Pepsin A, aspartic protease produced in gastric mucosa of mammals.
Pepsin, a well-known aspartic protease, is produced by the human gastric mucosa in seven different zymogen isoforms, subdivided into two types: pepsinogen A and pepsinogen C. The prosequence of the zymogens are self cleaved under acidic pH. The mature enzymes are called pepsin A and pepsin C, correspondingly. The well researched porcine pepsin is also in this pepsin A family. Pepsins play an integral role in the digestion process of vertebrates. Pepsins are bilobal enzymes, each lobe contributing a catalytic Asp residue, with an extended active site cleft localized between the two lobes of the molecule. One lobe may be evolved from the other through ancient gene-duplication event. More recently evolved enzymes have similar three-dimensional structures, however their amino acid sequences are more divergent except for the conserved catalytic site motif. Pepsins specifically cleave bonds in peptides which have 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. This family of aspartate proteases is classified by MEROPS as the peptidase family A1 (pepsin A, clan AA).