Nitrosomonas europaea is an aerobic, obligate chemolithoautotroph. It uses the oxidation of ammonia to nitrite to provide its sole source of reducing equivalents for molecular biosynthesis, as well as generating a proton gradient for energy transduction. This nitrification process accounts for the majority of ammonia oxidation in the biosphere, with the second step being catalyzed by hydroxylamine oxidoreductase (HAO). HAO is a homo trimer containing 21 c-type hemes and three catalytic heme P460s. Heme P460 is unique in biology as the only known heme to withdraw electrons from substrate. N. europaea also contains a soluble 19kD periplasmic mono-heme P460 cytochrome (cyt P460), whose physiological function is still unclear. It has a weak hydroxylamine oxidation/cyt-c reduction activity (about 3% that of HAO). Heme P460 is unusual among c-type hemes in that it contains three covalent links to the protein scaffold. Two are to cysteine residues in the CXXCH motif, typical for cytochromes c. In HAO the third linkage is between a porphyrin meso carbon and a tyrosine. Mass spectrometry, mutation studies and optical spectroscopy indicate that in cyt P460 the third bond is to a lysine residue, K70. Mutation of K70 leads to the heme in cyt P460 becoming equivalent to that of a cytochrome c'. In this presentation, the structure of N. europaea cyt P460 is described to 1.7Å resolution.