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). 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 mppA is required for catalytic activity. Heterodimer of mppA (alpha) and mppB (beta) subunits, forming the mitochondrial processing protease (MPP) in which mppA is involved in substrate recognition and binding and mppB is the catalytic subunit. Belongs to the peptidase M16 family.