Cytochrome b (N-terminus)/b6/petB: Cytochrome b is a subunit of cytochrome bc1, an 11-subunit mitochondrial respiratory enzyme. Cytochrome b spans the mitochondrial membrane with 8 transmembrane helices (A-H) in eukaryotes. In plants and cyanobacteria, cytochrome b6 is analogous to eukaryote cytochrome b, containing two chains: helices A-D are encoded by the petB gene and helices E-H are encoded by the petD gene in these organisms. Cytochrome b/b6 contains two bound hemes and two ubiquinol/ubiquinone binding sites. The C-terminal portion of cytochrome b is described in a separate CD.