Monovalent Cation Activation of the Radical SAM Enzyme Pyruvate Formate-Lyase Activating Enzyme

The Fe-S cluster biosynthesis in Enterococcus faecium is essential for anaerobic growth and gastrointestinal colonization

The facultative anaerobic Gram-positive bacterium Enterococcus faecium is a ubiquitous member of the human gut microbiota. However, it has gradually evolved into a pathogenic and multidrug resistant lineage that causes nosocomial infections. The establishment of high-level intestinal colonization by enterococci represents a critical step of infection. The majority of current research on Enterococcus has been conducted under aerobic conditions, while limited attention has been given to its physiological characteristics in anaerobic environments, which reflects its natural colonization niche in the gut. In this study, a high-density transposon mutant library containing 26,620 distinct insertion sites was constructed. Tn-seq analysis identified six genes that significantly contribute to growth under anaerobic conditions. Under anaerobic conditions, deletion of sufB (encoding Fe-S cluster assembly protein B) results in more extensive and significant impairments on carbohydrate metabolism compared to aerobic conditions. Consistently, the pathways involved in this utilization-restricted carbohydrates were mostly expressed at significantly lower levels in mutant compared to wild-type under anaerobic conditions. Moreover, deletion of sufB or pflA (encoding pyruvate formate lyase-activating protein A) led to failure of gastrointestinal colonization in mice. These findings contribute to our understanding of the mechanisms by which E. faecium maintains proliferation under anaerobic conditions and establishes colonization in the gut.

Improving pathway prediction accuracy of constraints-based metabolic network models by treating enzymes as microcompartments

Metabolic network models have become increasingly precise and accurate as the most widespread and practical digital representations of living cells. The prediction functions were significantly expanded by integrating cellular resources and abiotic constraints in recent years. However, if unreasonable modeling methods were adopted due to a lack of consideration of biological knowledge, the conflicts between stoichiometric and other constraints, such as thermodynamic feasibility and enzyme resource availability, would lead to distorted predictions. In this work, we investigated a prediction anomaly of EcoETM, a constraints-based metabolic network model, and introduced the idea of enzyme compartmentalization into the analysis process. Through rational combination of reactions, we avoid the false prediction of pathway feasibility caused by the unrealistic assumption of free intermediate metabolites. This allowed us to correct the pathway structures of l-serine and l-tryptophan. A specific analysis explains the application method of the EcoETM-like model and demonstrates its potential and value in correcting the prediction results in pathway structure by resolving the conflict between different constraints and incorporating the evolved roles of enzymes as reaction compartments. Notably, this work also reveals the trade-off between product yield and thermodynamic feasibility. Our work is of great value for the structural improvement of constraints-based models.

Pyruvate formate-lyase activating enzyme: The catalytically active 5'-deoxyadenosyl radical caught in the act of H-atom abstraction

Enzymes of the radical S-adenosyl-l-methionine (radical SAM, RS) superfamily, the largest in nature, catalyze remarkably diverse reactions initiated by H-atom abstraction. Glycyl radical enzyme activating enzymes (GRE-AEs) are a growing class of RS enzymes that generate the catalytically essential glycyl radical of GREs, which in turn catalyze essential reactions in anaerobic metabolism. Here, we probe the reaction of the GRE-AE pyruvate formate-lyase activating enzyme (PFL-AE) with the peptide substrate RVSG734YAV, which mimics the site of glycyl radical formation on the native substrate, pyruvate formate-lyase. Time-resolved freeze-quench electron paramagnetic resonance spectroscopy shows that at short mixing times reduced PFL-AE + SAM reacts with RVSG734YAV to form the central organometallic intermediate, Ω, in which the adenosyl 5'C is covalently bound to the unique iron of the [4Fe-4S] cluster. Freeze-trapping the reaction at longer times reveals the formation of the peptide G734• glycyl radical product. Of central importance, freeze-quenching at intermediate times reveals that the conversion of Ω to peptide glycyl radical is not concerted. Instead, homolysis of the Ω Fe-C5' bond generates the nominally "free" 5'-dAdo• radical, which is captured here by freeze-trapping. During cryoannealing at 77 K, the 5'-dAdo• directly abstracts an H-atom from the peptide to generate the G734• peptide radical trapped in the PFL-AE active site. These observations reveal the 5'-dAdo• radical to be a well-defined intermediate, caught in the act of substrate H-atom abstraction, providing new insights into the mechanistic steps of radical initiation by RS enzymes.

Mechanism of Radical Initiation in the Radical SAM Enzyme Superfamily

Radical S-adenosylmethionine (SAM) enzymes use a site-differentiated [4Fe-4S] cluster and SAM to initiate radical reactions through liberation of the 5'-deoxyadenosyl (5'-dAdo•) radical. They form the largest enzyme superfamily, with more than 700,000 unique sequences currently, and their numbers continue to grow as a result of ongoing bioinformatics efforts. The range of extremely diverse, highly regio- and stereo-specific reactions known to be catalyzed by radical SAM superfamily members is remarkable. The common mechanism of radical initiation in the radical SAM superfamily is the focus of this review. Most surprising is the presence of an organometallic intermediate, Ω, exhibiting an Fe-C5'-adenosyl bond. Regioselective reductive cleavage of the SAM S-C5' bond produces 5'-dAdo• to form Ω, with the regioselectivity originating in the Jahn-Teller effect. Ω liberates the free 5'-dAdo• as the catalytically active intermediate through homolysis of the Fe-C5' bond, in analogy to Co-C5' bond homolysis in B12, which was once viewed as biology's choice of radical generator.

Computational engineering of previously crystallized pyruvate formate-lyase activating enzyme reveals insights into SAM binding and reductive cleavage

Radical S-adenosyl-l-methionine (SAM) enzymes are ubiquitous in nature and carry out a broad variety of difficult chemical transformations initiated by hydrogen atom abstraction. Although numerous radical SAM (RS) enzymes have been structurally characterized, many prove recalcitrant to crystallization needed for atomic-level structure determination using X-ray crystallography, and even those that have been crystallized for an initial study can be difficult to recrystallize for further structural work. We present here a method for computationally engineering previously observed crystallographic contacts and employ it to obtain more reproducible crystallization of the RS enzyme pyruvate formate-lyase activating enzyme (PFL-AE). We show that the computationally engineered variant binds a typical RS [4Fe-4S]2+/+ cluster that binds SAM, with electron paramagnetic resonance properties indistinguishable from the native PFL-AE. The variant also retains the typical PFL-AE catalytic activity, as evidenced by the characteristic glycyl radical electron paramagnetic resonance signal observed upon incubation of the PFL-AE variant with reducing agent, SAM, and PFL. The PFL-AE variant was also crystallized in the [4Fe-4S]2+ state with SAM bound, providing a new high-resolution structure of the SAM complex in the absence of substrate. Finally, by incubating such a crystal in a solution of sodium dithionite, the reductive cleavage of SAM is triggered, providing us with a structure in which the SAM cleavage products 5'-deoxyadenosine and methionine are bound in the active site. We propose that the methods described herein may be useful in the structural characterization of other difficult-to-resolve proteins.

Radical SAM enzymes: Nature's choice for radical reactions

Enzymes that use a [4Fe-4S]1+ cluster plus S-adenosyl-l-methionine (SAM) to initiate radical reactions (radical SAM) form the largest enzyme superfamily, with over half a million members across the tree of life. This review summarizes recent work revealing the radical SAM reaction pathway, which ultimately liberates the 5'-deoxyadenosyl (5'-dAdo•) radical to perform extremely diverse, highly regio- and stereo-specific, transformations. Most surprising was the discovery of an organometallic intermediate Ω exhibiting an Fe-C5'-adenosyl bond. Ω liberates 5'-dAdo• through homolysis of the Fe-C5' bond, in analogy to Co-C5' bond homolysis in B12 , previously viewed as biology's paradigmatic radical generator. The 5'-dAdo• has been trapped and characterized in radical SAM enzymes via a recently discovered photoreactivity of the [4Fe-4S]+ /SAM complex, and has been confirmed as a catalytically active intermediate in enzyme catalysis. The regioselective SAM S-C bond cleavage to produce 5'-dAdo• originates in the Jahn-Teller effect. The simplicity of SAM as a radical precursor, and the exquisite control of 5'-dAdo• reactivity in radical SAM enzymes, may be why radical SAM enzymes pervade the tree of life, while B12 enzymes are only a few.

Chitosan-regulated biomimetic hybrid nanoflower for efficiently immobilizing enzymes to enhance stability and by-product tolerance

Organic-inorganic hybrid nano-materials have been considered to be promising immobilization matrixes for enzymes due to their significantly enhanced reusability and stability of enzymes. Herein, we constructed a novel organic-inorganic hybrid nanoflower via biomacromolecule-regulated biomimetic mineralization to immobilize sucrose phosphorylase (SPase). It was found that chitosan (CS) effectively regulated the biomimetic mineralization of calcium phosphate (CaP), leading to the formation of flower-like hybrid materials for the entrapment of SPase via self-assembly to establish a nano-biocatalyst (CS-CaP@SPase). Upon immobilization, the obtained CS-CaP@SPase exhibited excellent pH, by-product and organic solvents tolerance, and storage stability. Specifically, at acidic condition (pH 4), CS-CaP@SPase performed over 80 % of initial activity, which was 2.42-folds higher than that of free SPase. The catalytic activity of free SPase was severely inhibited about 30 % in the presence of fructose (1.2 M), but CS-CaP@SPase only lost 5 % relative activity. The CS-CaP@SPase retained over 80 % of its relative activity, while the free SPase maintained <20 % of its relative activity in acetonitrile. The relative activity of CS-CaP@SPase was still retained about 80 % after 10 cycles and maintained 75 % after 15 days. Based on Raman spectra analysis, it was also found that the increased β-folding component of SPase in the secondary structure after immobilization was the main factor for its enhanced stability. It is reasonable to believe that biomacromolecule-regulated biomimetic mineralization could be potentially used as a promising method to immobilize enzymes with excellent stability and recyclability, thereby facilitating the preparation of highly efficient catalysts for industrial biocatalysts, biosensing, and biomedicine.

Activation of Glycyl Radical Enzymes─Multiscale Modeling Insights into Catalysis and Radical Control in a Pyruvate Formate-Lyase-Activating Enzyme

Pyruvate formate-lyase (PFL) is a glycyl radical enzyme (GRE) playing a pivotal role in the metabolism of strict and facultative anaerobes. Its activation is carried out by a PFL-activating enzyme, a member of the radical S-adenosylmethionine (rSAM) superfamily of metalloenzymes, which introduces a glycyl radical into the Gly radical domain of PFL. The activation mechanism is still not fully understood and is structurally based on a complex with a short model peptide of PFL. Here, we present extensive molecular dynamics simulations in combination with quantum mechanics/molecular mechanics (QM/MM)-based kinetic and thermodynamic reaction evaluations of a more complete activation model comprising the 49 amino acid long C-terminus region of PFL. We reveal the benefits and pitfalls of the current activation model, providing evidence that the bound peptide conformation does not resemble the bound protein-protein complex conformation with PFL, with implications for the activation process. Substitution of the central glycine with (S)- and (R)-alanine showed excellent binding of (R)-alanine over unstable binding of (S)-alanine. Radical stabilization calculations indicate that a higher radical stability of the glycyl radical might not be the sole origin of the evolutionary development of GREs. QM/MM-derived radical formation kinetics further demonstrate feasible activation barriers for both peptide and C-terminus activation, demonstrating why the crystalized model peptide system is an excellent inhibitory system for natural activation. This new evidence supports the theory that GREs converged on glycyl radical formation due to the better conformational accessibility of the glycine radical loop, rather than the highest radical stability of the formed peptide radicals.

Cobalamin-Dependent Radical S-Adenosylmethionine Enzymes: Capitalizing on Old Motifs for New Functions

The members of the radical S-adenosylmethionine (SAM) enzyme superfamily are responsible for catalyzing a diverse set of reactions in a multitude of biosynthetic pathways. Many members of this superfamily accomplish their transformations using the catalytic power of a 5'-deoxyadenosyl radical (5'-dAdo•), but there are also enzymes within this superfamily that bind auxiliary cofactors and extend the catalytic repertoire of SAM. In particular, the cobalamin (Cbl)-dependent class synergistically uses Cbl to facilitate challenging methylation and radical rearrangement reactions. Despite identification of this class by Sofia et al. 20 years ago, the low sequence identity between members has led to difficulty in predicting function of uncharacterized members, pinpointing catalytic residues, and elucidating reaction mechanisms. Here, we capitalize on the three recent structures of Cbl-dependent radical SAM enzymes that use common cofactors to facilitate ring contraction as well as radical-based and non-radical-based methylation reactions. With these three structures as a framework, we describe how the Cbl-dependent radical SAM enzymes repurpose the traditional SAM- and Cbl-binding motifs to form an active site where both Cbl and SAM can participate in catalysis. In addition, we describe how, in some cases, the classic SAM- and Cbl-binding motifs support the diverse functionality of this enzyme class, and finally, we define new motifs that are characteristic of Cbl-dependent radical SAM enzymes.

Purification and structural elucidation of a cobalamin-dependent radical SAM enzyme

The cobalamin (Cbl)-dependent radical S-adenosylmethionine (SAM) enzymes use a [4Fe-4S] cluster, SAM, and Cbl to carry out remarkable catalytic feats in a large number of biosynthetic pathways. However, despite the abundance of annotated Cbl-dependent radical SAM enzymes, relatively few molecular details exist regarding how these enzymes function. Traditionally, challenges associated with purifying and reconstituting Cbl-dependent radical SAM enzymes have hindered biochemical studies aimed at elucidating the structures and mechanisms of these enzymes. Herein, we describe a bottom-up approach that was used to crystallize OxsB, learn about the overall architecture of a Cbl-dependent radical SAM enzyme, and facilitate mechanistic studies. We report lessons learned from the crystallization of different states of OxsB, including the apo-, selenomethionine (SeMet)-labeled, and fully reconstituted form of OxsB that has a [4Fe-4S] cluster, SAM, and Cbl bound. Further, we suggest that, when appropriate, this bottom-up method can be used to facilitate studies on enzymes in this class for which there are challenges associated with purifying and reconstituting the active enzyme.

Crystallographic snapshots of a B12-dependent radical SAM methyltransferase

By catalysing the microbial formation of methane, methyl-coenzyme M reductase has a central role in the global levels of this greenhouse gas^1,2. The activity of methyl-coenzyme M reductase is profoundly affected by several unique post-translational modifications^3–6, such as  a unique C-methylation reaction catalysed by methanogenesis marker protein 10 (Mmp10), a radical S-adenosyl-l-methionine (SAM) enzyme^7,8. Here we report the spectroscopic investigation and atomic resolution structure of Mmp10 from Methanosarcina acetivorans, a unique B₁₂ (cobalamin)-dependent radical SAM enzyme⁹. The structure of Mmp10 reveals a unique enzyme architecture with four metallic centres and critical structural features involved in the control of catalysis. In addition, the structure of the enzyme–substrate complex offers a glimpse into a B₁₂-dependent radical SAM enzyme in a precatalytic state. By combining electron paramagnetic resonance spectroscopy, structural biology and biochemistry, our study illuminates the mechanism by which the emerging superfamily of B₁₂-dependent radical SAM enzymes catalyse chemically challenging alkylation reactions and identifies distinctive active site rearrangements to provide a structural rationale for the dual use of the SAM cofactor for radical and nucleophilic chemistry.

Structural and spectroscopic studies show how a B₁₂-dependent radical SAM enzyme catalyses unique and challenging alkylation chemistry, including protein post-translational modification required for methane biosynthesis.

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.

Occurrence, Evolution and Specificities of Iron-Sulfur Proteins and Maturation Factors in Chloroplasts from Algae

Iron-containing proteins, including iron-sulfur (Fe-S) proteins, are essential for numerous electron transfer and metabolic reactions. They are present in most subcellular compartments. In plastids, in addition to sustaining the linear and cyclic photosynthetic electron transfer chains, Fe-S proteins participate in carbon, nitrogen, and sulfur assimilation, tetrapyrrole and isoprenoid metabolism, and lipoic acid and thiamine synthesis. The synthesis of Fe-S clusters, their trafficking, and their insertion into chloroplastic proteins necessitate the so-called sulfur mobilization (SUF) protein machinery. In the first part, we describe the molecular mechanisms that allow Fe-S cluster synthesis and insertion into acceptor proteins by the SUF machinery and analyze the occurrence of the SUF components in microalgae, focusing in particular on the green alga Chlamydomonas reinhardtii. In the second part, we describe chloroplastic Fe-S protein-dependent pathways that are specific to Chlamydomonas or for which Chlamydomonas presents specificities compared to terrestrial plants, putting notable emphasis on the contribution of Fe-S proteins to chlorophyll synthesis in the dark and to the fermentative metabolism. The occurrence and evolutionary conservation of these enzymes and pathways have been analyzed in all supergroups of microalgae performing oxygenic photosynthesis.

S-Adenosyl-l-ethionine is a Catalytically Competent Analog of S-Adenosyl-l-methione (SAM) in the Radical SAM Enzyme HydG

Radical S-adenosyl-l-methionine (SAM) enzymes initiate biological radical reactions with the 5'-deoxyadenosyl radical (5'-dAdo•). A [4Fe-4S]+ cluster reductively cleaves SAM to form the Ω organometallic intermediate in which the 5'-deoxyadenosyl moiety is directly bound to the unique iron of the [4Fe-4S] cluster, with subsequent liberation of 5'-dAdo•. Here we present synthesis of the SAM analog S-adenosyl-l-ethionine (SAE) and show SAE is a mechanistically-equivalent SAM-alternative for HydG, both supporting enzymatic turnover of substrate tyrosine and forming the organometallic intermediate Ω. Photolysis of SAE bound to HydG forms an ethyl radical trapped in the active site. The ethyl radical withstands prolonged storage at 77 K and its EPR signal is only partially lost upon annealing at 100 K, making it significantly less reactive than the methyl radical formed by SAM photolysis. Upon annealing above 77K, the ethyl radical adds to the [4Fe-4S]2+ cluster, generating an ethyl-[4Fe-4S]3+ organometallic species termed ΩE.

d-Alanine-d-alanine ligase as a model for the activation of ATP-grasp enzymes by monovalent cations

The ATP-grasp superfamily of enzymes shares an atypical nucleotide-binding site known as the ATP-grasp fold. These enzymes are involved in many biological pathways in all domains of life. One ATP-grasp enzyme, d-alanine-d-alanine ligase (Ddl), catalyzes ATP-dependent formation of the d-alanyl-d-alanine dipeptide essential for bacterial cell wall biosynthesis and is therefore an important antibiotic drug target. Ddl is activated by the monovalent cation (MVC) K⁺, but despite its clinical relevance and decades of research, how this activation occurs has not been elucidated. We demonstrate here that activating MVCs bind adjacent to the active site of Ddl from Thermus thermophilus and used a combined biochemical and structural approach to characterize MVC activation. We found that TtDdl is a type II MVC-activated enzyme, retaining activity in the absence of MVCs. However, the efficiency of TtDdl increased ∼20-fold in the presence of activating MVCs, and it was maximally activated by K⁺ and Rb⁺ ions. A strict dependence on ionic radius of the MVC was observed, with Li⁺ and Na⁺ providing little to no TtDdl activation. To understand the mechanism of MVC activation, we solved crystal structures of TtDdl representing distinct catalytic stages in complex with K⁺, Rb⁺, or Cs⁺ Comparison of these structures with apo TtDdl revealed no evident conformational change on MVC binding. Of note, the identified MVC binding site is structurally conserved within the ATP-grasp superfamily. We propose that MVCs activate Ddl by altering the charge distribution of its active site. These findings provide insight into the catalytic mechanism of ATP-grasp enzymes.

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.

Mechanism of Radical Initiation in the Radical S-Adenosyl-l-methionine Superfamily

The seeds for recognition of the vast superfamily of radical S-adenosyl-l-methionine (SAM) enzymes were sown in the 1960s, when Joachim Knappe found that the dissimilation of pyruvate was dependent on SAM and Fe(II), and Barker and co-workers made similar observations for lysine 2,3-aminomutase. These intriguing observations, coupled with the evidence that SAM and Fe were cofactors in radical catalysis by these enzyme systems, drew us in the 1990s to explore how Fe(II) and SAM initiate radical reactions. Our early work focused on the same enzyme Knappe had originally characterized: the pyruvate formate-lyase activating enzyme (PFL-AE). Our discovery of an iron-sulfur cluster in this enzyme, together with similar findings for other SAM-dependent enzymes at the time, led to the recognition of an emerging class of enzymes that use iron-sulfur clusters to cleave SAM, liberating the 5'-deoxyadenosyl radical (5'-dAdo•) that initiates radical reactions. A major bioinformatics study by Heidi Sofia and co-workers identified the enzyme superfamily denoted Radical SAM, now known to span all kingdoms of life with more than 100,000 unique sequences encoding enzymes that catalyze remarkably diverse reactions. Despite the limited sequence similarity and vastly divergent reactions catalyzed, the radical SAM enzymes appear to employ a common mechanism for initiation of radical chemistry, a mechanism we have helped to clarify over the last 25 years. A reduced [4Fe-4S]⁺ cluster provides the electron needed for the reductive cleavage of SAM. The resulting [4Fe-4S]²⁺ cluster can be rereduced either by an external reductant, with SAM acting as a cosubstrate, or by an electron provided during the reformation of SAM in cases where SAM is used as a cofactor. The amino and carboxylate groups of SAM bind to the unique iron of the catalytic [4Fe-4S] cluster, placing the sulfonium of SAM in close proximity to the cluster. Surprising recent results have shown that the initiating enzymatic cleavage of SAM generates an organometallic intermediate prior to liberation of 5'-dAdo•, which initiates radical chemistry on the substrate. This organometallic intermediate, denoted Ω, has a 5'-deoxyadenosyl moiety directly bound to the unique iron of the [4Fe-4S] cluster via the 5'-C, giving a structure that is directly analogous to the Co-(5'-C) bond of the organometallic cofactor adenosylcobalamin. Our observation that this intermediate Ω is formed throughout the superfamily suggests that this is a key intermediate in initiating radical SAM reactions, and that organometallic chemistry is much more broadly relevant in biology than previously thought.

Alkyl Radical Generation by an Intramolecular Homolytic Substitution Reaction between Iron(II) and Trialkylsulfonium Groups

Intramolecular, homolytic substitution reactions between iron(II) species and various trialkylsulfonium groups were directly observed in the gas phase upon collision-induced dissociation. In spite of the notoriously low reduction potential of trialkylsulfonium species and the mismatched oxidation potential of iron(II), the reactions proceed at moderate collision energies, forming an alkyl radical as well as a thioether coordinated to the iron. In contrast to classical homolytic substitutions, the attacking radical is a "metalloradical", namely iron(II) that is oxidized to iron(III) during the reaction. With this process we demonstrate that the conceptually analogous, putative radical generation step in radical S-adenosyl methionine (SAM) enzymes is possible and plausible. Further, we show that this kind of reaction only occurs in constrained systems with a defined geometry. Combining experimental measurements with DFT studies and NBO analyses allowed us to gain insights into the reactivity and transition states of these systems. Based on our findings, we challenge the notion of a collinear transition state in the radical generation step of radical SAM enzymes and propose it to be bent instead.