Pepsin-like aspartic proteases, bilobal enzymes that cleave bonds in peptides at acidic pH.Pepsin-like aspartic proteases are found in mammals, plants, fungi and bacteria. These well known and extensively characterized enzymes include pepsins, chymosin, renin, cathepsins, and fungal aspartic proteases. Several have long been known to be medically (renin, cathepsin D and E, pepsin) or commercially (chymosin) important. Structurally, aspartic proteases are bilobal enzymes, each lobe contributing a catalytic Aspartate residue, with an extended active site cleft localized between the two lobes of the molecule. The N- and C-terminal domains, although structurally related by a 2-fold axis, have only limited sequence homology except the vicinity of the active site. This suggests that the enzymes evolved by an ancient duplication event. Most members of the pepsin family specifically cleave bonds in peptides that are at least six residues in length, with hydrophobic residues in both the P1 and P1' positions. The active site is located at the groove formed by the two lobes, with an extended loop projecting over the cleft to form an 11-residue flap, which encloses substrates and inhibitors in the active site. Specificity is determined by nearest-neighbor hydrophobic residues surrounding the catalytic aspartates, and by three residues in the flap.The enzymes are mostly secreted from cells as inactive proenzymes that activate autocatalytically at acidic pH. This family of aspartate proteases is classified by MEROPS as the peptidase family A1 (pepsin A, clan AA).