Metalloendopeptidase of the mitochondrial matrix that functions in peptide cleavage and degradation rather than in protein processing. Has an ATP-independent activity. Specifically cleaves peptides in the range of 5 to 65 residues. Shows a preference for cleavage after small polar residues and before basic residues, but without any positional preference. Degrades the transit peptides of mitochondrial proteins after their cleavage. Also degrades other unstructured peptides. Binds 1 zinc ion per subunit. Mainly exists in a closed and catalytically competent conformation but a closed-to-open switch allows substrate entry into the catalytic chamber. Substrate binding induces closure and dimerization. A disulfide bond may lock the enzyme in a closed conformation preventing substrate entry into the catalytic chamber, participating in redox regulation of the enzyme. Inhibited by metal-chelating agents. Inhibited by nickel and zinc excess, and slightly activated by manganese. Monomer and homodimer; homodimerization is induced by binding of the substrate. A disulfide bond locks the enzyme in the closed conformation preventing substrate entry into the catalytic chamber. Belongs to the peptidase M16 family. PreP subfamily.