How does hydrochloric acid (HCl) in the stomach facilitate proteolysis by pepsin?

HCl chemically cleaves polypeptide bonds directly into amino acids, and pepsin only functions to neutralize excess acid preventing over‑digestion.

HCl lowers gastric pH which unfolds protein structures exposing peptide bonds and triggers conversion of pepsinogen to pepsin, the active protease that cleaves peptide bonds to generate peptides.

HCl activates pancreatic enzymes that retrograde into the stomach where they hydrolyze proteins into individual amino acids without involvement of pepsin.

HCl serves mainly to sterilize ingested food while pepsinogen is converted to pepsin by intestinal alkaline secretions which then digest proteins back in the duodenum.