Protein structure of the complex III catalytic centre (the cytochrome bc1 complex) with annotated locations for the variants in cytochrome B. (A) The catalytic centre of complex III is shown, represented by three interacting subunits, cytochrome B (yellow), cytochrome C1 (blue) and the Rieske iron-sulfur protein (green). The key redox cofactors are Heme bL and bH, which reside within cytochrome B, and heme c1 in cytochrome C1. Oxidation of ubihydroquinone (QH2) occurs at the Qo site, and reduction of ubiquinone to ubiquinol at the Qi site. Following these events, a membrane potential is generated across the inner mitochondrial membrane. The Rieske iron-sulfur protein interconnects the cytochrome B and C1 subunits. (B) Location of the proline to leucine (P173L) and threonine to serine (T174S) mutations are highlighted on cytochrome B. Both of these reside on a transmembrane helical region of the subunit. (C) Magnified view of the adjacent P173L and T174S mutations.