has also been used to identify key enzymes that are closely related to diseases and develop covalent inhibitors for use in disease treatment. The technology used in proteome analysis ranges from gel electrophoresis to high-throughput MS due to the progress of MS technology. ABPP strategies combined with chemical probe labeling and quantitative MS enable the characterization of amino acid activity, which may enhance the discovery of novel drug targets and the development of lead compounds. Amino acid residues play critical roles in protein structures and functions, and covalent drugs targeting these amino acids are effective in treating numerous diseases. There are 20 main types of natural amino acids, with different reactivities, in the proteins in the human body. In addition, the proteins and amino acids are affected by the spatial microenvironment, leading to significant differences in their spatial reactivities. The key in evaluating the reactivities of amino acids via ABPP is to select those with high reactivities. The core of the ABPP strategy is the use of chemical probes to label amino acid sites that exhibit higher activities in certain environments. The activity-based probe (ABP) at the core of ABPP consists of three components: reactive, reporter groups and a linker. The reactive group is the basis of the ABP and anchors the drug target via strong forces, such as covalent bonds. The reaction exhibits a high specificity and conversion rate and should display a good biocompatibility. Activity probes based on different amino acid residues have been developed, and the screening of amino acid activity combined with isotope labeling is a new focus of research. Currently, different types of ABPs have been developed to target amino acids and characterize amino acid reactivity, such as cysteine labeled with an electrophilic iodoacetamide probe and lysine labeled with activated esters. ABPP facilitates the discovery of potentially therapeutic protein targets, the screening of lead compounds, and the identification of drug targets, thus aiding the design of novel drugs. This review focuses on the development of ABPP methods and the progress in the screening of amino acid reactivity using ABPs, which should be promising methods for use in designing targeted drugs with covalent interactions.

A widespread group of large plasmids in methanotrophic Methanoperedens archaea

Anaerobic methanotrophic (ANME) archaea obtain energy from the breakdown of methane, yet their extrachromosomal genetic elements are little understood. Here we describe large plasmids associated with ANME archaea of the Methanoperedens genus in enrichment cultures and other natural anoxic environments. By manual curation we show that two of the plasmids are large (155,605 bp and 191,912 bp), circular, and may replicate bidirectionally. The plasmids occur in the same copy number as the main chromosome, and plasmid genes are actively transcribed. One of the plasmids encodes three tRNAs, ribosomal protein uL16 and elongation factor eEF2; these genes appear to be missing in the host Methanoperedens genome, suggesting an obligate interdependence between plasmid and host. Our work opens the way for the development of genetic vectors to shed light on the physiology and biochemistry of Methanoperedens, and potentially genetically edit them to enhance growth and accelerate methane oxidation rates.

Mössbauer-based molecular-level decomposition of the Saccharomyces cerevisiae ironome, and preliminary characterization of isolated nuclei

One hundred proteins in Saccharomyces cerevisiae are known to contain iron. These proteins are found mainly in mitochondria, cytosol, nuclei, endoplasmic reticula, and vacuoles. Cells also contain non-proteinaceous low-molecular-mass labile iron pools (LFePs). How each molecular iron species interacts on the cellular or systems' level is underdeveloped as doing so would require considering the entire iron content of the cell-the ironome. In this paper, Mössbauer (MB) spectroscopy was used to probe the ironome of yeast. MB spectra of whole cells and isolated organelles were predicted by summing the spectral contribution of each iron-containing species in the cell. Simulations required input from published proteomics and microscopy data, as well as from previous spectroscopic and redox characterization of individual iron-containing proteins. Composite simulations were compared to experimentally determined spectra. Simulated MB spectra of non-proteinaceous iron pools in the cell were assumed to account for major differences between simulated and experimental spectra of whole cells and isolated mitochondria and vacuoles. Nuclei were predicted to contain ∼30 μM iron, mostly in the form of [Fe4S4] clusters. This was experimentally confirmed by isolating nuclei from 57Fe-enriched cells and obtaining the first MB spectra of the organelle. This study provides the first semi-quantitative estimate of all concentrations of iron-containing proteins and non-proteinaceous species in yeast, as well as a novel approach to spectroscopically characterizing LFePs.

RaS-RiPPs in Streptococci and the Human Microbiome

Radical S-adenosylmethionine (RaS) enzymes have quickly advanced to one of the most abundant and versatile enzyme superfamilies known. Their chemistry is predicated upon reductive homolytic cleavage of a carbon–sulfur bond in cofactor S-adenosylmethionine forming an oxidizing carbon-based radical, which can initiate myriad radical transformations. An emerging role for RaS enzymes is their involvement in the biosynthesis of ribosomally synthesized and post-translationally modified peptides (RiPPs), a natural product family that has become known as RaS-RiPPs. These metabolites are especially prevalent in human and mammalian microbiomes because the complex chemistry of RaS enzymes gives rise to correspondingly complex natural products with minimal cellular energy and genomic fingerprint, a feature that is advantageous in microbes with small, host-adapted genomes in competitive environments. Herein, we review the discovery and characterization of RaS-RiPPs from the human microbiome with a focus on streptococcal bacteria. We discuss the varied chemical modifications that RaS enzymes introduce onto their peptide substrates and the diverse natural products that they give rise to. The majority of RaS-RiPPs remain to be discovered, providing an intriguing avenue for future investigations at the intersection of metalloenzymology, chemical ecology, and the human microbiome.

Translational fidelity and growth of Arabidopsis require stress-sensitive diphthamide biosynthesis

Diphthamide, a post-translationally modified histidine residue of eukaryotic TRANSLATION ELONGATION FACTOR2 (eEF2), is the human host cell-sensitizing target of diphtheria toxin. Diphthamide biosynthesis depends on the 4Fe-4S-cluster protein Dph1 catalyzing the first committed step, as well as Dph2 to Dph7, in yeast and mammals. Here we show that diphthamide modification of eEF2 is conserved in Arabidopsis thaliana and requires AtDPH1. Ribosomal -1 frameshifting-error rates are increased in Arabidopsis dph1 mutants, similar to yeast and mice. Compared to the wild type, shorter roots and smaller rosettes of dph1 mutants result from fewer formed cells. TARGET OF RAPAMYCIN (TOR) kinase activity is attenuated, and autophagy is activated, in dph1 mutants. Under abiotic stress diphthamide-unmodified eEF2 accumulates in wild-type seedlings, most strongly upon heavy metal excess, which is conserved in human cells. In summary, our results suggest that diphthamide contributes to the functionality of the translational machinery monitored by plants to regulate growth.

Context-specific roles of diphthamide deficiency in hepatocellular carcinogenesis

Diphthamide biosynthesis protein 1 (DPH1) is biochemically involved in the first step of diphthamide biosynthesis, a post-translational modification of eukaryotic elongation factor 2 (EEF2). Earlier studies showed that DPH1, also known as ovarian cancer-associated gene 1 (OVCA1), is involved in ovarian carcinogenesis. However, the role of DPH1 in hepatocellular carcinoma (HCC) remains unclear. To investigate the impact of DPH1 in hepatocellular carcinogenesis, we have performed data mining from The Cancer Genome Atlas Liver Hepatocellular Carcinoma (TCGA-LIHC) dataset. We found that reduced DPH1 levels were associated with advanced stages and poor survival of patients with HCC. Also, we generated hepatocyte-specific Dph1 deficient mice and showed that diphthamide deficient EEF2 resulted in a reduced translation elongation rate in the hepatocytes and let to mild liver damage with fatty accumulation. After N-diethylnitrosamine (DEN) -induced acute liver injury, p53-mediated pericentral hepatocyte death was increased, and compensatory proliferation was reduced in Dph1-deficient mice. Consistent with these effects, Dph1 deficiency decreased the incidence of DEN-induced pericentral-derived HCC and revealed a protective effect against p53 loss. In contrast, Dph1 deficiency combined with Trp53- or Trp53/Pten-deficient hepatocytes led to increased tumor loads associated with KRT19 (K19)-positive periportal-like cell expansion in mice. Further gene set enrichment analysis also revealed that HCC patients with lower levels of DPH1 and TP53 expression had enriched gene-sets related to the cell cycle and K19-upregulated HCC. Additionally, liver tumor organoids obtained from 6-month-old Pten/Trp53/Dph1-triple-mutant mice had a higher frequency of organoid re-initiation cells and higher proliferative index compared with those of the Pten/Trp53-double-mutant. Pten/Trp53/Dph1-triple-mutant liver tumor organoids showed expression of genes associated with stem/progenitor phenotypes, including Krt19 and Prominin-1 (Cd133) progenitor markers, combined with low hepatocyte-expressed fibrinogen genes. These findings indicate that diphthamide deficiency differentially regulates hepatocellular carcinogenesis, which inhibits pericentral hepatocytes-derived tumor and promotes periportal progenitors-associated liver tumors. This article is protected by copyright. All rights reserved.

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

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

RadicalSAM.org: A Resource to Interpret Sequence-Function Space and Discover New Radical SAM Enzyme Chemistry

The radical SAM superfamily (RSS), arguably the most functionally diverse enzyme superfamily, is also one of the largest with ~700K members currently in the UniProt database. The vast majority of the members have uncharacterized enzymatic activities and metabolic functions. In this Perspective, we describe RadicalSAM.org, a new web-based resource that enables a user-friendly genomic enzymology strategy to explore sequence-function space in the RSS. The resource attempts to enable identification of isofunctional groups of radical SAM enzymes using sequence similarity networks (SSNs) and the genome context of the bacterial, archaeal, and fungal members provided by genome neighborhood diagrams (GNDs). Enzymatic activities and in vivo functions frequently can be inferred from genome context given the tendency for genes of related function to be clustered. We invite the scientific community to use RadicalSAM.org to (i) guide their experimental studies to discover new enzymatic activities and metabolic functions, (ii) contribute experimentally verified annotations to RadicalSAM.org to enhance the ability to predict novel activities and functions, and (iii) provide suggestions for improving this resource.

The B12-independent glycerol dehydratase activating enzyme from Clostridium butyricum cleaves SAM to produce 5'-deoxyadenosine and not 5'-deoxy-5'-(methylthio)adenosine

Glycerol dehydratase activating enzyme (GD-AE) is a radical S-adenosyl-l-methionine (SAM) enzyme that installs a catalytically essential amino acid backbone radical onto glycerol dehydratase in bacteria under anaerobic conditions. Although GD-AE is closely homologous to other radical SAM activases that have been shown to cleave the S-C(5') bond of SAM to produce 5'-deoxyadenosine (5'-dAdoH) and methionine, GD-AE from Clostridium butyricum has been reported to instead cleave the S-C(γ) bond of SAM to yield 5'-deoxy-5'-(methylthio)adenosine (MTA). Here we re-investigate the SAM cleavage reaction catalyzed by GD-AE and show that it produces the widely observed 5'-dAdoH, and not the less conventional product MTA.

Mechanistic investigation of B12-independent glycerol dehydratase and its activating enzyme GD-AE

GD-AE is a classical radical SAM enzyme that cleaves SAM to form 5′-deoxyadenosine (5′-dA) and a glycyl radical on B12-independent GD. GD catalyzes the glycerol dehydration reaction by direct elimination of the 2-OH group rather than migration.

Flagellin lysine methyltransferase FliB catalyzes a [4Fe-4S] mediated methyl transfer reaction

The methyltransferase FliB posttranslationally modifies surface-exposed ɛ-N-lysine residues of flagellin, the protomer of the flagellar filament in Salmonella enterica (S. enterica). Flagellin methylation, reported originally in 1959, was recently shown to enhance host cell adhesion and invasion by increasing the flagellar hydrophobicity. The role of FliB in this process, however, remained enigmatic. In this study, we investigated the properties and mechanisms of FliB from S. enterica in vivo and in vitro. We show that FliB is an S-adenosylmethionine (SAM) dependent methyltransferase, forming a membrane associated oligomer that modifies flagellin in the bacterial cytosol. Using X-band electron paramagnetic resonance (EPR) spectroscopy, zero-field 57Fe Mössbauer spectroscopy, methylation assays and chromatography coupled mass spectrometry (MS) analysis, we further found that FliB contains an oxygen sensitive [4Fe-4S] cluster that is essential for the methyl transfer reaction and might mediate a radical mechanism. Our data indicate that the [4Fe-4S] cluster is coordinated by a cysteine rich motif in FliB that is highly conserved among multiple genera of the Enterobacteriaceae family.

Identification of the Biosynthetic Gene Cluster for the Organoarsenical Antibiotic Arsinothricin

The soil bacterium Burkholderia gladioli GSRB05 produces the natural compound arsinothricin [2-amino-4-(hydroxymethylarsinoyl) butanoate] (AST), which has been demonstrated to be a broad-spectrum antibiotic. To identify the genes responsible for AST biosynthesis, a draft genome sequence of B. gladioli GSRB05 was constructed. Three genes, arsQML, in an arsenic resistance operon were found to be a biosynthetic gene cluster responsible for synthesis of AST and its precursor, hydroxyarsinothricin [2-amino-4-(dihydroxyarsinoyl) butanoate] (AST-OH). The arsL gene product is a noncanonical radical S-adenosylmethionine (SAM) enzyme that is predicted to transfer the 3-amino-3-carboxypropyl (ACP) group from SAM to the arsenic atom in inorganic arsenite, forming AST-OH, which is methylated by the arsM gene product, a SAM methyltransferase, to produce AST. Finally, the arsQ gene product is an efflux permease that extrudes AST from the cells, a common final step in antibiotic-producing bacteria. Elucidation of the biosynthetic gene cluster for this novel arsenic-containing antibiotic adds an important new tool for continuation of the antibiotic era. IMPORTANCE Antimicrobial resistance is an emerging global public health crisis, calling for urgent development of novel potent antibiotics. We propose that arsinothricin and related arsenic-containing compounds may be the progenitors of a new class of antibiotics to extend our antibiotic era. Here, we report identification of the biosynthetic gene cluster for arsinothricin and demonstrate that only three genes, two of which are novel, are required for the biosynthesis and transport of arsinothricin, in contrast to the phosphonate counterpart, phosphinothricin, which requires over 20 genes. Our discoveries will provide insight for the development of more effective organoarsenical antibiotics and illustrate the previously unknown complexity of the arsenic biogeochemical cycle, as well as bring new perspective to environmental arsenic biochemistry.

Spin-Regulated Electron Transfer and Exchange-Enhanced Reactivity in Fe4 S4 -Mediated Redox Reaction of the Dph2 Enzyme During the Biosynthesis of Diphthamide

The [4Fe-4S]-dependent radical S-adenosylmethionine (SAM) proteins is one of large families of redox enzymes that are able to carry a panoply of challenging transformations. Despite the extensive studies of structure-function relationships of radical SAM (RS) enzymes, the electronic state-dependent reactivity of the [4Fe-4S] cluster in these enzymes remains elusive. Using combined MD simulations and QM/MM calculations, we deciphered the electronic state-dependent reactivity of the [4Fe-4S] cluster in Dph2, a key enzyme involved in the biosynthesis of diphthamide. Our calculations show that the reductive cleavage of the S-C_(γ) bond is highly dependent on the electronic structure of [4Fe-4S]. Interestingly, the six electronic states can be classified into a low-energy and a high-energy groups, which are correlated with the net spin of Fe4 atom ligated to SAM. Due to the driving force of Fe4-C_(γ) bonding, the net spin on the Fe4 moiety dictate the shift of the opposite spin electron from the Fe1-Fe2-Fe3 block to SAM. Such spin-regulated electron transfer results in the exchange-enhanced reactivity in the lower-energy group compared with those in the higher-energy group. This reactivity principle provides fundamental mechanistic insights into reactivities of [4Fe-4S] cluster in RS enzymes.

Radical SAM Enzymes and Ribosomally-Synthesized and Post-translationally Modified Peptides: A Growing Importance in the Microbiomes

To face the current antibiotic resistance crisis, novel strategies are urgently required. Indeed, in the last 30 years, despite considerable efforts involving notably high-throughput screening and combinatorial libraries, only few antibiotics have been launched to the market. Natural products have markedly contributed to the discovery of novel antibiotics, chemistry and drug leads, with more than half anti-infective and anticancer drugs approved by the FDA being of natural origin or inspired by natural products. Among them, thanks to their modular structure and simple biosynthetic logic, ribosomally synthesized and posttranslationally modified peptides (RiPPs) are promising scaffolds. In addition, recent studies have highlighted the pivotal role of RiPPs in the human microbiota which remains an untapped source of natural products. In this review, we report on recent developments in radical SAM enzymology and how these unique biocatalysts have been shown to install complex and sometimes unprecedented posttranslational modifications in RiPPs with a special focus on microbiome derived enzymes.

Eukaryotic TYW1 Is a Radical SAM Flavoenzyme

TYW1 is a radical S-adenosyl-l-methionine (SAM) enzyme that catalyzes the condensation of pyruvate and N-methylguanosine-containing tRNA^Phe, forming 4-demethylwyosine-containing tRNA^Phe. Homologues of TYW1 are found in both archaea and eukarya; archaeal homologues consist of a single domain, while eukaryal homologues contain a flavin binding domain in addition to the radical SAM domain shared with archaeal homologues. In this study, TYW1 from Saccharomyces cerevisiae (ScTYW1) was heterologously expressed in Escherichia coli and purified to homogeneity. ScTYW1 is purified with 0.54 ± 0.07 and 4.2 ± 1.9 equiv of flavin mononucleotide (FMN) and iron, respectively, per mole of protein, suggesting the protein is ∼50% replete with Fe–S clusters and FMN. While both NADPH and NADH are sufficient for activity, significantly more product is observed when used in combination with flavin nucleotides. ScTYW1 is the first example of a radical SAM flavoenzyme that is active with NAD(P)H alone.

Dph3 Enables Aerobic Diphthamide Biosynthesis by Donating One Iron Atom to Transform a [3Fe-4S] to a [4Fe-4S] Cluster in Dph1-Dph2

All radical S-adenosylmethionine (radical-SAM) enzymes, including the noncanonical radical-SAM enzyme diphthamide biosynthetic enzyme Dph1-Dph2, require at least one [4Fe-4S](Cys)₃ cluster for activity. It is well-known in the radical-SAM enzyme community that the [4Fe-4S](Cys)₃ cluster is extremely air-sensitive and requires strict anaerobic conditions to reconstitute activity in vitro. Thus, how such enzymes function in vivo in the presence of oxygen in aerobic organisms is an interesting question. Working on yeast Dph1-Dph2, we found that consistent with the known oxygen sensitivity, the [4Fe-4S] cluster is easily degraded into a [3Fe-4S] cluster. Remarkably, the small iron-containing protein Dph3 donates one Fe atom to convert the [3Fe-4S] cluster in Dph1-Dph2 to a functional [4Fe-4S] cluster during the radical-SAM enzyme catalytic cycle. This mechanism to maintain radical-SAM enzyme activity in aerobic environments is likely general, and Dph3-like proteins may exist to keep other radical-SAM enzymes functional in aerobic environments.

Development and application of novel electrophilic warheads in target identification and drug discovery

Nucleophilic amino acids play important roles in maintenance of protein structure and function, covalent modification of such amino acid residues by therapeutic agents is an efficient way to treat human diseases. Most of current clinical drugs are structurally limited to α,β-unsaturated amide as an electrophilic warhead. To alleviate this issue, many novel electrophiles have been developed in recent years that can covalently bind to different amino acid residues and provides a unique way to interrogate proteins, including "undruggable" targets. With an activity-based protein profiling (ABPP) approach, the activity and functionality of a protein and its binding sites can be assessed. This facilitates an understanding of protein function, and contributes to the discovery of new druggable targets and lead compounds. Meanwhile, many novel inhibitors bearing new reactive warhead were developed and displayed remarkable pharmaceutical properties. In this perspective, we have reviewed the recent remarkable progress of novel electrophiles and their applications in target identification and drug discovery.

Menaquinone Biosynthesis: New Strategies to Trap Radical Intermediates in the MqnE-Catalyzed Reaction

Aminofutalosine synthase (MqnE) is a radical SAM enzyme that catalyzes the conversion of 3-((1-carboxyvinyl)oxy)benzoic acid to aminofutalosine during the futalosine-dependent menaquinone biosynthesis. In this Communication, we report the trapping of a radical intermediate in the MqnE-catalyzed reaction using sodium dithionite, molecular oxygen, or 5,5-dimethyl-1-pyrroline-N-oxide. These radical trapping strategies are potentially of general utility in the study of other radical SAM enzymes.

Iron in Translation: From the Beginning to the End

Iron is an essential element for all eukaryotes, since it acts as a cofactor for many enzymes involved in basic cellular functions, including translation. While the mammalian iron-regulatory protein/iron-responsive element (IRP/IRE) system arose as one of the first examples of translational regulation in higher eukaryotes, little is known about the contribution of iron itself to the different stages of eukaryotic translation. In the yeast Saccharomyces cerevisiae, iron deficiency provokes a global impairment of translation at the initiation step, which is mediated by the Gcn2-eIF2α pathway, while the post-transcriptional regulator Cth2 specifically represses the translation of a subgroup of iron-related transcripts. In addition, several steps of the translation process depend on iron-containing enzymes, including particular modifications of translation elongation factors and transfer RNAs (tRNAs), and translation termination by the ATP-binding cassette family member Rli1 (ABCE1 in humans) and the prolyl hydroxylase Tpa1. The influence of these modifications and their correlation with codon bias in the dynamic control of protein biosynthesis, mainly in response to stress, is emerging as an interesting focus of research. Taking S. cerevisiae as a model, we hereby discuss the relevance of iron in the control of global and specific translation steps.

Gene atlas of iron-containing proteins in Arabidopsis thaliana

Iron (Fe) is an essential element for the development and physiology of plants, owing to its presence in numerous proteins involved in central biological processes. Here, we established an exhaustive, manually curated inventory of genes encoding Fe-containing proteins in Arabidopsis thaliana, and summarized their subcellular localization, spatiotemporal expression and evolutionary age. We have currently identified 1068 genes encoding potential Fe-containing proteins, including 204 iron-sulfur (Fe-S) proteins, 446 haem proteins and 330 non-Fe-S/non-haem Fe proteins (updates of this atlas are available at https://conf.arabidopsis.org/display/COM/Atlas+of+Fe+containing+proteins). A fourth class, containing 88 genes for which iron binding is uncertain, is indexed as 'unclear'. The proteins are distributed in diverse subcellular compartments with strong differences per category. Interestingly, analysis of the gene age index showed that most genes were acquired early in plant evolutionary history and have progressively gained regulatory elements, to support the complex organ-specific and development-specific functions necessitated by the emergence of terrestrial plants. With this gene atlas, we provide a valuable and updateable tool for the research community that supports the characterization of the molecular actors and mechanisms important for Fe metabolism in plants. This will also help in selecting relevant targets for breeding or biotechnological approaches aiming at Fe biofortification in crops.

Characterization and Mechanistic Study of the Radical SAM Enzyme ArsS Involved in Arsenosugar Biosynthesis

Arsenosugars are a group of arsenic-containing ribosides that are found predominantly in marine algae but also in terrestrial organisms. It has been proposed that arsenosugar biosynthesis involves a key intermediate 5'-deoxy-5'-dimethylarsinoyl-adenosine (DDMAA), but how DDMAA is produced remains elusive. Now, we report characterization of ArsS as a DDMAA synthase, which catalyzes a radical S-adenosylmethionine (SAM)-mediated alkylation (adenosylation) of dimethylarsenite (DMAs^III ) to produce DDMAA. This radical-mediated reaction is redox neutral, and multiple turnover can be achieved without external reductant. Phylogenomic and biochemical analyses revealed that DDMAA synthases are widespread in distinct bacterial phyla with similar catalytic efficiencies; these enzymes likely originated from cyanobacteria. This study reveals a key step in arsenosugar biosynthesis and also a new paradigm in radical SAM chemistry, highlighting the catalytic diversity of this superfamily of enzymes.

Construction of Peptide Macrocycles via Radical-Mediated Intramolecular C-H Alkylations

Enzyme-catalyzed radical-mediated C-H functionalization reactions allow nature to create natural products of unusual three-dimensional structures from simple linear peptide precursors. In comparison, chemist's ability to harness radical C-H functionalization reactions for synthesis of complex peptides remains limited. In this work, new methods have been developed to construct peptide macrocycles via radical-mediated intramolecular C-H alkylation reactions under photoredox catalysis. Linear peptide precursors equipped with a C-terminal N-(acyloxy)phthalimide ester can cyclize with the α C-H bond of N-terminal glycine or aryl C-H bond of N-heteroarene capping units in high yield and selectivity under mild conditions. The strategy uses the C-H cyclization step to incorporate lysine, homolysine, and various heteroarene-derived amino acid linchpins into peptide macrocycles, enabling convergent and flexible synthesis of complex peptide macrocycles from simple building blocks.

Active-Site Controlled, Jahn-Teller Enabled Regioselectivity in Reductive S-C Bond Cleavage of S-Adenosylmethionine in Radical SAM Enzymes

Catalysis by canonical radical S-adenosyl-l-methionine (SAM) enzymes involves electron transfer (ET) from [4Fe-4S]+ to SAM, generating an R3S0 radical that undergoes regioselective homolytic reductive cleavage of the S-C5' bond to generate the 5'-dAdo· radical. However, cryogenic photoinduced S-C bond cleavage has regioselectively yielded either 5'-dAdo· or ·CH3, and indeed, each of the three SAM S-C bonds can be regioselectively cleaved in an RS enzyme. This diversity highlights a longstanding central question: what controls regioselective homolytic S-C bond cleavage upon SAM reduction? We here provide an unexpected answer, founded on our observation that photoinduced S-C bond cleavage in multiple canonical RS enzymes reveals two enzyme classes: in one, photolysis forms 5'-dAdo·, and in another it forms ·CH3. The identity of the cleaved S-C bond correlates with SAM ribose conformation but not with positioning and orientation of the sulfonium center relative to the [4Fe-4S] cluster. We have recognized the reduced-SAM R3S0 radical is a (2E) state with its antibonding unpaired electron in an orbital doublet, which renders R3S0 Jahn-Teller (JT)-active and therefore subject to vibronically induced distortion. Active-site forces induce a JT distortion that localizes the odd electron in a single priority S-C antibond, which undergoes regioselective cleavage. In photolytic cleavage those forces act through control of the ribose conformation and are transmitted to the sulfur via the S-C5' bond, but during catalysis thermally induced conformational changes that enable ET from a cluster iron generate dominant additional forces that specifically select S-C5' for cleavage. This motion also can explain how 5'-dAdo· subsequently forms the organometallic intermediate Ω.

Diphthamide-deficiency syndrome: a novel human developmental disorder and ribosomopathy

We describe a novel type of ribosomopathy that is defined by deficiency in diphthamidylation of translation elongation factor 2. The ribosomopathy was identified by correlating phenotypes and biochemical properties of previously described patients with diphthamide biosynthesis gene 1 (DPH1) deficiencies with a new patient that carried inactivating mutations in both alleles of the human diphthamide biosynthesis gene 2 (DPH2). The human DPH1 syndrome is an autosomal recessive disorder associated with developmental delay, abnormal head circumference (microcephaly or macrocephaly), short stature, and congenital heart disease. It is defined by variants with reduced functionality of the DPH1 gene observed so far predominantly in consanguineous homozygous patients carrying identical mutant alleles of DPH1. Here we report a child with a very similar phenotype carrying biallelic variants of the human DPH2. The gene products DPH1 and DPH2 are components of a heterodimeric enzyme complex that mediates the first step of the posttranslational diphthamide modification on the nonredundant eukaryotic translation elongation factor 2 (eEF2). Diphthamide deficiency was shown to reduce the accuracy of ribosomal protein biosynthesis. Both DPH2 variants described here severely impair diphthamide biosynthesis as demonstrated in human and yeast cells. This is the first report of a patient carrying compound heterozygous DPH2 loss-of-function variants with a DPH1 syndrome-like phenotype and implicates diphthamide deficiency as the root cause of this patient's clinical phenotype as well as of DPH1-syndrome. These findings define "diphthamide-deficiency syndrome" as a special ribosomopathy due to reduced functionality of components of the cellular machinery for eEF2-diphthamide synthesis.

Potential Correlation between Dietary Fiber-Suppressed Microbial Conversion of Choline to Trimethylamine and Formation of Methylglyoxal

Dietary interventions alter the formation of the disease-associated metabolite, trimethylamine (TMA), via intestinal microbial TMA lyase activity. Nevertheless, the mechanisms regulating microbial enzyme production are still unclear. Sequencing of the gut bacteria 16S rDNA demonstrated that dietary intervention changed the composition of the gut microbiota and the functional metagenome involved in the choline utilization pathway. Characterization of the functional profile of the metagenomes and metabonomics analysis revealed that a series of Kyoto Encyclopedia of Genes and Genomes orthologous groups and enzyme groups related to accumulation of methylglyoxal (MG) and glycine were enriched in red meat diet-fed animals, whereas fiber-rich diet suppressed glycine formation via the MG-dependent pathway. Our observations suggest associations between choline-TMA lyase expression and MG formation, which are indicative of a novel role of the gut microbiota in choline metabolism and highlight it as a potential target for inhibiting TMA production.

The Known, the New, and a Possible Surprise: A Re-Evaluation of the Nucleomorph-Encoded Proteome of Cryptophytes

Nucleomorphs are small nuclei that evolved from the nucleus of former eukaryotic endosymbionts of cryptophytes and chlorarachniophytes. These enigmatic organelles reside in their complex plastids and harbor the smallest and most compacted eukaryotic genomes investigated so far. Although the coding capacity of the nucleomorph genomes is small, a significant percentage of the encoded proteins (predicted nucleomorph-encoded proteins, pNMPs) is still not functionally annotated. We have analyzed pNMPs with unknown functions via Phyre2, a bioinformatic tool for prediction and modeling of protein structure, resulting in a functional annotation of 215 pNMPs out of 826 uncharacterized open reading frames of cryptophytes. The newly annotated proteins are predicted to participate in nucleomorph-specific functions such as chromosome organization and expression, as well as in modification and degradation of nucleomorph-encoded proteins. Additionally, we have functionally assigned nucleomorph-encoded, putatively plastid-targeted proteins among the reinvestigated pNMPs. Hints for a putative function in the periplastid compartment, the cytoplasm surrounding the nucleomorphs, emerge from the identification of pNMPs that might be homologs of endomembrane system-related proteins. These proteins are discussed in respect to their putative functions.

The Crystal Structure of Dph2 in Complex with Elongation Factor 2 Reveals the Structural Basis for the First Step of Diphthamide Biosynthesis

Elongation factor 2 (EF-2), a five-domain, GTP-dependent ribosomal translocase of archaebacteria and eukaryotes, undergoes post-translational modification to form diphthamide on a specific histidine residue in domain IV prior to binding the ribosome. The first step of diphthamide biosynthesis in archaebacteria is catalyzed by Dph2, a homodimeric radical S-adenosylmethionine (SAM) enzyme having a noncanonical architecture. Here, we describe a 3.5 Å resolution crystal structure of the Methanobrevibacter smithii (Ms) Dph2 homodimer bound to two molecules of MsEF-2, one of which is ordered and the other largely disordered. MsEF-2 is bound to both protomers of MsDph2, with domain IV bound to the active site of one protomer and domain III bound to a surface α-helix of an adjacent protomer. The histidine substrate of domain IV is inserted into the active site, which reveals for the first time the architecture of the Dph2 active site in complex with its target substrate. We also determined a high-resolution crystal structure of isolated MsDph2 bound to 5'-methylthioadenosine that shows a conserved arginine residue preoriented by conserved phenylalanine and aspartate residues for binding the carboxylate group of SAM. Mutagenesis experiments suggest that the arginine plays an important role in the first step of diphthamide biosynthesis.

Photoinduced Electron Transfer in a Radical SAM Enzyme Generates an S-Adenosylmethionine Derived Methyl Radical

Radical SAM (RS) enzymes use S-adenosyl-l-methionine (SAM) and a [4Fe-4S] cluster to initiate a broad spectrum of radical transformations throughout all kingdoms of life. We report here that low-temperature photoinduced electron transfer from the [4Fe-4S]¹⁺ cluster to bound SAM in the active site of the hydrogenase maturase RS enzyme, HydG, results in specific homolytic cleavage of the S-CH₃ bond of SAM, rather than the S-C5' bond as in the enzyme-catalyzed (thermal) HydG reaction. This result is in stark contrast to a recent report in which photoinduced ET in the RS enzyme pyruvate formate-lyase activating enzyme cleaved the S-C5' bond to generate a 5'-deoxyadenosyl radical, and provides the first direct evidence for homolytic S-CH₃ bond cleavage in a RS enzyme. Photoinduced ET in HydG generates a trapped ^•CH₃ radical, as well as a small population of an organometallic species with an Fe-CH₃ bond, denoted Ω_M. The ^•CH₃ radical is surprisingly found to exhibit rotational diffusion in the HydG active site at temperatures as low as 40 K, and is rapidly quenched: whereas 5'-dAdo^• is stable indefinitely at 77 K, ^•CH₃ quenches with a half-time of ∼2 min at this temperature. The rapid quenching and rotational/translational freedom of ^•CH₃ shows that enzymes would be unable to harness this radical as a regio- and stereospecific H atom abstractor during catalysis, in contrast to the exquisite control achieved with the enzymatically generated 5'-dAdo^•.

A unifying view of the broad-spectrum antiviral activity of RSAD2 (viperin) based on its radical-SAM chemistry

RSAD2 (cig-5), also known as viperin (virus inhibitory protein, endoplasmic reticulum associated, interferon inducible), is a member of the radical S-adenosylmethionine (SAM) superfamily of enzymes. Since the discovery of this enzyme more than a decade ago, numerous studies have shown that it exhibits antiviral activity against a wide range of viruses. However, there is no clear picture demonstrating the mechanism by which RSAD2 restricts the replication process of different viruses, largely because there is no direct evidence describing its in vivo enzymatic activity. As a result, a multifunctionality model has emerged. According to this model the mechanism by which RSAD2 restricts replication of different viruses varies and in many cases is not dependent on the radical-SAM chemistry of RSAD2. If the radical-SAM activity of RSAD2 is not required for its antiviral function, the question worth asking is: why does the cellular defence mechanism induce the expression of the radical-SAM enzyme RSAD2, which is metabolically expensive due to the requirement for a [4Fe-4S] cluster and usage of SAM? Here, in contrast to the multifunctionality view, I put forward a unifying model. I postulate that the radical-SAM activity of RSAD2 modulates cellular metabolic pathways essential for viral replication and/or cell proliferation and survival. As a result, its catalytic activity restricts the replication of a wide range of viruses via a common cellular function. This view is based on recent discoveries hinting towards possible substrates of RSAD2, re-evaluation of previous studies regarding the antiviral activity of RSAD2, and accumulating evidence suggesting a role of human RSAD2 in the metabolic reprogramming of cells.

The asymmetric function of Dph1-Dph2 heterodimer in diphthamide biosynthesis

Diphthamide, the target of diphtheria toxin, is a post-translationally modified histidine residue found in archaeal and eukaryotic translation elongation factor 2 (EF2). In the first step of diphthamide biosynthesis, a [4Fe-4S] cluster-containing radical SAM enzyme, Dph1-Dph2 heterodimer in eukaryotes or Dph2 homodimer in archaea, cleaves S-adenosylmethionine and transfers the 3-amino-3-carboxypropyl group to EF2. It was demonstrated previously that for the archaeal Dph2 homodimer, only one [4Fe-4S] cluster is necessary for the in vitro activity. Here, we demonstrate that for the eukaryotic Dph1-Dph2 heterodimer, the [4Fe-4S] cluster-binding cysteine residues in each subunit are required for diphthamide biosynthesis to occur in vivo. Furthermore, our in vitro reconstitution experiments with Dph1-Dph2 mutants suggested that the Dph1 cluster serves a catalytic role, while the Dph2 cluster facilitates the reduction of the Dph1 cluster by the physiological reducing system Dph3/Cbr1/NADH. Our results reveal the asymmetric functional roles of the Dph1-Dph2 heterodimer and may help to understand how the Fe-S clusters in radical SAM enzymes are reduced in biology.

Analysis of RNA Methylation by Phylogenetically Diverse Cfr Radical S-Adenosylmethionine Enzymes Reveals an Iron-Binding Accessory Domain in a Clostridial Enzyme

Cfr is a radical S-adenosylmethionine (SAM) RNA methylase linked to multidrug antibiotic resistance in bacterial pathogens. It catalyzes a chemically challenging C-C bond-forming reaction to methylate C8 of A2503 (Escherichia coli numbering) of 23S rRNA during ribosome assembly. The cfr gene has been identified as a mobile genetic element in diverse bacteria and in the genome of select Bacillales and Clostridiales species. Despite the importance of Cfr, few representatives have been purified and characterized in vitro. Here we show that Cfr homologues from Bacillus amyloliquefaciens, Enterococcus faecalis, Paenibacillus lautus, and Clostridioides difficile act as C8 adenine RNA methylases in biochemical assays. C. difficile Cfr contains an additional Cys-rich C-terminal domain that binds a mononuclear Fe²⁺ ion in a rubredoxin-type Cys₄ motif. The C-terminal domain can be truncated with minimal impact on C. difficile Cfr activity, but the rate of turnover is decreased upon disruption of the Fe²⁺-binding site by Zn²⁺ substitution or ligand mutation. These findings indicate an important purpose for the observed C-terminal iron in the native fusion protein. Bioinformatic analysis of the C. difficile Cfr Cys-rich domain shows that it is widespread (∼1400 homologues) as a stand-alone gene in pathogenic or commensal Bacilli and Clostridia, with >10% encoded adjacent to a predicted radical SAM RNA methylase. Although the domain is not essential for in vitro C. difficile Cfr activity, the genomic co-occurrence and high abundance in the human microbiome suggest a possible functional role for a specialized rubredoxin in certain radical SAM RNA methylases that are relevant to human health.