Helicobacter mustelae, a gastric pathogen of ferrets, synthesizes a distinct iron-dependent urease in addition to its archetypical nickel-containing enzyme. The iron-urease is oxygen-labile, with the inactive protein exhibiting a methemerythrin-like electronic spectrum. Significantly, incubation of the oxidized protein with dithionite under anaerobic conditions leads to restoration of activity and bleaching of the spectrum. Structural analysis of the oxidized species reveals a dinuclear iron metallocenter bridged by a lysine carbamate, closely resembling the traditional nickelurease active site. Although the iron-urease is less active than the nickel-enzyme, its activity allows H. mustelae to survive the carnivore's low-nickel gastric environment.diiron | hydrolase | metalloprotein | isoenzyme | enzyme activation