The breakdown of dietary protein begins in the stomach where gastric hydrochloric acid (HCl) assumes an important role in protein digestion and other processes. Hydrochloric acid acts as a catalyst in the activation of pepsin from pepsinogen. Active pepsin is a proteolytic enzyme which splits large protein molecules into smaller polypeptides. Additionally, hydrochloric acid acts directly on dietary proteins by loosening up their macromolecular structure (denaturing). Gastric hydrochloric acid also helps reduce poorly available trivalent ferric iron into more soluble and absorbable divalent ferrous iron.