Catalytic subunit of the essential mitochondrial processing protease (MPP), which cleaves the mitochondrial sequence off newly imported precursors proteins (By similarity). Preferentially, cleaves after an arginine at position P2 (By similarity). Component of the ubiquinol-cytochrome c oxidoreductase, a multisubunit transmembrane complex that is part of the mitochondrial electron transport chain which drives oxidative phosphorylation. The respiratory chain contains 3 multisubunit complexes succinate dehydrogenase (complex II, CII), ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII) and cytochrome c oxidase (complex IV, CIV), that cooperate to transfer electrons derived from NADH and succinate to molecular oxygen, creating an electrochemical gradient over the inner membrane that drives transmembrane transport and the ATP synthase. The cytochrome b-c1 complex catalyzes electron transfer from ubiquinol to cytochrome c, linking this redox reaction to translocation of protons across the mitochondrial inner membrane, with protons being carried across the membrane as hydrogens on the quinol. In the process called Q cycle, 2 protons are consumed from the matrix, 4 protons are released into the intermembrane space and 2 electrons are passed to cytochrome c. Release of N-terminal transit peptides from precursor proteins imported into the mitochondrion, typically with Arg in position P2. Binds 1 zinc ion per subunit. Binding to the alpha subunit is required for catalytic activity. Heterodimer of an alpha subunit and a beta subunit subunits, forming the mitochondrial processing protease (MPP) in which the alpha subunit is involved in substrate recognition and binding and the beta subunit is the catalytic subunit (By similarity). Component of the ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII), a multisubunit enzyme composed of 10 subunits. The complex is composed of 3 respiratory subunits cytochrome b (MT-CYB), cytochrome c1 (CYC1-1 or CYC1-2) and Rieske protein (UCR1-1 or UCR1-2), 2 core protein subunits MPPalpha1 (or MPPalpha2) and MPPB, and 5 low-molecular weight protein subunits QCR7-1 (or QCR7-2), UCRQ-1 (or UCRQ-2), QCR9, UCRY and probably QCR6-1 (or QCR6-2) (PubMed:18189341, PubMed:18305213). The complex exists as an obligatory dimer and forms supercomplexes (SCs) in the inner mitochondrial membrane with NADH-ubiquinone oxidoreductase (complex I, CI), resulting in different assemblies (supercomplexes SCI(1)III(2) and SCI(2)III(4)) (PubMed:12970493). May be due to a competing donor splice site. Belongs to the peptidase M16 family.