<p>Nature Catalysis, Published online: 26 March 2026; <a href="https://www.nature.com/articles/s41929-026-01498-8">doi:10.1038/s41929-026-01498-8</a></p>Biological nitrogen fixation is vital for sustainable agriculture, yet nitrogenase engineering is hindered by limited insight into their essential metallocluster cofactor assembly and transfer. Here, by capturing the nitrogenase biosynthetic component NifEN in multiple structural states, a tunnel-and-switch mechanism that coordinates receipt, maturation and delivery of the FeMo-cofactor precursor is revealed.