Molecular movie of penicillin biosynthesis. X-ray free electron laser experiments on isopenicillin N synthase microcrystals capture structural snapshots of key intermediates during the penicillin forming reaction. The film strip illustrates progression through the catalytic cycle, with the monocyclic β-lactam intermediate and reactive high valent ferryl iron species highlighted at the center of the molecular movie. [University of Oxford/Greg Stewart/SLAC National Accelerator Laboratory]
Researchers from the University of Oxford and international collaborators have revealed previously unseen rapid chemical stages in the formation of β-lactam antibiotics such as penicillin, offering insights that could help to support future antibiotic development.
Rather than relying on static X-ray crystallographic structures of the enzyme, the Oxford University team, collaborating with partners from Diamond Light Source, Lawrence Berkeley National Laboratory, PAL-XFEL, and SLAC National Accelerator Laboratory, followed the reaction in real time. To do this they used X-ray free-electron lasers (XFEL) to observe the enzyme isopenicillin N synthase, (IPNS) as it converts its linear peptide substrate into the β-lactam ring system of penicillin.
The study brought together expertise in structural biology, spectroscopy, chemistry, computation and enzyme engineering. The approach, they say, provides a powerful way to study fleeting chemical intermediates that have previously been inaccessible, both in antibiotic biosynthesis and in other biologically important enzyme reactions.
The team captured several rapid stages in the reaction. These included a thioaldehyde intermediate formed just before the β-lactam ring is created, and a monocyclic β-lactam intermediate, representing the first ring-shaped structure on the way to forming the complete penicillin scaffold. These steps provide the clearest picture yet of how the IPNS enzyme assembles the penicillin scaffold. Subtle movements throughout the enzyme help guide these chemical steps, showing that both the enzyme’s shape and its chemistry work together to control penicillin formation.
The results show, in unprecedented detail, how the enzyme IPNS achieves an exceptionally complex transformation in a single step. Christopher Schofield, PhD, Professor of Chemistry, University of Oxford, said, “Penicillin has shaped modern medicine, but there is still much to learn about how nature builds this important antibiotic structure. By capturing these fleeting steps, we can better understand how enzymes control complex chemistry with remarkable precision … As rates of antimicrobial resistance continue to rise, understanding this process will ultimately help us make existing antibiotics more efficient and design new antibiotic structures.”
Allen M Orville, PhD, group leader of the XFEL Hub, further commented, “The XFEL Hub at Diamond brought specialist expertise in time-resolved X-ray methods and XFEL-based structural biology to this study. Working closely with our colleagues in Oxford and at partner facilities, we helped bring together the experimental approaches needed to capture these fleeting stages of enzyme catalysis. The results show the power of combining complementary expertise and X-ray techniques to move beyond static structures and reveal how enzymes work in real time. It is a great example of how time-resolved structural biology can uncover new principles of enzyme function and, ultimately, inform the design of new catalysts and therapeutics.”
Schofield is co-senior author of the researchers’ published paper in Nature Catalysis, titled “Unanticipated intermediates during isopenicillin N synthase catalysis identified by time-resolved X-ray free-electron laser studies.”
Ever since penicillin was developed into a working drug at the University of Oxford in the early 1940s, β-lactam antibiotics have been among the most important medicines for treating bacterial infections. Their activity depends on a special ring—the β-lactam ring—a highly strained chemical ring system that interferes with bacterial cell wall synthesis, ultimately causing the cell wall to fail and the bacteria to die. “Isopenicillin N synthase (IPNS) catalyses the biosynthesis of isopenicillin N (IPN), forming the fused four-membered β-lactam and five-membered thiazolidine ring system that is characteristic of the penicillins,” the authors explained.
Rising rates of antimicrobial resistance are undermining the effectiveness of existing antibiotics. Today, one in six bacterial infections is resistant to antibiotics, the authors noted, citing WHO figures. With too few new antibiotics in development, understanding how nature builds these molecules is an important step towards replenishing the antibiotic pipeline.
Scientists have been studying how nature constructs this β-lactam ring for decades, but the key fleeting reaction intermediates have been too difficult to observe directly.
To capture the reaction in real time, the researchers used a system in which thousands of tiny droplets containing anaerobic enzyme microcrystals were deposited onto a moving 2 mm wide tape. As the tape entered an oxygen filled chamber, oxygen rapidly diffused into the crystals and initiated the reaction simultaneously across the sample. By precisely controlling the speed of the tape, the researchers could determine how long each crystal reacted before reaching the X-ray interaction point, where an ultrafast XFEL pulse recorded an atomic resolution snapshot.
Combining thousands of these snapshots allowed the team to build a frame-by-frame “molecular movie” of penicillin biosynthesis. This method allows individual reaction intermediates that exist only for tiny fractions of a second to be observed at atomic resolution, and under physiological temperature and pressure, before they disappear.
The newly reported findings help to resolve a long-standing mechanistic question that has remained unanswered for more than four decades. The work also showed that water molecules inside the enzyme play a key role in guiding the reaction. The combined results of their studies, they noted, “… reveal unexpected insight into the IPNS reaction mechanism including the vital roles of active-site water molecules.”
The study ultimately builds on the work of Nobel prize-winning Oxford chemist Dorothy Hodgkin, PhD, who first solved the structure of penicillin in 1945 using X-ray crystallography.
Beyond penicillin, IPNS belongs to a large family of iron-dependent oxygenase enzymes involved in human biology, including those that enable us to sense and respond to changes in oxygen availability. The mechanistic insights from this work could therefore have implications beyond antibiotics, offering new principles for enzyme engineering and catalyst design.
“These findings provide structural insight into IPNS catalysis, inform on the wider Fe(II) oxygenase superfamily and highlight the power of X-ray free-electron laser methods for resolving transient enzymatic intermediates,” the team stated in their paper. First author Patrick Rabe, PhD, Wellcome Career Development Award Investigator, University of Oxford, said, “These insights matter because they can inform how we think about antibiotic biosynthesis and future strategies for developing antibacterial medicines. The ability to capture structural snapshots over milliseconds to seconds of reaction time allows us to connect enzyme motion, iron chemistry and water-mediated proton transfer. This gives us a much richer view of how IPNS controls a difficult chemical transformation. By understanding this process in atomic detail, we can begin to think about engineering these enzymes to produce new or improved antibiotic scaffolds.”
