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.
Aggarwal et al. (Mon,) studied this question.