A crystal structure of the enzyme nitrogenase, which some microorganisms use to turn nitrogen into useful ammonia, has at last been captured with a substrate analog bound to its catalytic metal cluster. The structural snapshot reveals that CO can displace a S atom from the enzyme’s FeMo cofactor, a cagelike assembly of Fe, S, C, and Mo, reports a team led by Thomas Spatzal and Douglas C. Rees of Caltech (Science 2014, DOI: 10.1126/science.1256679). Researchers have struggled to unravel how nitrogenase does its N2-reduction chemistry, which could point to better ways to make NH3 than the energy-intensive Haber-Bosch process. CO is isoelectronic with N2 and is a reversible nitrogenase inhibitor. The molecule’s binding to two iron atoms in the cofactor resembles N2 binding on the surface of the iron catalyst used in the Haber-Bosch process, the researchers note.