Final answer:
Hydrochloric acid (HCl) denatures proteins in the stomach, facilitating their subsequent digestion by the enzyme pepsin, which breaks down proteins into smaller peptides.
Step-by-step explanation:
The substance that denatures protein in the stomach during digestion is hydrochloric acid (HCl). HCl is secreted by the parietal cells in the stomach lining and is essential in the process of protein digestion. It unfolds protein molecules, making them more accessible to enzymatic action. The principal digestive enzyme in the stomach is pepsin. Pepsinogen, an inactive form of pepsin, is converted to its active form when it comes into contact with the acidic environment of the stomach. Pepsin accelerates protein digestion by cleaving the peptide bonds within protein molecules, resulting in smaller chains known as peptides tryptophan, tyrosine, phenylalanine, methionine, and leucine. These peptides are then further broken down into free amino acids by other proteases in the small intestine.