Pepsin is a proteolytic enzyme (protease) that is active in an acidic medium. It is secreted as its inactive precursor, pepsinogen, by the chief cells of the gastric glands in the stomach lining. Hydrochloric acid (HCl), produced by the parietal (oxyntic) cells of the stomach, converts pepsinogen into active pepsin and maintains the stomach contents at a pH of approximately 1.5 to 2.5. Pepsin functions optimally within this strongly acidic pH range, where it breaks down proteins into shorter polypeptide chains.
In an alkaline medium, pepsin is denatured and loses its catalytic activity. It does not function in a neutral medium either. The statement that pepsin works in both alkaline and acidic media is incorrect because the enzyme's three-dimensional structure, which is essential for substrate binding, is disrupted outside its narrow acidic pH range.
Each digestive enzyme has a specific optimum pH: pepsin requires acid (stomach), while enzymes like trypsin and pancreatic lipase require an alkaline environment (small intestine, pH approximately 7.5 to 8.5).
