Identification and characterization of a novel member of the radical AdoMet enzyme superfamily and implications for the biosynthesis of the Hmd hydrogenase active site cofactor

[Fe]-Hydrogenase, Cofactor Biosynthesis and Engineering

[Fe]-hydrogenase catalyzes the heterolytic cleavage of H2 and reversible hydride transfer to methenyl-tetrahydromethanopterin. The iron-guanylylpyridinol (FeGP) cofactor is the prosthetic group of this enzyme, in which mononuclear Fe(II) is ligated with a pyridinol and two CO ligands. The pyridinol ligand fixes the iron by an acyl carbon and a pyridinol nitrogen. Biosynthetic proteins for this cofactor are encoded in the hmd co-occurring (hcg) genes. The function of HcgB, HcgC, HcgD, HcgE, and HcgF was studied by using structure-to-function analysis, which is based on the crystal structure of the proteins and subsequent enzyme assays. Recently, we reported the catalytic properties of HcgA and HcgG, novel radical S-adenosyl methionine enzymes, by using an in vitro biosynthesis assay. Here, we review the properties of [Fe]-hydrogenase and the FeGP cofactor, and the biosynthesis of the FeGP cofactor. Finally, we discuss the expected engineering of [Fe]-hydrogenase and the FeGP cofactor.

The Function of Two Radical-SAM Enzymes, HcgA and HcgG, in the Biosynthesis of the [Fe]-Hydrogenase Cofactor

In the biosynthesis of the iron-guanylylpyridinol (FeGP) cofactor, 6-carboxymethyl-5-methyl-4-hydroxy-2-pyridinol (1) is 3-methylated to form 2, then 4-guanylylated to form 3, and converted into the full cofactor. HcgA-G proteins catalyze the biosynthetic reactions. Herein, we report the function of two radical S-adenosyl methionine enzymes, HcgA and HcgG, as uncovered by in vitro complementation experiments and the use of purified enzymes. In vitro biosynthesis using the cell extract from the Methanococcus maripaludis ΔhcgA strain was complemented with HcgA or precursors 1, 2 or 3. The results suggested that HcgA catalyzes the biosynthetic reaction that forms 1. We demonstrated the formation of 1 by HcgA using the 3 kDa cell extract filtrate as the substrate. Biosynthesis in the ΔhcgG system was recovered by HcgG but not by 3, which indicated that HcgG catalyzes the reactions after the biosynthesis of 3. The data indicated that HcgG contributes to the formation of CO and completes biosynthesis of the FeGP cofactor.

Proteomic Analysis of Methanococcus voltae Grown in the Presence of Mineral and Nonmineral Sources of Iron and Sulfur

Iron sulfur (Fe-S) proteins are essential and ubiquitous across all domains of life, yet the mechanisms underpinning assimilation of iron (Fe) and sulfur (S) and biogenesis of Fe-S clusters are poorly understood. This is particularly true for anaerobic methanogenic archaea, which are known to employ more Fe-S proteins than other prokaryotes. Here, we utilized a deep proteomics analysis of Methanococcus voltae A3 cultured in the presence of either synthetic pyrite (FeS2) or aqueous forms of ferrous iron and sulfide to elucidate physiological responses to growth on mineral or nonmineral sources of Fe and S. The liquid chromatography-mass spectrometry (LCMS) shotgun proteomics analysis included 77% of the predicted proteome. Through a comparative analysis of intra- and extracellular proteomes, candidate proteins associated with FeS2 reductive dissolution, Fe and S acquisition, and the subsequent transport, trafficking, and storage of Fe and S were identified. The proteomic response shows a large and balanced change, suggesting that M. voltae makes physiological adjustments involving a range of biochemical processes based on the available nutrient source. Among the proteins differentially regulated were members of core methanogenesis, oxidoreductases, membrane proteins putatively involved in transport, Fe-S binding ferredoxin and radical S-adenosylmethionine proteins, ribosomal proteins, and intracellular proteins involved in Fe-S cluster assembly and storage. This work improves our understanding of ancient biogeochemical processes and can support efforts in biomining of minerals. IMPORTANCE Clusters of iron and sulfur are key components of the active sites of enzymes that facilitate microbial conversion of light or electrical energy into chemical bonds. The proteins responsible for transporting iron and sulfur into cells and assembling these elements into metal clusters are not well understood. Using a microorganism that has an unusually high demand for iron and sulfur, we conducted a global investigation of cellular proteins and how they change based on the mineral forms of iron and sulfur. Understanding this process will answer questions about life on early earth and has application in biomining and sustainable sources of energy.

Highlighting the Unique Roles of Radical S-Adenosylmethionine Enzymes in Methanogenic Archaea

Radical S-adenosylmethionine (SAM) enzymes catalyze an impressive variety of difficult biochemical reactions in various pathways across all domains of life. These metalloenzymes employ a reduced [4Fe-4S] cluster and SAM to generate a highly reactive 5'-deoxyadenosyl radical that is capable of initiating catalysis on otherwise unreactive substrates. Interestingly, the genomes of methanogenic archaea encode many unique radical SAM enzymes with underexplored or completely unknown functions. These organisms are responsible for the yearly production of nearly 1 billion tons of methane, a potent greenhouse gas as well as a valuable energy source. Thus, understanding the details of methanogenic metabolism and elucidating the functions of essential enzymes in these organisms can provide insights into strategies to decrease greenhouse gas emissions as well as inform advances in bioenergy production processes. This minireview provides an overview of the current state of the field regarding the functions of radical SAM enzymes in methanogens and discusses gaps in knowledge that should be addressed.

The Chemistry behind ThiC Rearrangement

Thiamine (vitamin B1) is crucial for life and plays a central role in metabolism. It contains thiazole and pyrimidine moieties that are constructed independently and then assembled together to generate thiamine phosphate. The study of the thiazole moiety is relatively clear, but deciphering the mechanistic enzymology of thiamine pyrimidine is more difficult. This review aims to summarize the recent research progress on ThiC rearrangement, mainly including the mechanism, related enzymes, and genes involved in the rearrangement.

A Rich Man, Poor Man Story of S-Adenosylmethionine and Cobalamin Revisited

S-adenosylmethionine (AdoMet) has been referred to as both "a poor man's adenosylcobalamin (AdoCbl)" and "a rich man's AdoCbl," but today, with the ever-increasing number of functions attributed to each cofactor, both appear equally rich and surprising. The recent characterization of an organometallic species in an AdoMet radical enzyme suggests that the line that differentiates them in nature will be constantly challenged. Here, we compare and contrast AdoMet and cobalamin (Cbl) and consider why Cbl-dependent AdoMet radical enzymes require two cofactors that are so similar in their reactivity. We further carry out structural comparisons employing the recently determined crystal structure of oxetanocin-A biosynthetic enzyme OxsB, the first three-dimensional structural data on a Cbl-dependent AdoMet radical enzyme. We find that the structural motifs responsible for housing the AdoMet radical machinery are largely conserved, whereas the motifs responsible for binding additional cofactors are much more varied.

C3'-Deoxygenation of Paromamine Catalyzed by a Radical S-Adenosylmethionine Enzyme: Characterization of the Enzyme AprD4 and Its Reductase Partner AprD3

C3'-deoxygenation of aminoglycosides results in their decreased susceptibility to phosphorylation thereby increasing their efficacy as antibiotics. However, the biosynthetic mechanism of C3'-deoxygenation is unknown. To address this issue, aprD4 and aprD3 genes from the apramycin gene cluster in Streptomyces tenebrarius were expressed in E. coli and the resulting gene products were characterized in vitro. AprD4 is shown to be a radical S-adenosylmethionine (SAM) enzyme, catalyzing homolysis of SAM to 5'-deoxyadenosine (5'-dAdo) in the presence of paromamine. [4'-(2) H]-Paromamine was prepared and used to show that its C4'-H is transferred to 5'-dAdo by AprD4, during which the substrate is dehydrated to a product consistent with 4'-oxolividamine. In contrast, paromamine is reduced to a deoxy product when incubated with AprD4/AprD3/NADPH. These results show that AprD4 is the first radical SAM diol-dehydratase and, along with AprD3, is responsible for 3'-deoxygenation in aminoglycoside biosynthesis.

Chemical and Biological Reduction of the Radical SAM Enzyme 7-Carboxy-7-deazaguanine [corrected] Synthase

The radical S-adenosyl-L-methionine (SAM) superfamily is a large and growing group of enzymes that conduct complex radical-mediated transformations. A one-electron reduction of SAM via the +1 state of the cubane [4Fe-4S] cluster generates a 5'-deoxyadenosyl radical, which initiates turnover. The [4Fe-4S] cluster must be reduced from its resting +2 state to the catalytically active +1 oxidation state by an electron. In practice, dithionite or the Escherichia coli flavodoxin (EcFldA)/ferredoxin (flavodoxin):NADP(+) oxidoreductase (Fpr)/NADPH system is used. Herein, we present a systematic investigation of the reductive activation of the radical SAM enzyme CDG synthase (BsQueE) from Bacillus subtilis comparing biological and chemical reductants. These data show that either of the flavodoxin homologues encoded by the B. subtilis genome, BsYkuN or BsYkuP, as well as a series of small molecule redox mediators, supports BsQueE activity. With dithionite as a reductant, the activity of BsQueE is ~75-fold greater in the presence of BsYkuN and BsYkuP compared to that in the presence of dithionite alone. By contrast, EcFldA supports turnover to ~10-fold greater levels than dithionite alone under the same conditions. Comparing the ratio of the rate of turnover to the apparent binding constant for the flavodoxin homologues reveals 10- and 240-fold preferences for BsYkuN over BsYkuP and EcFldA, respectively. The differential activation of the enzyme cannot be explained by the abortive cleavage of SAM. We conclude from these observations that the differential activation of BsQueE by Fld homologues may reside in the details of the interaction between the flavodoxin and the radical SAM enzyme.

Non-canonical active site architecture of the radical SAM thiamin pyrimidine synthase

Radical S-adenosylmethionine (SAM) enzymes use a [4Fe-4S] cluster to generate a 5'-deoxyadenosyl radical. Canonical radical SAM enzymes are characterized by a β-barrel-like fold and SAM anchors to the differentiated iron of the cluster, which is located near the amino terminus and within the β-barrel, through its amino and carboxylate groups. Here we show that ThiC, the thiamin pyrimidine synthase in plants and bacteria, contains a tethered cluster-binding domain at its carboxy terminus that moves in and out of the active site during catalysis. In contrast to canonical radical SAM enzymes, we predict that SAM anchors to an additional active site metal through its amino and carboxylate groups. Superimposition of the catalytic domains of ThiC and glutamate mutase shows that these two enzymes share similar active site architectures, thus providing strong evidence for an evolutionary link between the radical SAM and adenosylcobalamin-dependent enzyme superfamilies.

Radical S-adenosyl-L-methionine chemistry in the synthesis of hydrogenase and nitrogenase metal cofactors

Nitrogenase, [FeFe]-hydrogenase, and [Fe]-hydrogenase enzymes perform catalysis at metal cofactors with biologically unusual non-protein ligands. The FeMo cofactor of nitrogenase has a MoFe7S9 cluster with a central carbon, whereas the H-cluster of [FeFe]-hydrogenase contains a 2Fe subcluster coordinated by cyanide and CO ligands as well as dithiomethylamine; the [Fe]-hydrogenase cofactor has CO and guanylylpyridinol ligands at a mononuclear iron site. Intriguingly, radical S-adenosyl-L-methionine enzymes are vital for the assembly of all three of these diverse cofactors. This minireview presents and discusses the current state of knowledge of the radical S-adenosylmethionine enzymes required for synthesis of these remarkable metal cofactors.

An integrative computational model for large-scale identification of metalloproteins in microbial genomes: a focus on iron-sulfur cluster proteins

Metalloproteins represent a ubiquitous group of molecules which are crucial to the survival of all living organisms. While several metal-binding motifs have been defined, it remains challenging to confidently identify metalloproteins from primary protein sequences using computational approaches alone. Here, we describe a comprehensive strategy based on a machine learning approach to design and assess a penalized generalized linear model. We used this strategy to detect members of the iron-sulfur cluster protein family. A new category of descriptors, whose profile is based on profile hidden Markov models, encoding structural information was combined with public descriptors into a linear model. The model was trained and tested on distinct datasets composed of well-characterized iron-sulfur protein sequences, and the resulting model provided higher sensitivity compared to a motif-based approach, while maintaining a good level of specificity. Analysis of this linear model allows us to detect and quantify the contribution of each descriptor, providing us with a better understanding of this complex protein family along with valuable indications for further experimental characterization. Two newly-identified proteins, YhcC and YdiJ, were functionally validated as genuine iron-sulfur proteins, confirming the prediction. The computational model was then applied to over 550 prokaryotic genomes to screen for iron-sulfur proteomes; the results are publicly available at: . This study represents a proof-of-concept for the application of a penalized linear model to identify metalloprotein superfamilies on a large-scale. The application employed here, screening for iron-sulfur proteomes, provides new candidates for further biochemical and structural analysis as well as new resources for an extensive exploration of iron-sulfuromes in the microbial world.

A possible iron delivery function of the dinuclear iron center of HcgD in [Fe]-hydrogenase cofactor biosynthesis

HcgD, a homolog of the ubiquitous Nif3-like protein family, is found in a gene cluster involved in the biosynthesis of the iron-guanylylpyridinol (FeGP) cofactor of [Fe]-hydrogenase. The presented crystal structure and biochemical analyses indicated that HcgD has a dinuclear iron-center, which provides a pronounced binding site for anionic ligands. HcgD contains a stronger and a weaker bound iron; the latter being removable by chelating reagents preferentially in the oxidized state. Therefore, we propose HcgD as an iron chaperone in FeGP cofactor biosynthesis, which might also stimulate investigations on the functionally unknown but physiologically important eukaryotic Nif3-like protein family members.

Glycyl radical activating enzymes: structure, mechanism, and substrate interactions

The glycyl radical enzyme activating enzymes (GRE-AEs) are a group of enzymes that belong to the radical S-adenosylmethionine (SAM) superfamily and utilize a [4Fe-4S] cluster and SAM to catalyze H-atom abstraction from their substrate proteins. GRE-AEs activate homodimeric proteins known as glycyl radical enzymes (GREs) through the production of a glycyl radical. After activation, these GREs catalyze diverse reactions through the production of their own substrate radicals. The GRE-AE pyruvate formate lyase activating enzyme (PFL-AE) is extensively characterized and has provided insights into the active site structure of radical SAM enzymes including GRE-AEs, illustrating the nature of the interactions with their corresponding substrate GREs and external electron donors. This review will highlight research on PFL-AE and will also discuss a few GREs and their respective activating enzymes.

Radical SAM enzyme QueE defines a new minimal core fold and metal-dependent mechanism

7-carboxy-7-deazaguanine synthase (QueE) catalyzes a key S-adenosyl-L-methionine (AdoMet)- and Mg(2+)-dependent radical-mediated ring contraction step, which is common to the biosynthetic pathways of all deazapurine-containing compounds. QueE is a member of the AdoMet radical superfamily, which employs the 5'-deoxyadenosyl radical from reductive cleavage of AdoMet to initiate chemistry. To provide a mechanistic rationale for this elaborate transformation, we present the crystal structure of a QueE along with structures of pre- and post-turnover states. We find that substrate binds perpendicular to the [4Fe-4S]-bound AdoMet, exposing its C6 hydrogen atom for abstraction and generating the binding site for Mg(2+), which coordinates directly to the substrate. The Burkholderia multivorans structure reported here varies from all other previously characterized members of the AdoMet radical superfamily in that it contains a hypermodified (β6/α3) protein core and an expanded cluster-binding motif, CX14CX2C.

Spectroscopic, steady-state kinetic, and mechanistic characterization of the radical SAM enzyme QueE, which catalyzes a complex cyclization reaction in the biosynthesis of 7-deazapurines

7-Carboxy-7-deazaguanine (CDG) synthase (QueE) catalyzes the complex heterocyclic radical-mediated conversion of 6-carboxy-5,6,7,8-tetrahydropterin (CPH(4)) to CDG in the third step of the biosynthetic pathway to all 7-deazapurines. Here we present a detailed characterization of QueE from Bacillus subtilis to delineate the mechanism of conversion of CPH(4) to CDG. QueE is a member of the radical S-adenosyl-l-methionine (SAM) superfamily, all of which use a bound [4Fe-4S](+) cluster to catalyze the reductive cleavage of the SAM cofactor to generate methionine and a 5'-deoxyadenosyl radical (5'-dAdo(•)), which initiates enzymatic transformations requiring hydrogen atom abstraction. The ultraviolet-visible, electron paramagnetic resonance, and Mössbauer spectroscopic features of the homodimeric QueE point to the presence of a single [4Fe-4S] cluster per monomer. Steady-state kinetic experiments indicate a K(m) of 20 ± 7 μM for CPH(4) and a k(cat) of 5.4 ± 1.2 min(-1) for the overall transformation. The kinetically determined K(app) for SAM is 45 ± 1 μM. QueE is also magnesium-dependent and exhibits a K(app) for the divalent metal ion of 0.21 ± 0.03 mM. The SAM cofactor supports multiple turnovers, indicating that it is regenerated at the end of each catalytic cycle. The mechanism of rearrangement of QueE was probed with CPH(4) isotopologs containing deuterium at C-6 or the two prochiral positions at C-7. These studies implicate 5'-dAdo(•) as the initiator of the ring contraction reaction catalyzed by QueE by abstraction of the H atom from C-6 of CPH(4).

Emerging themes in radical SAM chemistry

Enzymes in the radical SAM (RS) superfamily catalyze a wide variety of reactions through unique radical chemistry. The characteristic markers of the superfamily include a [4Fe-4S] cluster coordinated to the protein via a cysteine triad motif, typically CX(3)CX(2)C, with the fourth iron coordinated by S-adenosylmethionine (SAM). The SAM serves as a precursor for a 5'-deoxyadenosyl radical, the central intermediate in nearly all RS enzymes studied to date. The SAM-bound [4Fe-4S] cluster is located within a partial or full triosephosphate isomerase (TIM) barrel where the radical chemistry occurs protected from the surroundings. In addition to the TIM barrel and a RS [4Fe-4S] cluster, many members of the superfamily contain additional domains and/or additional Fe-S clusters. Recently characterized superfamily members are providing new examples of the remarkable range of reactions that can be catalyzed, as well as new structural and mechanistic insights into these fascinating reactions.

The modular respiratory complexes involved in hydrogen and sulfur metabolism by heterotrophic hyperthermophilic archaea and their evolutionary implications

Hydrogen production is a vital metabolic process for many anaerobic organisms, and the enzyme responsible, hydrogenase, has been studied since the 1930s. A novel subfamily with unique properties was recently recognized, represented by the 14-subunit membrane-bound [NiFe] hydrogenase from the archaeon Pyrococcus furiosus. This so-called energy-converting hydrogenase links the thermodynamically favorable oxidation of ferredoxin with the formation of hydrogen and conserves energy in the form of an ion gradient. It is therefore a simple respiratory system within a single complex. This hydrogenase shows a modular composition represented by a Na(+)/H(+) antiporter domain (Mrp) and a [NiFe] hydrogenase domain (Mbh). An analysis of the large number of microbial genome sequences available shows that homologs of Mbh and Mrp tend to be clustered within the genomes of a limited number of archaeal and bacterial species. In several instances, additional genes are associated with the Mbh and Mrp gene clusters that encode proteins that catalyze the oxidation of formate, CO or NAD(P)H. The Mbh complex also shows extensive homology to a number of subunits within the NADH quinone oxidoreductase or complex I family. The respiratory-type membrane-bound hydrogenase complex appears to be closely related to the common ancestor of complex I and [NiFe] hydrogenases in general.

Enzyme catalyzed formation of radicals from S-adenosylmethionine and inhibition of enzyme activity by the cleavage products

A large superfamily of enzymes have been identified that make use of radical intermediates derived by reductive cleavage of S-adenosylmethionine. The primary nature of the radical intermediates makes them highly reactive and potent oxidants. They are used to initiate biotransformations by hydrogen atom abstraction, a process that allows a particularly diverse range of substrates to be functionalized, including substrates with relatively inert chemical structures. In the first part of this review, we discuss the evidence supporting the mechanism of radical formation from S-adenosylmethionine. In the second part of the review, we examine the potential of reaction products arising from S-adenosylmethionine to cause product inhibition. The effects of this product inhibition on kinetic studies of 'radical S-adenosylmethionine' enzymes are discussed and strategies to overcome these issues are reviewed. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.

Radical AdoMet enzymes in complex metal cluster biosynthesis

Radical S-adenosylmethionine (AdoMet) enzymes comprise a large superfamily of proteins that engage in a diverse series of biochemical transformations through generation of the highly reactive 5'-deoxyadenosyl radical intermediate. Recent advances into the biosynthesis of unique iron-sulfur (FeS)-containing cofactors such as the H-cluster in [FeFe]-hydrogenase, the FeMo-co in nitrogenase, as well as the iron-guanylylpyridinol (FeGP) cofactor in [Fe]-hydrogenase have implicated new roles for radical AdoMet enzymes in the biosynthesis of complex inorganic cofactors. Radical AdoMet enzymes in conjunction with scaffold proteins engage in modifying ubiquitous FeS precursors into unique clusters, through novel amino acid decomposition and sulfur insertion reactions. The ability of radical AdoMet enzymes to modify common metal centers to unusual metal cofactors may provide important clues into the stepwise evolution of these and other complex bioinorganic catalysts. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.

Cfr and RlmN contain a single [4Fe-4S] cluster, which directs two distinct reactivities for S-adenosylmethionine: methyl transfer by SN2 displacement and radical generation

The radical SAM (RS) proteins RlmN and Cfr catalyze methylation of carbons 2 and 8, respectively, of adenosine 2503 in 23S rRNA. Both reactions are similar in scope, entailing the synthesis of a methyl group partially derived from S-adenosylmethionine (SAM) onto electrophilic sp(2)-hybridized carbon atoms via the intermediacy of a protein S-methylcysteinyl (mCys) residue. Both proteins contain five conserved Cys residues, each required for turnover. Three cysteines lie in a canonical RS CxxxCxxC motif and coordinate a [4Fe-4S]-cluster cofactor; the remaining two are at opposite ends of the polypeptide. Here we show that each protein contains only the one "radical SAM" [4Fe-4S] cluster and the two remaining conserved cysteines do not coordinate additional iron-containing species. In addition, we show that, while wild-type RlmN bears the C355 mCys residue in its as-isolated state, RlmN that is either engineered to lack the [4Fe-4S] cluster by substitution of the coordinating cysteines or isolated from Escherichia coli cultured under iron-limiting conditions does not bear a C355 mCys residue. Reconstitution of the [4Fe-4S] cluster on wild-type apo RlmN followed by addition of SAM results in rapid production of S-adenosylhomocysteine (SAH) and the mCys residue, while treatment of apo RlmN with SAM affords no observable reaction. These results indicate that in Cfr and RlmN, SAM bound to the unique iron of the [4Fe-4S] cluster displays two reactivities. It serves to methylate C355 of RlmN (C338 of Cfr), or to generate the 5'-deoxyadenosyl 5'-radical, required for substrate-dependent methyl synthase activity.

The methylthiolation reaction mediated by the Radical-SAM enzymes

Over the past 10 years, considerable progress has been made in our understanding of the mechanistic enzymology of the Radical-SAM enzymes. It is now clear that these enzymes appear to be involved in a remarkably wide range of chemically challenging reactions. This review article highlights mechanistic and structural aspects of the methylthiotransferases (MTTases) sub-class of the Radical-SAM enzymes. The mechanism of methylthio insertion, now observed to be performed by three different enzymes is an exciting unsolved problem. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.

Probing the origin of the metabolic precursor of the CO ligand in the catalytic center of [NiFe] hydrogenase

The O(2)-tolerant [NiFe] hydrogenases of Ralstonia eutropha are capable of H(2) conversion in the presence of ambient O(2). Oxygen represents not only a challenge for catalysis but also for the complex assembling process of the [NiFe] active site. Apart from nickel and iron, the catalytic center contains unusual diatomic ligands, namely two cyanides (CN(-)) and one carbon monoxide (CO), which are coordinated to the iron. One of the open questions of the maturation process concerns the origin and biosynthesis of the CO group. Isotope labeling in combination with infrared spectroscopy revealed that externally supplied gaseous (13)CO serves as precursor of the carbonyl group of the regulatory [NiFe] hydrogenase in R. eutropha. Corresponding (13)CO titration experiments showed that a concentration 130-fold higher than ambient CO (0.1 ppmv) caused a 50% labeling of the carbonyl ligand in the [NiFe] hydrogenase, leading to the conclusion that the carbonyl ligand originates from an intracellular metabolite. A novel setup allowed us to the study effects of CO depletion on maturation in vivo. Upon induction of CO depletion by addition of the CO scavenger PdCl(2), cells cultivated on H(2), CO(2), and O(2) showed severe growth retardation at low cell concentrations, which was on the basis of partially arrested hydrogenase maturation, leading to reduced hydrogenase activity. This suggests gaseous CO as a metabolic precursor under these conditions. The addition of PdCl(2) to cells cultivated heterotrophically on organic substrates had no effect on hydrogenase maturation. These results indicate at least two different pathways for biosynthesis of the CO ligand of [NiFe] hydrogenase.

More than 200 genes required for methane formation from H₂ and CO₂ and energy conservation are present in Methanothermobacter marburgensis and Methanothermobacter thermautotrophicus

The hydrogenotrophic methanogens Methanothermobacter marburgensis and Methanothermobacter thermautotrophicus can easily be mass cultured. They have therefore been used almost exclusively to study the biochemistry of methanogenesis from H₂ and CO₂, and the genomes of these two model organisms have been sequenced. The close relationship of the two organisms is reflected in their genomic architecture and coding potential. Within the 1,607 protein coding sequences (CDS) in common, we identified approximately 200 CDS required for the synthesis of the enzymes, coenzymes, and prosthetic groups involved in CO₂ reduction to methane and in coupling this process with the phosphorylation of ADP. Approximately 20 additional genes, such as those for the biosynthesis of F₄₃₀ and methanofuran and for the posttranslational modifications of the two methyl-coenzyme M reductases, remain to be identified.

Radicals from S-adenosylmethionine and their application to biosynthesis

The radical SAM superfamily of enzymes catalyzes a broad spectrum of biotransformations by employing a common obligate intermediate, the 5'-deoxyadenosyl radical (DOA). Radical formation occurs via the reductive cleavage of S-adenosylmethionine (SAM or AdoMet). The resultant highly reactive primary radical is a potent oxidant that enables the functionalization of relatively inert substrates, including unactivated C-H bonds. The reactions initiated by the DOA are breathtaking in their efficiency, elegance and in many cases, the complexity of the biotransformation achieved. This review describes the common features shared by enzymes that generate the DOA and the intriguing variations or modifications that have recently been reported. The review also highlights selected examples of the diverse biotransformations that ensue.

Mechanistic and functional versatility of radical SAM enzymes

Enzymes of the radical SAM (RS) superfamily catalyze a diverse assortment of reactions that proceed via intermediates containing unpaired electrons. The radical initiator is the common metabolite S-adenosyl-l-methionine (SAM), which is reductively cleaved to generate a 5′-deoxyadenosyl 5′-radical, a universal and obligate intermediate among enzymes within this class. A bioinformatics study that appeared in 2001 indicated that this superfamily contained over 600 members, many catalyzing reactions that were rich in novel chemical transformations. Since that seminal study, the RS superfamily has grown immensely, and new details about the scope of reactions and biochemical pathways in which its members participate have emerged. This review will highlight only a few of the most significant findings from the past 2-3 years, focusing primarily on: RS enzymes involved in complex metallocofactor maturation; characterized RS enzymes that lack the canonical CxxxCxxC motif; RS enzymes containing multiple iron-sulfur clusters; RS enzymes catalyzing reactions with compelling medical implications; and the energetics and mechanism of generating the 5′-deoxyadenosyl radical. A number of significant studies of RS enzymes will unfortunately be omitted, and it is hoped that the reader will access the relevant literature - particularly a number of superb review articles recently written on the subject - to acquire a deeper appreciation of this class of enzymes.

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

Archaeal and eukaryotic translation elongation factor 2 contain a unique post-translationally modified histidine residue called diphthamide, which is the target of diphtheria toxin. The biosynthesis of diphthamide was proposed to involve three steps, with the first being the formation of a C-C bond between the histidine residue and the 3-amino-3-carboxypropyl group of S-adenosyl-l-methionine (SAM). However, further details of the biosynthesis remain unknown. Here we present structural and biochemical evidence showing that the first step of diphthamide biosynthesis in the archaeon Pyrococcus horikoshii uses a novel iron-sulphur-cluster enzyme, Dph2. Dph2 is a homodimer and each of its monomers can bind a [4Fe-4S] cluster. Biochemical data suggest that unlike the enzymes in the radical SAM superfamily, Dph2 does not form the canonical 5'-deoxyadenosyl radical. Instead, it breaks the C(gamma,Met)-S bond of SAM and generates a 3-amino-3-carboxypropyl radical. Our results suggest that P. horikoshii Dph2 represents a previously unknown, SAM-dependent, [4Fe-4S]-containing enzyme that catalyses unprecedented chemistry.