The story begins with the molecule itself. 5-Azacytidine is a mimic of cytidine, one of the four canonical letters of RNA, but with a crucial difference: the carbon at position 5 of the pyrimidine ring is replaced by nitrogen, converting the base into 5-azacytosine, a member of the 1,3,5-triazine family. This single atom swap is what gives the drug its therapeutic power. Once incorporated into RNA, the analogue disrupts RNA metabolism and traps methyltransferase enzymes, ultimately reactivating tumor suppressor genes that cancer cells had silenced. Approved by the FDA in 2004 under the brand name Vidaza, the compound has become a mainstay of treatment for patients with myelodysplastic syndromes.

Because the drug was first made synthetically by Czech chemists in 1964 and only isolated from microbial cultures two years later, most researchers assumed its biosynthesis would never be found, or simply never thought to look. The prevailing view treated 5-azacytidine as a synthetic analogue that happened to also occur in nature, rather than as the product of an evolved metabolic pathway. That assumption, the new work shows, hid one of the most unusual enzymatic cascades yet discovered in natural product chemistry.

A team led by Yasushi Ogasawara and Tohru Dairi of Hokkaido University, together with Hiroyuki Morita of the University of Toyama and collaborators across Japan and Taiwan, set out to find the gene cluster. Through comparative genomics and biochemical screening of the producing organism, they pinpointed a set of genes designated azc that is both necessary and sufficient for assembling the triazine nucleobase. The identification immediately posed a puzzle: how does a cell convert ordinary purine and pyrimidine building blocks into a ring system with an extra nitrogen in an arrangement that is otherwise essentially unknown among natural nucleosides?

The answer begins with AzcE, an enzyme the researchers identify as a guanosine triphosphate cyclohydrolase. GTP cyclohydrolases are best known for opening the purine ring of GTP during the biosynthesis of folate and riboflavin, but AzcE repurposes this chemistry to generate 2,5,6-triaminopyrimidin-4(1H)-one, a pyrimidine intermediate carrying three amino groups. Structural analysis of AzcE, determined in complex with zinc and its product, revealed how the enzyme positions the substrate for ring opening, providing the first stepping stone on the path from a standard nucleotide precursor toward the triazine scaffold of the drug.

The centerpiece of the pathway, and of the study, is AzcA, a cupin domain-containing enzyme that performs what the authors describe as a skeletal editing reaction. Rather than building the triazine ring from scratch, AzcA takes the pyrimidine delivered by AzcE and surgically reworks it: the enzyme selectively cleaves the carbon-nitrogen framework of the heterocycle and reassembles it into 6-amino-4-oxo-1,4-dihydro-1,3,5-triazine-2-carboxylic acid. In effect, the protein performs molecular surgery on a ring, cutting specific bonds and stitching the fragments back together with an additional nitrogen inserted into the skeleton. X-ray crystal structures of AzcA bound to manganese and to reaction intermediates, combined with biochemical assays and density functional theory calculations, allowed the team to trace the mechanism in atomic detail, showing how the metal center and active-site residues orchestrate bond cleavage and reformation with remarkable selectivity.

The final step is equally unconventional. The carboxylic acid that AzcA installs must be removed to yield 5-azacytosine, and the enzyme pair AzcB/C accomplishes this through a thiamine pyrophosphate-dependent decarboxylation acting on the alpha-imino carboxylic acid moiety of the AzcA product. Thiamine-dependent enzymes typically handle alpha-keto acids, so acting on an imino acid represents an atypical use of this cofactor class. Cryo-electron microscopy structures of AzcB/C captured with thiamine pyrophosphate and substrate analogues revealed the active-site architecture that stabilizes the reactive intermediates, completing the mechanistic picture of how the triazine base is finished and presumably glycosylated to give the mature nucleoside antibiotic.

What makes the discovery resonate beyond one molecule is the concept of enzymatic skeletal editing itself. Synthetic chemists have in recent years celebrated skeletal editing, the late-stage insertion, deletion, or transmutation of single atoms within a molecular framework, as a frontier strategy for drug discovery. The demonstration that a cupin enzyme performs precisely this kind of single-atom logic on a heterocyclic ring during natural biosynthesis shows that nature arrived at the same idea long ago, and suggests that related enzymes scattered across bacterial genomes may perform similar transformations on other scaffolds. Genome mining guided by the azc gene cluster could therefore uncover new triazine and related azine natural products, or inspire engineered enzymes for late-stage functionalization of existing drugs.

There are also practical implications for the drug itself. Current industrial production of 5-azacytidine relies on multi-step chemical synthesis, and a biosynthetic route defined by just a handful of enzymes offers a potential platform for biocatalytic or fermentative manufacturing, possibly enabling access to analogues that are difficult to make chemically. The work also resolves a long-standing oddity in natural product history: a molecule used daily in oncology wards worldwide turns out to be a genuine bacterial metabolite, forged by enzymes that edit the very skeletons of nucleobases. For a drug whose clinical value stems from impersonating DNA’s own letters, it is fitting that its maker turns out to be a master of rewriting them.

Subject of Research: Enzymatic biosynthesis of the anticancer nucleoside analogue 5-azacytidine

Article Title: Enzymatic skeletal editing reaction forming the 1,3,5-triazine core during biosynthesis of the anticancer nucleoside analogue 5-azacytidine

Article References: Enzymatic skeletal editing reaction forming the 1,3,5-triazine core during biosynthesis of the anticancer nucleoside analogue 5-azacytidine. (n.d.). https://doi.org/10.1038/s41929-026-01611-x

Image Credits: AI Generated

DOI: 10.1038/s41929-026-01611-x

Keywords: 5-azacytidine, biosynthesis, natural products, enzymes, skeletal editing, 1,3,5-triazine, anticancer drugs, myelodysplastic syndrome, GTP cyclohydrolase, cupin enzyme, thiamine pyrophosphate, enzyme mechanisms

Redox cofactors are the molecular batteries of the cell. They accept and donate electrons in the tightly choreographed reactions that underpin respiration, photosynthesis, biosynthesis and detoxification. The canonical cofactors—molecules such as flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN), nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP)—are so widespread that their presence in an enzyme active site is often assumed rather than demonstrated. But over the past several years, researchers have identified a series of modified and entirely distinct cofactors: prenylated flavins such as flavin adenine dinucleotide modified with a prenyl group, deazaflavins like F420, quinone-derived cofactors such as topaquinone and tryptophan tryptophylquinone, and metal-organic species that defy easy classification. These noncanonical cofactors enable chemistries that standard flavins and nicotinamides cannot easily achieve, including hydride transfers at unusual redox potentials, radical-mediated rearrangements and C–C bond formations that would be difficult with conventional catalysis.

The central puzzle addressed in the new work is one of recognition and assembly. If an enzyme uses a noncanonical cofactor, how does the protein know to bind that cofactor rather than its more abundant canonical cousin? And how can bioinformaticians predict, from sequence alone, which of the millions of uncharacterized proteins in genomic databases depend on these exotic helpers? The answer, according to the study, lies in a short, recurring sequence motif—a conserved stretch of amino acids that acts as a molecular postcode, directing the enzyme’s cofactor-binding pocket toward noncanonical chemistry.

Sequence motifs have long served as the workhorses of computational biology. Short conserved patterns, such as the P-loop that binds nucleotide phosphates or the zinc-finger motifs that coordinate metal ions in DNA-binding proteins, allow researchers to assign function to proteins that have never been isolated in a laboratory. The newly identified motif performs a similar role for redox cofactor selection. By scanning families of flavin-dependent enzymes and comparing those known to use standard FAD or FMN with the smaller subset confirmed to use modified or alternative cofactors, the researchers identified a conserved pattern of residues that appears with striking regularity in the noncanonical group and is conspicuously absent from the canonical one. Mutational experiments confirmed that altering these residues in a noncanonical enzyme abolished its ability to accommodate the alternative cofactor, while introducing the motif into a canonical scaffold shifted its cofactor preference—a result that establishes the motif as a genuine determinant of cofactor identity rather than a coincidental correlation.

The implications of this finding extend well beyond the specific enzyme families examined in the study. Genomic surveys suggest that proteins carrying the motif are distributed across a remarkable range of organisms, from soil-dwelling actinobacteria—long recognized as prolific producers of bioactive natural products—to human-associated microbes and even some archaeal lineages. In many of these organisms, the motif-bearing enzymes cluster within biosynthetic gene clusters, the compact genomic neighborhoods that encode the assembly lines for antibiotics, antitumor agents and other specialized metabolites. This genomic context hints at a widespread and previously underappreciated role for noncanonical redox chemistry in natural product biosynthesis, suggesting that many of the structurally exotic metabolites isolated from microbes over the past half-century may owe their existence to enzymes quietly using cofactors that standard annotation pipelines would never flag.

One of the most exciting consequences of the work is predictive. Armed with the motif, researchers can now interrogate sequence databases with a simple pattern search and retrieve a curated list of candidate enzymes likely to use noncanonical cofactors. This transforms what has historically been a slow, serendipitous process—discover a strange metabolite, purify the enzyme responsible, and only then realize the cofactor is unusual—into a rational, hypothesis-driven workflow. Biochemistry can then be targeted at the most promising candidates, prioritizing enzymes from gene clusters associated with medicinally relevant compound classes. In an era when the rate of genome sequencing vastly outpaces the rate of experimental characterization, tools that convert sequence information into functional predictions are among the most valuable commodities in the life sciences.

The discovery also carries significant weight for synthetic biology and enzyme engineering. Noncanonical cofactors often possess redox potentials and reactivity profiles that canonical cofactors cannot match. F420, for example, the deazaflavin cofactor best known from methanogenic archaea, mediates hydride transfer reactions at potentials inaccessible to NAD and NADP, and engineered F420-dependent enzymes have already been explored for the degradation of persistent pollutants and the production of pharmaceutical intermediates. Prenylated flavins, meanwhile, catalyze photochemical reactions that ordinary flavins cannot, and their light-driven chemistry is being harnessed in optogenetic tools and photocatalytic cascades. A sequence-level handle on cofactor selection means that protein engineers can now rationally swap cofactor identity in designed enzymes, effectively reprogramming the electrochemical capabilities of a catalytic scaffold without altering its overall fold. This could accelerate the design of biocatalysts for green chemistry, where replacing metal catalysts and harsh reagents with enzyme-based alternatives is a major industrial goal.

From an evolutionary standpoint, the findings raise fascinating questions about how and why biology expanded its redox cofactor repertoire in the first place. The canonical cofactors are ancient, likely predating the last universal common ancestor, and their chemistry is deeply woven into core metabolism. Noncanonical cofactors, by contrast, appear to have arisen as evolutionary innovations in specific ecological and metabolic contexts—perhaps to exploit new redox niches, to escape the thermodynamic constraints of shared metabolic pools, or to protect specialized pathways from cross-talk with housekeeping chemistry. The presence of a dedicated sequence motif suggests that cofactor innovation was accompanied by co-evolution of the protein binding environment, producing a heritable, recognizable signature that could be propagated across enzyme families through duplication and divergence. In this sense, the motif is a fossil record of chemical innovation, preserving in amino acid sequence the memory of evolutionary experiments in electron transfer.

The study also serves as a cautionary tale for genome annotation. Most automated pipelines assign enzyme function by homology, and a protein that resembles a flavin-dependent monooxygenase is typically annotated as such, regardless of which cofactor it actually employs. If a substantial fraction of these enzymes in fact use noncanonical cofactors, then large swaths of existing functional annotations may be subtly or substantially wrong, with consequences for metabolic modeling, pathway reconstruction and the interpretation of gene-expression data. The motif provides a corrective lens, allowing annotators to flag proteins whose cofactor assignments deserve experimental scrutiny. As the authors and commentators in the field note, the lesson is broader: the most abundant cofactors are not necessarily the only ones, and assumptions baked into databases can obscure entire layers of biochemical diversity.

Looking forward, the identification of this sequence motif is likely to catalyze a wave of discovery across several fronts. Experimentalists will purify and characterize motif-bearing enzymes from diverse organisms, likely uncovering new cofactor structures and new reaction types. Computational biologists will refine the motif definition, searching for related patterns that govern the use of other exotic cofactors, and integrating these signals into machine-learning models of enzyme function. Structural biologists will determine how the motif residues reshape the cofactor-binding pocket at atomic resolution, providing design principles for engineered catalysts. And natural products chemists will revisit orphan biosynthetic gene clusters with fresh eyes, suspecting that many of the unexplained transformations encoded within them depend on redox chemistry that no one thought to look for. What began as a search for a short string of amino acids has ended with a map pointing toward a vast, unexplored territory of enzyme chemistry—one that has been hiding in plain sight within the genomes of organisms all around us, waiting only for the right pattern to reveal it.

Subject of Research: A conserved sequence motif that enables natural enzymes to use noncanonical redox cofactors

Article Title: A sequence motif enables widespread use of noncanonical redox cofactors in natural enzymes

Article References: Saleh, S., Hsu, N.-H., Luu, E., Martin, V. C., Ng, H. J. C., Black, W. B., Zhang, S., Kim, J.-K., Sankaran, B., Tran, A. H. T., Hayes, R. L., Siegel, J. B., Qiao, F., & Li, H. (2026). A sequence motif enables widespread use of noncanonical redox cofactors in natural enzymes. Nature Chemical Biology. https://doi.org/10.1038/s41589-026-02315-w

Image Credits: AI Generated

DOI: 10.1038/s41589-026-02315-w

Keywords: redox cofactors, sequence motif, enzymes, flavin, natural product biosynthesis, genome annotation, enzyme engineering, deazaflavin F420, biocatalysis, protein evolution, biosynthetic gene clusters, Nature Chemical Biology