Structural insights into urocanate reductase using room-temperature X-ray crystallography

Urocanate reductase (UrdA) is a bacterial enzyme that converts urocanic acid to imidazole propionate. Its catalytic residue Arg411 undergoes a large conformational change in the substrate-bound versus product-bound states. In contrast to previously studied cryo-conditions, the room-temperature X-ray data of UrdA presented here show that the occupancy distribution of Arg411 is affected by crystal cryocooling. We further provide evidence that a phosphate ion stabilizes the substrate complex and can bias the conformation of Arg411. Both room-temperature and cryogenic X-ray datasets were essential to elucidate the dynamic nature of the UrdA active site, highlighting the importance of collecting data at both temperatures, as each may reveal distinct conformational states.