Final answer:
Pepsin, secreted as pepsinogen by chief cells, is the enzyme in the stomach responsible for beginning the digestion of proteins into smaller peptides in the acidic environment caused by hydrochloric acid.
Step-by-step explanation:
The enzyme secreted by the gastric glands in the stomach that acts on proteins is called pepsin. Pepsin is initially secreted as an inactive enzyme known as pepsinogen by the chief cells located in the stomach wall. The acidic environment, with a pH of about 1.5 to 2.5, provided by hydrochloric acid (HCl) secreted by parietal cells, activates pepsinogen to pepsin. Pepsin then begins the process of protein digestion by cleaving the protein molecules into smaller polypeptides and peptides. This process is crucial for the absorption of amino acids later in the small intestine.