Structure of the guanidinase enzyme of the comammox species Nitrospira inopinata, which converts guanidine into urea. The presumed entrance to a tunnel leading to the active site is highlighted in the left image. In the right section, the tunnel is shown as a green line and guanidine as a stick model. The structure was elucidated by the team led by Kristina Djinović-Carugo.