The β-lactam ring is one of the most remarkable structures in all of chemistry. It is a four-membered ring under enormous strain, and that strain is precisely what gives β-lactam antibiotics their power. When the molecule reaches a bacterium, the reactive ring interferes with the enzymes that build the bacterial cell wall. Unable to maintain its protective envelope, the cell ruptures and dies. Every penicillin, cephalosporin and carbapenem in the clinical arsenal depends on this chemical trick. Yet for decades, scientists studying how nature assembles this ring have been frustrated by a fundamental obstacle: the key reaction intermediates exist for only tiny fractions of a second, far too briefly to be observed with conventional structural biology methods, which rely on static snapshots of stable states.

The Oxford-led team overcame this barrier with an ingenious experimental design built around X-ray free-electron lasers, instruments that generate ultrashort, extraordinarily intense X-ray pulses capable of recording atomic-resolution images before radiation damage destroys the sample. The researchers prepared thousands of tiny microcrystals of IPNS bound to its substrate under oxygen-free conditions, since the enzyme requires oxygen to trigger the reaction but must not encounter it prematurely. These crystals were deposited as microscopic droplets onto a moving tape just two millimeters wide. As the tape swept through an oxygen-filled chamber, oxygen diffused rapidly into the crystals, initiating the catalytic reaction simultaneously across the entire sample. By precisely tuning the speed of the tape, the team controlled exactly how long each crystal had been reacting by the time it reached the X-ray interaction point, where a single ultrafast pulse captured an atomic-resolution snapshot of that instant.

Repeating this procedure at many different reaction times and combining thousands of individual snapshots, the researchers assembled a complete, time-resolved sequence of the catalytic process, effectively a molecular movie spanning milliseconds to seconds. Crucially, the technique works at physiological temperature and pressure, meaning the enzyme was filmed under conditions close to those inside a living organism rather than in a frozen or chemically trapped state. Each frame revealed a distinct stage of the transformation, allowing the team to watch, in atomic detail, how IPNS folds and oxidizes its linear peptide substrate into the bicyclic penicillin scaffold in a single, extraordinarily complex catalytic step.

Among the stages captured were two intermediates never directly observed before. The first was a thioaldehyde, a reactive sulfur-containing species formed immediately before the β-lactam ring closes. The second was a monocyclic β-lactam intermediate, the first ring-shaped structure on the pathway to the finished antibiotic. These observations provide the clearest picture yet of how IPNS, an iron-dependent enzyme, choreographs one of the most demanding transformations in natural product biosynthesis. The results overturn long-held assumptions about the sequence of chemical events and demonstrate that the enzyme guides its substrate through a series of carefully controlled, high-energy states that no static structure could ever reveal.

Perhaps the most surprising finding concerns water. The study showed that water molecules buried within the enzyme play an active role in steering the reaction, shuttling protons at critical junctures. Subtle movements rippling through the protein scaffold help position these waters and the substrate itself, indicating that the enzyme’s shape and its chemistry operate as an integrated system. Enzyme motion, iron coordination chemistry and water-mediated proton transfer are thus woven together in a precisely timed sequence. This holistic view of catalysis, in which the protein architecture actively participates in the chemistry rather than merely hosting it, offers a much richer framework for understanding how enzymes achieve such remarkable precision.

The significance of the work extends well beyond satisfying historical curiosity. Antimicrobial resistance, the process by which bacteria and other microorganisms evolve to survive the drugs designed to kill them, is steadily undermining existing antibiotics. The World Health Organization has warned that widespread resistance to common antibiotics is now a global reality, with resistance reported in a substantial share of bacterial infections. Rising resistance threatens not only the treatment of infection itself but also the foundations of modern medicine: cancer chemotherapy, organ transplantation and major surgery all depend on the ability to prevent and control bacterial infection. With too few new antibiotics in the development pipeline, understanding exactly how nature builds these molecules is a strategic step toward replenishing it.

Professor Christopher Schofield, Professor of Chemistry at the University of Oxford and a senior author of the paper, emphasized that penicillin still holds lessons for modern science. By capturing these fleeting steps, he noted, researchers can better understand how enzymes control complex chemistry with remarkable precision, and as resistance rates continue to rise, that understanding will ultimately help make existing antibiotics more efficient and support the design of entirely new antibiotic structures. Dr Patrick Rabe, Wellcome Career Development Award Investigator at Oxford and first author of the study, added that the ability to capture structural snapshots across milliseconds to seconds of reaction time allows scientists to connect enzyme motion, iron chemistry and proton transfer in a single coherent picture, and that knowing the process in atomic detail opens the door to engineering these enzymes to produce new or improved antibiotic scaffolds.

The technical achievement also reflects a broader shift in structural biology. Dr Allen M. Orville, group leader of the XFEL Hub at Diamond Light Source and a co-author, described how the collaboration combined specialist expertise in time-resolved X-ray methods with the capabilities of partner facilities to move beyond static structures and reveal enzymes at work in real time. Experiments were carried out at X-ray free-electron laser facilities on three continents, including the Linac Coherent Light Source in the United States, PAL-XFEL in the Republic of Korea, SACLA in Japan and Diamond Light Source in the United Kingdom. The study drew together structural biology, spectroscopy, chemistry, computation and enzyme engineering, demonstrating a powerful general approach for observing fleeting chemical intermediates that were previously inaccessible, both in antibiotic biosynthesis and in other biologically important enzyme reactions.

There is a fitting historical resonance in the fact that Oxford has once again advanced the science of penicillin. It was there, during the Second World War, that the Nobel prize-winning scientists Howard Florey and Ernst Chain, together with their colleagues, developed penicillin into the first clinically useful antibiotic, building on Alexander Fleming’s original discovery. It was also at Oxford that the Nobel laureate Dorothy Hodgkin solved the structure of penicillin in 1945 using X-ray crystallography, laying the groundwork for the structural approach that made the new study possible. Beyond antibiotics, the findings carry implications for a wide family of iron-dependent oxygenase enzymes involved in human biology, including those that allow our cells to sense and respond to changes in oxygen availability. By revealing the principles through which one such enzyme executes an exceptionally difficult transformation, the molecular movie of penicillin biosynthesis offers new foundations for enzyme engineering and catalyst design, and a vivid reminder that even the most familiar medicines still guard secrets worth filming.

Subject of Research: Time-resolved X-ray free-electron laser observation of isopenicillin N synthase catalysis in penicillin biosynthesis

Article Title: Ingenious ‘molecular movie’ finally reveals how penicillin is made

Article References: Ingenious ‘molecular movie’ finally reveals how penicillin is made. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: penicillin, β-lactam antibiotics, isopenicillin N synthase, X-ray free-electron laser, time-resolved crystallography, enzyme catalysis, antimicrobial resistance, University of Oxford, Diamond Light Source, reaction intermediates, structural biology, Nature Catalysis

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 experimental organism, Chlorella vulgaris, is a globally distributed unicellular green alga that sits near the base of aquatic food webs. Because it photosynthesizes, grows quickly and responds rapidly to chemical stress, it is one of the standard test organisms used in ecotoxicology to evaluate how contaminants affect primary producers. If algal growth falters, the consequences cascade upward: less energy captured from sunlight means less food for zooplankton, fish and everything that depends on them. That is why the fate of a single microalgal species matters enormously when regulators try to judge whether a chemical in the water is safe.

The researchers focused on two members of the β-lactam family, the most widely prescribed class of antibiotics in human and veterinary medicine. The first was amoxicillin, a penicillin-derivative consumed in enormous quantities worldwide and frequently detected in effluent from hospitals, livestock operations and municipal treatment plants. The second was aztreonam, a monobactam antibiotic prized for its activity against gram-negative bacteria. Both drugs reach surface waters after being excreted by patients or discarded improperly, since unused antibiotics flushed down drains largely pass through conventional wastewater treatment intact. A recent body of literature has flagged improper disposal of unused antibiotics as an overlooked driver of antimicrobial resistance, but the Tianjin team was interested in a different consequence: what these molecules do to organisms that are not their targets.

The second contaminant was silver nanoparticles, engineered particles of metallic silver measuring mere billionths of a meter across. Silver nanoparticles are among the most commercially successful nanomaterials on the planet, embedded in textiles, food packaging, cosmetics, medical devices and water filters because of their potent antibacterial properties. That same potency, however, extends to non-target organisms, and the particles are known to shed silver ions as they age in the environment. Previous studies have documented toxicity of silver nanoparticles to bacteria, algae, fish and invertebrates, yet most regulatory risk assessments still evaluate chemicals one at a time, an approach that grossly underestimates exposure when multiple contaminants co-occur, as they inevitably do in real receiving waters downstream of wastewater outfalls.

The experimental design was straightforward but rigorous. Cultures of Chlorella vulgaris were exposed for 96 hours to each of the three substances individually across a gradient of concentrations, and then to binary combinations of the antibiotics with the silver nanoparticles. Growth inhibition was used as the primary endpoint, summarized by the median effect concentration, the dose that reduces algal growth by half over the exposure period. The ranking that emerged was unambiguous: silver nanoparticles were the most toxic of the three, aztreonam came second, and amoxicillin was the least harmful on its own. In every case, higher exposure concentrations produced progressively greater suppression of algal growth, confirming dose-dependent toxicity for each compound.

The crucial finding came from the cotreatments. When the antibiotics and silver nanoparticles were present together, their combined effect on the algae was synergistic, meaning the observed toxicity exceeded what would be predicted by simply adding the effects of each chemical alone. The nanoparticles did not merely contribute an independent share of damage; they actively magnified the harm caused by the antibiotics, and the antibiotics in turn intensified the damage caused by the nanoparticles. This kind of interaction is precisely what conventional single-substance testing misses, and it suggests that environmental concentrations of these pollutants, even when each falls below a formal toxicity threshold individually, could still endanger photosynthetic organisms in combination.

To understand what the synergy was doing at the cellular level, the team turned to scanning electron microscopy. The images told a stark story. Algae that had been exposed to a single contaminant showed visible but comparatively limited damage to their cell surfaces, while cells subjected to coexposure displayed markedly more severe structural disruption. The cell wall and membrane, the alga’s first lines of defense, appear to be a shared point of attack: β-lactam antibiotics interfere with cell wall integrity in bacteria, and silver nanoparticles are notorious for compromising membranes and generating reactive oxygen species. When both stressors arrive together, the cell’s protective architecture may be weakened from two directions at once, allowing each toxicant to penetrate more deeply and inflict more damage than either could alone.

Oxidative stress is likely a central player in this interaction. Aquatic toxicologists have long observed that antibiotics such as clarithromycin, erythromycin and roxithromycin trigger antioxidant responses in freshwater microalgae, forcing cells to divert resources into defensive enzymes at the expense of growth. Silver nanoparticles act through a complementary mechanism, releasing silver ions that disrupt electron transport and catalyze the formation of superoxide and other reactive oxygen species inside the cell. When both classes of compounds are present, the combined oxidative burden can overwhelm the antioxidant system, damaging photosynthetic machinery and lipids faster than the cell can repair them. The result is the synergy recorded in the growth data: a whole-cell collapse that neither contaminant could achieve independently at the same doses.

To make sense of the many physiological indicators measured across the study, the researchers applied grey correlation analysis, a statistical technique well suited to small datasets with uncertain or incomplete information. The method quantifies how strongly each measured parameter tracks with the overall toxic outcome, helping to identify which physiological responses are the best predictors of harm under combined exposure. By linking growth inhibition, ultrastructural damage and the underlying biochemical indicators within a single analytical framework, the analysis strengthens the mechanistic interpretation of the synergy and offers a template that other laboratories can use to disentangle complex, multi-stressor toxicity.

The wider implications reach well beyond one species of green alga. Antimicrobial resistance researchers have warned for years that antibiotics polluting aquatic environments do not simply disappear; they persist, transform and interact with other contaminants, including the engineered nanomaterials that society is releasing in ever-larger quantities. This study adds a critical ecological dimension to that concern, showing that the coexistence of β-lactam antibiotics and silver nanoparticles in water is not merely a matter of two pollutants sharing space but of chemically interactive stressors whose joint toxicity to primary producers is greater than their parts. The authors argue that their findings contribute substantially to understanding the joint effects of these contaminants on aquatic organisms and provide solid evidence for the ecological risk assessment of their coexistence in natural waters. For regulators, the message is sobering: risk assessments that test chemicals in isolation will systematically understate the threat, and future water-quality standards may need to account explicitly for mixtures of antibiotics and nanomaterials. For the rest of the ecosystem, the study is a reminder that the health of entire food webs can hinge on the microscopic cells that convert sunlight into life, and that those cells are quietly registering the cumulative burden of everything humanity sends downstream.

Subject of Research: Combined toxicity of β-lactam antibiotics and silver nanoparticles to the freshwater microalga Chlorella vulgaris

Article Title: Physiological responses of Chlorella vulgaris upon coexposure to β-lactam antibiotics and Ag NPs

Article References: Lu, X., Zhang, Z., Huang, R., & Ge, Y. (2026). Physiological responses of Chlorella vulgaris upon coexposure to β-lactam antibiotics and Ag NPs. Ecotoxicology, 35(7), Article 163. https://doi.org/10.1007/s10646-026-03150-4

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03150-4

Keywords: silver nanoparticles, β-lactam antibiotics, amoxicillin, aztreonam, Chlorella vulgaris, combined toxicity, synergistic effects, ecotoxicology, freshwater algae, oxidative stress, ecological risk assessment, water pollution