A22 aspartic endopeptidases belong to MEROPS family A22, which are membrane-inserted endopeptidases. They catalyse the hydrolysis of internal, alpha-peptide bonds in a polypeptide chain by a mechanism in which a water molecule bound by the side chains of aspartic residues at the active center acts as a nucleophile. The family includes presenilins and signal peptide peptidases. (Adapted from PMID:12966028.)