Complex III2 is an enzyme complex embedded in the inner membrane (light gray band) that separates the matrix (top) and the inter-membrane space (IMS; bottom) in mitochondria. One of the reactions it catalyzes is an oxidoreduction, where two molecules of ubiquinol (QH2) are sequentially converted into ubiquinone (Q), two pairs of protons are released into the mitochondrial intermembrane space (IMS), and two pairs of electrons are transferred onto complex III2. Two of these electrons are sequentially transferred onto cytochrome c (cyt c), while the other two electrons are used to convert one molecule ubiquinone into ubiquinol. The latter step is linked to the uptake of two protons from the mitochondrial matrix. The overall reaction results in protons being translocated across the inner mitochondrial membrane, which is a key process in cellular respiration. However, in plants, complex III2 is also involved in another reaction: it helps mitochondrial preproteins mature by removing the import sequence necessary for their movement into the compartment (top).
Image credit: Shape of the complex III2 is adapted from Maldonado et al., 2021.