Heme packing motifs revealed by the crystal structure of the tetra-heme cytochrome c554 from Nitrosomonas europaea

Cytochrome c554 (cyt c554), a tetra-heme cytochrome from Nitrosomonas europaea , is an essential component in the biological nitrification pathway. In N. europaea , ammonia is converted to hydroxylamine, which is then oxidized to nitrite by hydroxylamine oxidoreductase (HAO). Cyt c554 functions in the latter process by accepting pairs of electrons from HAO and transferring them to a cytochrome acceptor. The crystal structure of cyt c554 at 2.6 Å resolution shows a predominantly α-helical protein with four covalently attached hemes. The four hemes are arranged in two pairs such that the planes of the porphyrin rings are almost parallel and overlapping at the edge; corresponding heme arrangements are observed in other multi-heme proteins. Striking structural similarities are evident between the tetra-heme core of cyt c554 and hemes 3–6 of HAO, which suggests an evolutionary relationship between these redox partners.