The napB gene encodes a dihaem cytochrome c, the small subunit of a heterodimeric periplasmic nitrate reductase. The NapB protein is essential in transferring electrons to the large catalytic subunit NapA, which subsequently reduces nitrate to nitrite. [].
In proteins belonging to the c-type cytochrome family [], the haem group is covalently attached by thioether bonds to two conserved cysteine residues located in the cytochrome c centre. Cytochromes c typically function in electron transfer, but c-type cytochrome centres are also found in the active sites of many enzymes.This domain contains multiple CxxCH motifs. There are a variable number of helices, as well as a little beta structure present in multihaem cytochromes, but they do not form a true structural fold. Cytochrome (cyt) c3-like proteins are multihaem cytochromes, including cyt c3 with four haem groups [], cyt c7 (cyt c551.5) with three haem groups (deletion of one cyt c3 haem-binding site), nine-haem cyt c (tandem repeat of two cyt c3-like domains with an additional haem-binding site), and 16-haem cyt c HmcA (tandem repeat of four cyt c3-like domains). The photosynthetic reaction centre composed of a cytochrome subunit is also a multihaem cytochrome []. In addition, the di-haem elbow motif shows a similar structure, the main characteristic feature of this motif being the packing of its two haems; many members of this group of proteins contain one or more complete motifs flanked by incomplete motifs and/or other domains. For example, the di-haem elbow motif is present in the multihaem cytochrome domain found in the periplasmic nitrate reductase subunit NapB [], in hydroxylamine oxioreductase HAO (contains 3 complete motifs), in cyt c554 (contains one complete motif), in cyt c nitrite reductase, and in the N-terminal domain of flavocytochrome c3 (respiratory fumarate reductase).
