Pepsin

Gastric juice enzyme (EC 3.4.23.1), which breaks down proteins. It is a phosphoprotein and the main stomach protease. It is an endopeptidase. It hydrolyzes those peptide bonds formed by aromatic amino acids (tyrosine, phenylalanine, tryptophan) amino groups and dicarboxylic amino acids (glutamine). The optimal pH for pepsin is 1.5–2. The enzyme is stable only in an acidic environment. Pepsin is formed from its precursor pepsinogen, which is secreted by the chief cells of the stomach. Human pepsinogen consists of 388 amino acid residues. Unlike pepsin, it is stable in neutral and weakly alkaline solutions. Its activation begins at a pH of about 5.4 and reaches a maximum at pH 2. Pepsin exhibits autolysis. During activation, several peptides are cleaved from pepsinogen, one of which is pepsin inhibitor, consisting of 34 amino acids, and has alkaline properties. Once this peptide is cleaved, pepsin acquires acidic properties. The isoelectric point is pI<1. The pH of pure gastric juice is 1.0–2.0. The despiralization of molecule parts is important for pepsinogen activation. Active pepsin has an unfolded polypeptide chain. The molecular weight of pepsin is about 34,000–37,000 (pepsinogen – 40,000). Pepsin breaks down almost all natural proteins, except certain keratins, protamines, histones, mucoproteins (mucins and mucoids). When catalyzed by pepsin, small peptides are formed in the stomach, not amino acids.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė