Molecular Movie Captures Penicillin Being Built Atom by Atom

More than eighty years after scientists at the University of Oxford transformed penicillin from a curious mold extract into a medicine that saved countless lives during the Second World War, researchers have finally filmed the invisible drama at the heart of its creation. In a study published in Nature Catalysis, a team led by the University of Oxford, working with collaborators at Diamond Light Source, Lawrence Berkeley National Laboratory, PAL-XFEL and SLAC National Accelerator Laboratory, has captured the fleeting chemical stages through which nature constructs the defining feature of penicillin: the β-lactam ring. Using X-ray free-electron lasers, the researchers recorded a frame-by-frame molecular movie of the enzyme isopenicillin N synthase, or IPNS, as it converts a simple linear peptide into the strained ring system that makes these antibiotics lethal to bacteria. The work resolves a mechanistic puzzle that has stood open for more than four decades, and it arrives at a moment when the world urgently needs new ideas for antibiotic design.

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