Pepsiin

Pepsiin, commonly referred to as pepsin, is a digestive endopeptidase of the stomach belonging to the aspartic protease family. It is produced in the stomach as the inactive precursor pepsinogen by chief cells and is activated to pepsin when exposed to the highly acidic environment created by gastric hydrochloric acid. In its active form, pepsiin initiates the digestion of dietary proteins by cleaving peptide bonds, producing smaller peptides that are further degraded in the small intestine.

Pepsiin operates best at very low pH, with activity maximized around pH 1.5 to 2.0. It exhibits broad substrate specificity, preferring peptide bonds adjacent to hydrophobic amino acids. The enzyme's catalytic mechanism relies on a dyad of aspartic acid residues in the active site, enabling acid-base catalysis to hydrolyze peptide bonds. Pepsiin is relatively stable in acidic conditions but loses activity as the pH rises.

Structure and activation: Pepsiin is derived from a larger zymogen, pepsinogen, which is secreted into the gastric lumen. Proteolytic cleavage removes portions of pepsinogen to form the active single-chain enzyme. The enzyme can be inhibited by specific protease inhibitors such as pepstatin.

Occurrence and significance: Pepsiin is produced by the stomachs of many vertebrates and plays a central role in initiating protein digestion. Its activity complements other digestive proteases in the small intestine. In laboratory and industry, pepsiin is used as a model protease for studying enzyme function and protein hydrolysis under acidic conditions.