Fig. 1. Structure and function of heterodimeric cytochromes bc₁.

A. A hypothetical structural model for a non-native heterodimeric cytochrome b-bc₁. The 3D structure of cytochrome bc₁ (PDB entry 1ZRT) was modified to include the Q_i site H212N mutation (blue) on one monomer (M1) and the Q_o site G158W mutation (red) on the other (M2) to depict a heterodimeric enzyme produced by the one-plasmid system [29]. The carboxyl end of one cytochrome b (blue) is linked to the amino end of the other (yellow) with a peptide (green) indicated by an arrow to create cytochrome b-b that is not naturally present in cytochrome bc₁. S refers to the Strep–tag on the carboxyl terminal end of cytochrome b-b. The Fe/S and cytochrome c₁ subunits are shown in light blue and light brown, with the cofactors ([2Fe2S] and hemes b_H, b_L and c₁) in monomer M1 in blue and those in M2 in red. The black arrows indicate the electron transfer pathways across the monomers, Q and QH₂ correspond to quinone and hydroquinone, respectively. The black ellipsoid indicates the absence of heme b_H on monomer M1. B. A hypothetical structural model for a native-like heterodimeric cytochrome bc₁. The 3D structure of cytochrome bc₁ (PDB entry 1ZRT) was modified to include the Q_i site H212N mutation (blue) on one monomer (M1) and the Q_o site Y147A mutation (red) on the other (M2) to depict a heterodimeric enzyme produced by the two-plasmids system [32]. The labels are as in A, and note the absence (indicated by an arrow) of the peptide linking together the cytochromes b, and the presence of Flag- (F) and Strep (S)-tags on the carboxyl terminals of two cytochrome b on monomers M1 and M2, respectively.