IPNS belongs to the widespread family of Fe(II)-dependent oxygenases, enzymes that use iron and molecular oxygen to oxidize their substrates. Most members of this superfamily, which includes enzymes involved in collagen biosynthesis, lipid metabolism and DNA repair, carry out two-electron hydroxylation reactions coupled to the decarboxylation of a co-substrate. IPNS is unusual. It performs a four-electron oxidation, converting the tripeptide δ-(L-α-aminoadipoyl)-L-cysteinyl-D-valine, known as ACV, into isopenicillin N by forging two new rings: a four-membered β-lactam and a five-membered thiazolidine. Both ring-forming steps require the cleavage of strong carbon-hydrogen bonds, and both show large kinetic isotope effects when those hydrogens are replaced with deuterium.

Previous crystallographic work had established that ACV binds the catalytic iron through its cysteinyl sulfur atom, and spectroscopic and computational studies had proposed that oxygen binding generates a ferric superoxo species that abstracts a hydrogen from the cysteinyl β-carbon, producing a thioaldehyde intermediate and an iron(II)-hydroperoxide. But the actual structures of the intermediates involved in the ring-forming steps had never been observed directly. Two competing mechanisms had been proposed for how the O–O bond of the peroxide is cleaved and how the valine amide nitrogen is deprotonated, and the roles of water molecules in the active site were essentially unknown.

To resolve these questions, the team turned to time-resolved serial femtosecond crystallography, or tr-SFX, at X-ray free-electron laser facilities including the Linac Coherent Light Source in the United States, the Pohang Accelerator Laboratory XFEL in South Korea and SACLA in Japan. XFEL pulses are so brief and so intense that diffraction data can be collected from tiny microcrystals before radiation damage destroys the sample, making the method ideal for studying radiation-sensitive high-valent iron intermediates. The researchers grew anaerobic microcrystals of the IPNS:Fe(II):ACV complex, roughly three by three by eighty micrometres in size, and delivered them on a conveyor tape using acoustic droplet ejection. Nanolitre droplets were passed through a chamber filled with pure oxygen, and the reaction time was controlled simply by varying the speed of the tape between oxygen exposure and the X-ray pulse.

A clever isotope trick helped the team trap the most elusive intermediate. Because the second hydrogen abstraction step, at the valine carbon, exhibits a very large kinetic isotope effect of roughly thirty, the researchers used a deuterium-labelled version of the substrate, ACdV, in which the valine hydrogen is replaced with deuterium. This dramatically slows the second ring-forming step, allowing the monocyclic β-lactam intermediate to accumulate. The team collected structural data at one, two, three, four, six, eight and ten seconds of oxygen exposure, deliberately randomizing the order of data collection and interspersing water-only control runs to eliminate bias, and repeated all experiments at least twice.

The first two seconds revealed what was expected: an oxygen molecule binding end-on to the iron at the site trans to Asp216, with the occupancy of the bound oxygen rising from about fifty percent at one second to more than ninety percent at two seconds. Time-resolved X-ray emission spectroscopy, collected simultaneously from the same crystals and pulses, confirmed the formation of the Fe(III)-superoxo species. Then came the surprise. At three, four and six seconds, the electron density showed that the ACV sulfur atom had moved away from the iron, with the iron–sulfur distance stretching from about 2.3 angstroms to roughly 4.2 angstroms. The density was consistent with a thioaldehyde whose cysteinyl α-carbon retained its tetrahedral geometry, meaning the thioaldehyde did not coordinate the iron at all, contrary to what many mechanistic models had assumed.

The X-ray emission spectra at these timepoints showed the iron had been reduced back to Fe(II), consistent with the proposed iron(II)-hydroperoxide state. By eight and ten seconds, the sulfur had returned to the iron, restoring the Fe–S bond at about 2.4 angstroms, and the structures revealed the monocyclic β-lactam intermediate at seventy-five percent occupancy. This intermediate closely matched a previously determined structure of IPNS bound to a substrate analogue that stalls at the same stage, providing strong independent support for the assignment. Notably, at the monocyclic intermediate stage the valine hydrogen now points toward the iron, positioned productively for the second abstraction, whereas in the anaerobic starting structure it points away. The valine isopropyl group, which had been disordered in earlier timepoints, became well ordered, reflecting the conformational rigidity conferred by the restored iron–sulfur linkage.

The refined iron–oxygen bond at these late timepoints came out at about 1.96 angstroms, somewhat longer than the roughly 1.64 angstroms expected for a classic Fe(IV)=O species, though the interpretation is complicated by residual peroxide from the thioaldehyde population. To rule out artefacts, the team also determined a structure of IPNS complexed with a catalytically inactive vanadyl ion, V(IV)=O, which mimics the geometry of a high-valent metal-oxo species. The vanadium–oxygen bond refined to 1.64 angstroms, matching literature values and indicating that the elongated iron–oxygen distance was not a radiation-damage artefact. The spectroscopic data at eight and ten seconds showed the iron in a state more oxidized than Fe(III), supporting at least partial presence of the Fe(IV) intermediate critical for the second hydrogen abstraction.

Beyond the iron coordination sphere itself, the study revealed that water molecules and distant parts of the protein play decisive roles. B-factor analysis, a measure of atomic mobility, showed that the α3 and α10 helices become progressively more mobile during catalysis, with α3 returning to its starting conformation in the product complex while α10 remains dynamic, consistent with the movement needed to release the product. A chain of three water molecules extending from the iron-bound water W1 through W2 and W3 proved essential. Mutating the second-shell residue Asn252, which hydrogen-bonds to W2, produced a striking result: the conservative N252D substitution, which preserved the water chain structurally, abolished penicillin production in solution and instead yielded a hydrated aldehyde shunt product, while the bulkier N252Q and N252E variants disrupted W2 binding and impaired catalysis. A separate variant, H270E, in which the iron-coordinating His270 was replaced with glutamate, displaced the iron-bound water W1 and stalled the reaction at the thioaldehyde stage, with the released intermediate tautomerizing to enethiol products identified by comparison with synthetic standards.

Quantum mechanics/molecular mechanics calculations tied the observations together, supporting a pathway in which the iron-bound water W1 mediates peroxide cleavage to generate the Fe(IV)=O species, followed by water-assisted proton transfer to form the monocyclic intermediate. The computations reproduced the transient elongation of the iron–sulfur bond at the thioaldehyde stage and the low barrier for the final radical recombination that delivers isopenicillin N in a strongly exothermic step. Together, the structural, spectroscopic, mutagenic and computational evidence paints a revised picture of penicillin biosynthesis in which protein dynamics, a delicate water network and an unexpected release of sulfur from the iron choreograph the formation of one of medicine’s most important molecular scaffolds. The findings not only illuminate IPNS itself but offer a framework for understanding the broader Fe(II) oxygenase superfamily, and they demonstrate the power of X-ray free-electron laser methods to resolve transient enzymatic intermediates that no other experimental or computational approach can currently capture at such resolution.

Subject of Research: Time-resolved structural studies of intermediates in isopenicillin N synthase catalysis during β-lactam antibiotic biosynthesis

Article Title: Unanticipated intermediates during isopenicillin N synthase catalysis identified by time-resolved X-ray free-electron laser studies

Article References: Rabe, P., Makita, H., Southwart, R., Thomas, M. G., Kazaks, M., Basak, S., Zhou, T., Myers, W. K., Doyle, M. D., Simon, P. S., Hayama, S., Mosselmans, J. F. W., Stead, A. T., Oluwole, A. O., Sami, M., Clifton, I., Aller, P., Bhowmick, A., Tumber, A., … Schofield, C. J. (2026). Unanticipated intermediates during isopenicillin N synthase catalysis identified by time-resolved X-ray free-electron laser studies. Nature Catalysis. https://doi.org/10.1038/s41929-026-01618-4

Image Credits: AI Generated

DOI: 10.1038/s41929-026-01618-4

Keywords: isopenicillin N synthase, penicillin biosynthesis, X-ray free-electron laser, serial femtosecond crystallography, enzyme mechanism, iron oxygenase, β-lactam antibiotics, thioaldehyde intermediate, X-ray emission spectroscopy, protein dynamics, active-site water, structural biology

The DprE1–DprE2 complex performs a deceptively simple but absolutely essential chemical transformation. It converts decaprenylphosphoribose into decaprenylphosphoarabinose, the activated sugar donor required to build arabinan, a critical polysaccharide component of the mycobacterial cell wall. The reaction is a two-step epimerization: DprE1, a flavin-dependent oxidase, oxidizes the ribose moiety of the lipid-linked substrate, and DprE2, an NADH-dependent reductase belonging to the short-chain dehydrogenase/reductase superfamily, then reduces the intermediate back to yield the arabinose epimer. Because arabinan is indispensable for the bacterium’s survival and integrity, blocking either half of this two-enzyme relay starves the cell wall of its building blocks and kills the pathogen. This vulnerability has made the epimerase one of the most intensively pursued targets in modern anti-tubercular drug discovery, yet the physical architecture of the complex and the mechanism of its inhibition by clinical compounds had remained poorly defined.

The structural work now published reveals that the functional unit of the epimerase is far more elaborate than a simple pair of enzymes. Cryo-EM analysis shows that DprE1 and DprE2 assemble into a membrane-associated tetramer in which two DprE2 molecules form a central homodimer, flanked on each side by a single DprE1 subunit. Each DprE2 monomer cradles a molecule of NADH in its catalytic core, while the neighboring DprE1 subunits carry their own FAD cofactors and bind the lipid-like decaprenylphosphoribose substrate. The interfaces holding this four-subunit assembly together were mapped in detail: the DprE2 dimer is stabilized through contacts involving its alpha-5 and alpha-7 helices and its C-terminal region, while the heterodimeric contacts between DprE1 and DprE2 involve the N-terminal loop of DprE2, residues 2 through 10, reaching into the FAD-binding and substrate-binding domains of DprE1. This arrangement positions the two active sites in close proximity, providing a structural rationale for how the oxidized intermediate can be handed efficiently from DprE1 to DprE2 without diffusing away into the membrane.

Perhaps the most consequential finding concerns pretomanid and delamanid, two nitroimidazole prodrugs that have become cornerstones of therapy for multidrug-resistant tuberculosis. Both compounds require activation by the bacterial deazaflavin F420-dependent nitroreductase Ddn, a process that generates reactive des-nitro metabolites. Earlier work had established that the activated derivatives of these drugs ultimately target DprE2, but the structural basis of that interaction was unknown. The new structures show that both activated pretomanid and activated delamanid bind to DprE2 in the form of covalent NADH adducts. Mass spectrometric analysis confirmed the formation of these adducts, with the drugs attached to the nicotinamide ribose of NADH, producing an inhibitory species that occupies an extended, conserved binding pocket in DprE2. Strikingly, this pocket stretches all the way from the NADH-binding site at the catalytic center to the substrate-binding site, meaning the drug adduct simultaneously blocks cofactor chemistry and substrate access in one motion.

This mechanism is a striking echo of the way isoniazid, the oldest frontline tuberculosis drug, kills mycobacteria. Isoniazid is also a prodrug, activated by the catalase-peroxidase KatG, and its activated form forms a covalent NAD adduct that potently inhibits the enoyl reductase InhA, halting mycolic acid synthesis. Pretomanid and delamanid, it now appears, deploy essentially the same chemical strategy against a different cell wall pathway: they hijack the cell’s own NADH to forge a tight-binding inhibitor inside the DprE2 active site. The structural overlays show that the drug portion of each adduct sits in a hydrophobic channel adapted to accept the long lipophilic tail of the decaprenyl substrate, with hydrogen-bonding interactions anchoring the drug in place. The catalytic triad of DprE2, comprising serine 147, tyrosine 160 and lysine 164, is highly conserved across the SDR superfamily, and the structures explain how the adduct wedges into the space adjacent to this catalytic machinery.

The third drug interrogated in the study, quabodepistat, acts on the other half of the complex. Unlike the nitroimidazoles, quabodepistat is a noncovalent inhibitor of DprE1, the oxidase component, and it is currently being evaluated in phase 2b/c clinical trials in combination with delamanid and bedaquiline. The cryo-EM structure of the quabodepistat-bound complex reveals a distinctive binding mode. The inhibitor occupies the substrate-binding site of DprE1, with its head, trunk and tail regions engaging separate polar and nonpolar pockets, and it induces a conformational change in two surface loops of DprE1 that adopt an open, ordered conformation not seen in structures with other noncovalent DprE1 inhibitors. Differential scanning fluorimetry experiments showed that wild-type DprE1 binds quabodepistat completely, whereas a catalytically relevant C387S mutant shows only partial binding, underscoring the specificity of the interaction and distinguishing quabodepistat’s binding mode from that of covalent inhibitors such as the benzothiazinones, which attack the same cysteine residue.

The technical achievement underlying these structures should not be understated. DprE1 and DprE2 are membrane-associated proteins, and capturing the intact tetramer in a homogeneous, grid-ready state required careful biochemical optimization, including crosslinking of the complex. The researchers collected cryo-EM datasets for four distinct states: the substrate-bound complex, the complex bound to the NADH-pretomanid adduct, the complex bound to the NADH-delamanid adduct, and the quabodepistat-bound complex. Atomic coordinates were deposited in the Protein Data Bank under accession codes 21GR, 21GQ, 43FX and 21GT, with corresponding maps in the Electron Microscopy Data Bank. The resulting models reveal not only ligand-binding details but also the electrostatic character of DprE2’s surface, including a loop region that likely helps position the enzyme at the membrane interface where its long lipid substrate resides.

The therapeutic implications are significant. By showing exactly which residues line the DprE2 drug pocket and how the NADH-adduct form of pretomanid and delamanid nestles within it, the structures give medicinal chemists a template for designing next-generation nitroimidazoles with improved potency and a reduced likelihood of resistance. Resistance to nitroimidazoles in the clinic is already known to arise through mutations in the F420 biosynthetic pathway or in the nitroreductase Ddn, which prevent drug activation; the new structures now make it possible to anticipate and monitor resistance mutations directly in DprE2 as well. Similarly, the unique binding mode of quabodepistat in DprE1, with its ordered surface loops and its three-part engagement of the substrate pocket, offers a scaffold for optimizing noncovalent DprE1 inhibitors, a class that includes the advanced clinical candidate OPC-167832 and has been the focus of intense structure-based drug design for over a decade.

More broadly, the study closes a long-standing gap in mycobacterial cell wall biology. Since the discovery in 2005 that decaprenylphosphoarabinose is formed by a two-step epimerization, the field has studied DprE1 and DprE2 largely as separate enzymes, crystallizing DprE1 with numerous inhibitors and probing DprE2 biochemically. The tetrameric complex captured here, a DprE2 dimer flanked by two DprE1 subunits, provides the first complete architectural picture of the functional epimerase and suggests how its two active sites are coordinated in a single membrane-associated assembly. As tuberculosis continues to claim more than a million lives each year and drug resistance spreads, structures of this quality do more than satisfy curiosity: they convert a validated but opaque drug target into an engineerable one, and they explain at atomic resolution why some of the newest weapons against the world’s deadliest bacterial infection work the way they do. The path from these electron density maps to better tuberculosis regimens now runs directly through rational design, guided by the pocket architecture the researchers have laid bare.

Subject of Research: Structural and mechanistic analysis of the Mycobacterium tuberculosis DprE1–DprE2 epimerase complex and its inhibition by anti-tubercular drugs

Article Title: Mechanism of the M. tuberculosis DprE1–DprE2 epimerase complex and inhibition by anti-tubercular drugs

Article References: Gao, S., Wu, F., Zhang, Y., Batt, S. M., Wei, C., Wang, Z., Yang, X., Abrahams, K. A., Fütterer, K., Liu, X., Wang, P., Besra, G. S., Rao, Z., & Zhang, L. (2026). Mechanism of the M. tuberculosis DprE1–DprE2 epimerase complex and inhibition by anti-tubercular drugs. Nature Chemical Biology. https://doi.org/10.1038/s41589-026-02317-8

Image Credits: AI Generated

DOI: 10.1038/s41589-026-02317-8

Keywords: tuberculosis, Mycobacterium tuberculosis, DprE1, DprE2, cryo-EM, pretomanid, delamanid, quabodepistat, cell wall synthesis, drug resistance, structural biology, enzyme mechanism

Prenyl groups, the five-carbon isoprene-derived units installed by prenyltransferases across all domains of life, are among the most common chemical decorations found on natural products. They modulate membrane affinity, target binding, and the photochemical and oxidative stability of their host molecules. Conventionally, the further diversification of prenyl side chains has been understood to proceed through oxidation at the allylic positions, cyclization, or hydration chemistry that preserves or extends the preexisting double bond. The newly described enzymes break with that logic. Rather than modifying the prenyl unit from its periphery, they remove one of its methyl substituents in a demethylation reaction that reorganizes the bonding framework of the side chain itself, converting what was a simple isoprenoid branch into an allene or an alkyne.

The investigative team, whose analysis appears in the journal’s September 2026 issue, combined comparative genomics, in vivo gene activation, purified-enzyme biochemistry, isotopic labeling, and computational modeling to trace the reaction from gene to product. The biosynthetic gene clusters in question were flagged because they encode proteins annotated as radical S-adenosylmethionine enzymes alongside partners resembling methyltransferases and oxidative tailoring catalysts. When the clusters were expressed in heterologous bacterial hosts, the researchers observed the accumulation of metabolites bearing allene and terminal alkyne functionalities, structures that could only be explained if the prenyl substituents had lost a methyl group and undergone a formal dehydrogenation in the process.

At the heart of the discovery is the finding that the demethylating enzyme acts on the methyl group at the branched position of the prenyl chain. Isotopic feeding experiments, in which cultures were supplied with methyl-labeled precursors, demonstrated that the carbon of that methyl group is released from the product scaffold, while the hydrogen atoms retained on the adjacent carbons undergo stereospecific removal. The net result is a desaturation achieved by C–C bond cleavage followed by reorganization of the pi system, a sequence that organic chemists would typically accomplish with strong bases or transition-metal catalysts under strictly anhydrous conditions. The enzyme accomplishes the same transformation in water, at ambient temperature and neutral pH, using nothing more exotic than the cofactors already standard in secondary metabolism.

Mechanistically, the evidence supports a pathway in which an initial methylation or hydroxymethylation prime of the prenyl unit sets up an elimination-competent intermediate. Computational docking and quantum mechanical calculations suggest that the enzyme positions the methyl substituent adjacent to a catalytic base, enabling proton abstraction that drives fragmentation of the C–C bond to the methyl carbon. The resulting conjugated intermediate collapses either to an allene, when the geometry of the active site permits cumulation of the two double bonds, or to an alkyne, when successive dehydrogenation steps flatten the terminus into a linear acetylenic arrangement. The divergence between the two outcomes appears to be governed by subtle differences in the active-site architecture among enzyme variants, a conclusion the authors support with site-directed mutagenesis in which single residue swaps shifted product profiles from allene-bearing to alkyne-bearing metabolites.

The biological context of these transformations is as intriguing as the chemistry itself. Allene and alkyne motifs are rare but consequential in pharmacologically relevant molecules. The cytotoxic natural product families that contain them often owe their potency to the electrophilicity and strained geometry of the unsaturation, which can engage biological nucleophiles or undergo controlled activation to generate cytotoxic species. By revealing a biosynthetic route to these groups that runs through demethylation, the study supplies a missing link in the catalog of enzymatic reactions available to natural product assembly lines and suggests that many undiscovered metabolites bearing these motifs may be encoded in silent or poorly annotated gene clusters across microbial genomes.

From a biocatalysis standpoint, the implications are immediate. Synthetic routes to allenic and acetylenic compounds frequently demand multiple steps, protecting groups, and careful control of regioselectivity and stereochemistry. An enzyme that installs these functionalities regioselectively from a prenylated precursor offers a shortcut that synthetic chemists can borrow. The demonstrated tolerance of the enzymes for varied prenylated substrates raises the prospect of chemoenzymatic pipelines in which readily assembled prenylated intermediates are converted into allene- or alkyne-containing analogs for medicinal chemistry screening. Because the reactions proceed under mild aqueous conditions, they are compatible with sensitive molecular frameworks that would not survive conventional synthetic desaturation chemistry.

The study also contributes to a broader reassessment of what demethylation can mean in enzyme chemistry. Demethylases are conventionally viewed as deactivating catalysts: they remove methyl groups in the course of detoxification, epigenetic regulation, or catabolism, restoring a parent structure without altering the carbon skeleton beyond the excised methyl. The enzymes described here instead use demethylation as a constructive act, coupling the loss of a one-carbon unit to a deep reorganization of unsaturation elsewhere in the molecule. This reframing widens the mechanistic repertoire attributed to S-adenosylmethionine-dependent and methyl-oxidizing enzyme families and predicts that other examples of desaturative demethylation await discovery, particularly in gene clusters encoding enzymes of mixed annotation whose functions have not been experimentally interrogated.

The authors buttress their mechanistic proposals with a combination of substrate analog studies and structural modeling that delineates how the active site discriminates between the two possible unsaturated outcomes. Key residues lining the substrate channel appear to enforce the trajectory of the departing methyl group and the orientation of the nascent pi system, effectively templating the geometry of the product. When the researchers perturbed these residues, the enzyme’s output shifted in ways consistent with the computed energy landscapes, reinforcing the picture of an active site that does not merely accommodate a reaction but actively choreographs which of two chemically plausible unsaturations emerges. That level of product control, achieved without metal cofactors beyond those required for the initial radical or oxidative priming steps, underscores the sophistication with which enzyme pockets can steer reactive intermediates toward defined outcomes.

Looking forward, the work opens several avenues. Genome-mining campaigns can now be retargeted to search for homologs of the demethylating enzymes, prioritizing clusters whose neighborhoods encode prenyltransferases and tailoring oxidases suggestive of comparable chemistry. Protein engineering efforts can explore whether the allene-versus-alkyne decision can be rationally inverted, converting these enzymes into programmable instruments for installing either motif at will. And for natural products research more broadly, the finding is a reminder that the chemical vocabulary of metabolism remains incompletely inventoried: even a modification as familiar as the prenyl group, appended to countless molecules and studied for decades, conceals transformations that redefine what enzymes can be asked to do. In removing a methyl group, these catalysts add two new bonds of unsaturation to the biosynthetic lexicon, and they invite chemists to reconsider how many other routine modifications might harbor equally unexpected chemistry.

Subject of Research: Enzymatic prenyl demethylation that generates allene and alkyne functionalities in microbial natural product biosynthesis

Article Title: Biosynthetic allene and alkyne formation by enzymatic prenyl demethylation

Article References: Liu, M., Ohashi, M., Han, W., Zhou, Q., Houk, K. N., & Tang, Y. (2026). Biosynthetic allene and alkyne formation by enzymatic prenyl demethylation. Nature Chemical Biology. https://doi.org/10.1038/s41589-026-02323-w

Image Credits: AI Generated

DOI: 10.1038/s41589-026-02323-w

Keywords: biosynthesis, allene, alkyne, prenylation, demethylation, radical SAM enzymes, natural products, biocatalysis, enzyme mechanism, secondary metabolism, isotopic labeling, genome mining

Transfer RNA molecules are not naked RNA strands; they are heavily decorated with chemical groups that fine-tune their stability, folding, and ability to recognize codons on the ribosome. More than a hundred distinct modified nucleosides have been catalogued across the three domains of life, ranging from simple methyl groups to elaborate, multi-ring structures that require whole cascades of enzymes to assemble. Some of the most chemically ambitious modifications are found in bacteria, where they help organisms survive in hostile environments, tune translation in response to stress, and even contribute to antibiotic resistance. The aminovaleramide modification reported by the AvaS research joins this growing catalogue as a striking example of amide chemistry carried out directly on RNA.

Pyridoxal phosphate, commonly abbreviated PLP, is a cofactor with a legendary reputation in enzymology. It sits at the reactive heart of enzymes that make, break, and rearrange amino acids, enabling transformations that would otherwise demand harsh laboratory conditions. PLP achieves this by forming an internal aldimine, a Schiff base linkage, with a lysine residue in the enzyme’s active site. When an amino acid substrate arrives, the linkage is exchanged in a transamination step that produces an external aldimine, positioning the substrate for reactions ranging from decarboxylation to side-chain cleavage. What makes the AvaS discovery remarkable is that this canonical amino acid chemistry appears to have been recruited for RNA biosynthesis, in effect coupling amino acid metabolism to the chemical maturation of tRNA.

According to the study, AvaS catalyzes the formation of aminovaleramide through a PLP-dependent route that resembles pathways used to synthesize certain amino acid-derived metabolites. The reaction logic involves the cofactor-mediated processing of an amino acid precursor, likely through condensation and rearrangement steps that generate an activated intermediate, followed by transfer of the resulting aminovaleramide moiety onto the tRNA scaffold. This kind of cofactor-dependent construction on RNA is rare. Most known tRNA modification enzymes rely on S-adenosylmethionine for methyl and threonylcarbamoyl chemistry, or on ATP-driven activation reactions to ligate smaller groups onto nucleotides. A PLP enzyme acting on tRNA therefore represents a mechanistic surprise, suggesting that the boundary between primary metabolism and RNA modification biochemistry is more porous than previously appreciated.

The identification of AvaS also illustrates the power of modern bioinformatic screens. Rather than stumbling across the enzyme by chance, the researchers were able to trace the modification’s occurrence by following the distribution of gene clusters whose sequence features hinted at PLP-dependent chemistry linked to RNA processing. Genes encoding tRNA modification enzymes frequently cluster with partner genes, exporter proteins, or resistance determinants, a pattern known as neighboring gene logic. By scanning bacterial genomes for such clusters, and by asking which organisms contain both the predicted modification machinery and the chemical signature of aminovaleramide-containing nucleosides, the team narrowed the search to a manageable set of candidate enzymes and then validated their predictions experimentally.

Once AvaS was confirmed as the biosynthetic enzyme, the structural and mechanistic characterization proceeded along classical enzymological lines, with modern tools. Recombinant production of the protein allowed the researchers to test its activity in vitro, demonstrating that purified AvaS could carry out the key chemical steps without the rest of the cellular milieu. Mass spectrometry of digested nucleosides confirmed the identity of the aminovaleramide product, while comparisons with catalytic mutants and cofactor-free controls established that PLP is genuinely required, not merely tolerated. These experiments collectively argue that a single enzyme can perform a multi-step biosynthesis, assembling the modification before or during its attachment to RNA, rather than relying on a separate pathway to pre-build the transferable group.

Why would a bacterium invest energy in constructing such an elaborate modification? The most likely answers relate to translation fidelity and stress physiology. Modified bases near the anticodon loop of tRNA influence how reliably the molecule pairs with messenger RNA codons, and disruptions to these modifications typically cause ribosomal frameshifting, slowed growth, or heightened sensitivity to environmental challenges. Amide-bearing modifications can also alter the local geometry and hydrogen-bonding pattern of the tRNA in ways that stabilize particular conformations of the anticodon loop. In pathogenic or environmental bacteria, such fine-tuning can mean the difference between thriving and failing under thermal, oxidative, or nutrient stress, which in turn makes the underlying enzymes attractive subjects for study as potential antimicrobial targets.

The discovery carries implications beyond microbiology. RNA chemical biology has been undergoing a renaissance, driven partly by interest in modified nucleosides as biomarkers, as regulators of gene expression, and as engineering targets for synthetic biology. Finding that a cofactor as central as PLP participates directly in RNA modification suggests that other seemingly improbable chemistries may also be lurking in unexplored genomic corners. The aminovaleramide modification itself, featuring a terminal amino group on a five-carbon chain linked through an amide bond, is chemically rich, and understanding how enzymes build and install such groups could inspire new methods for site-specific RNA labeling or the design of modified oligonucleotide therapeutics.

There are also evolutionary questions raised by the work. PLP-dependent enzymes form large and ancient protein superfamilies, and the AvaS result adds a new functional branch to that family tree. Determining whether the RNA-modifying activity arose by divergence from an amino acid biosynthetic ancestor, or through convergent recruitment of PLP chemistry into an unrelated scaffold, will require broader phylogenetic analysis. The study’s genomic survey provides a starting point, mapping where AvaS homologs occur across bacterial phyla and hinting at horizontal gene transfer events that may have spread the capability between distantly related organisms. Such analyses often reveal that RNA modification systems evolve rapidly, shaped by the arms races between microbes, their viruses, and their chemical environments.

For the field of tRNA biology, the AvaS report is a reminder of how much chemical diversity remains undocumented. Decades of focused work on well-studied model organisms, such as Escherichia coli and Saccharomyces cerevisiae, produced detailed maps of their modification landscapes, but the vast majority of bacterial species have never been surveyed with modern nucleoside mass spectrometry. As high-throughput analytical methods and genome-mining approaches converge, enzymes like AvaS are expected to surface with increasing frequency, each one a potential new tool for manipulating RNA and a potential window into unexplored metabolic logic. The PLP-dependent construction of aminovaleramide stands as an early and vivid example of what that exploration is likely to yield.

Subject of Research: Pyridoxal phosphate-dependent biosynthesis of the tRNA modification aminovaleramide by the bacterial enzyme AvaS

Article Title: Pyridoxal-phosphate-dependent biosynthesis of aminovaleramide by AvaS in tRNA

Article References: Sun, J., Wu, J., Yuan, Y., Balamkundu, S., Dziergowska, A., Chay Suen Suen, H., Dwijapriya, Liang, C., Hardy, L., Lee, M. E., Leszczynska, G., Liu, C.-F., Baharoglu, Z., Drouard, L., Bruner, S. D., Begley, T. J., de Crécy-Lagard, V., & Dedon, P. C. (2026). Pyridoxal-phosphate-dependent biosynthesis of aminovaleramide by AvaS in tRNA. Nature Chemical Biology. https://doi.org/10.1038/s41589-026-02303-0

Image Credits: AI Generated

DOI: 10.1038/s41589-026-02303-0

Keywords: tRNA modification, AvaS, pyridoxal phosphate, aminovaleramide, vitamin B6, enzyme mechanism, RNA chemical biology, bacterial genomes, translation fidelity, nucleoside mass spectrometry, bioinformatic gene mining, Nature Chemical Biology