Fig. 2.—

Structural composition of cyt b. The subunit can be divided in N- and C-terminal regions. Cyt b is encoded as single protein in mitochondria and most bacteria (left, pdb 2ibz). In cyt b₆f complexes, these two regions are encoded as two separate polypeptides (right, pdb 1q90) and are denoted cyt b₆ and SUIV. Transmembrane and relevant surface helices are labeled with upper and lower case letters, respectively. The Q_i site of cyt bc₁ complexes is occupied by the natural substrate ubiquinone-6. The Q_o site of both cyt bc₁ and cyt b₆f complexes are, respectively, indicated by their inhibitors, stigmatellin and tridecylstigmatellin, in hydrogen bonding distance to the Glu272 equivalent residue of the Q_o motif. The structural composition of cyt b is shown schematically in the lower panel. The four “H” denote the four strictly conserved histidine axial ligands of heme b_H and heme b_L, and the Q_o motif is labeled on the C-terminal region and SUIV. Cyt b₆f complexes possess a heme c_i at the Q_i site, which is absent in cyt bc₁ complexes.