We have isolated a soluble cytochrome from Shewanella oneidensis that contains eight covalently attached heme groups and determined its crystal structure. One of these hemes exhibits novel ligation of the iron atom by the epsilon-amino group of a lysine residue, despite its attachment via a typical CXXCH motif. This heme is most likely the active site for tetrathionate reduction, a reaction catalyzed efficiently by this enzyme.