Carbapenem antibiotics kill diverse bacteria typically without the resistance problems that plague their fellow β-lactam antibiotics. A recent spike in bacterial resistance to carbapenems suggests that may not be true for much longer, however. To make new antibiotics that keep resistance at bay, researchers have been seeking to understand how carbapenems are made in nature. Now, chemists have determined how CarC, an enzyme from microbes that make carbapenems, inverts the stereochemistry at a chiral carbon center crucial for antibiotic activity (Science 2014, DOI: 10.1126/science.1248000). Previous efforts to characterize CarC have been thwarted by the instability of the substrate molecule that the enzyme acts on. Wei-Chen Chang, working in the joint lab of Carsten Krebs and J. Martin Bollinger Jr. at Pennsylvania State University, stabilized the substrate molecule enough to perform a combination of biophysical studies. The stabilization also made possible an X-ray structure, obtained in collaboration with Penn State’s Amie K. Boal and Northwestern University’s Amy C. Rosenzweig. The work indicates that tyrosine 165, an amino acid in the enzyme that’s not visible in prior structures, is essential in the enzyme’s antibiotic-producing mechanism. After an iron(IV)-oxo intermediate abstracts a hydrogen atom from the chiral center, tyrosine 165 donates a hydrogen atom to the opposite face of the substrate molecule, flipping the stereochemistry.