Cytochrome c_(554)

Cytochrome c_(554) (cyt c_(554)) is a tetra-heme c-type cytochrome that participates in the nitrification pathway of Nitrosomonas europaea. In this process, cyt c_(554) functions as the electron acceptor from the enzyme hydroxylamine oxidoreductase that catalyzes the oxidation of hydroxylamine to nitrite. Cyt c_(554) is 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 nearly parallel and overlapping at the edges. This type of heme-stacking has been observed in two other nitrogen cycle proteins: hydroxylamine oxidoreductase and cytochrome c nitrite reductase, an enzyme that catalyzes the reduction of nitrite to ammonia. The relatively unusual spectral properties of cyt c_(554) may reflect interactions between these pairs of stacked hemes.