Cytochrome c552 (cytochrome c nitrite reductase) is a crucial enzyme in the nitrogen cycle catalysing the reduction of nitrite to ammonia. The crystal structure of cytochrome c552 reveals it to be a dimer, with with 10 close-packed type c haem groups.

1: Structure of cytochrome c nitrite reductase. Einsle O, Messerschmidt A, Stach P, Bourenkov GP, Bartunik HD, Huber R, Kroneck PM; Nature 1999;400:476-480. PMID:10440380