In the stomach, what effect does hydrochloric acid have on proteins?

• A. Denatures proteins and activates pepsin
• B. Neutralizes stomach acid
• C. Initiates protein absorption
• D. Hydrolyzes fats

Answer: (A)

Hydrochloric acid in the stomach creates a very acidic environment that unfolds proteins, denaturing their complex structures and exposing peptide bonds to enzymatic attack. This denaturation, together with the acidic pH, converts the inactive zymogen pepsinogen into active pepsin, the enzyme that begins breaking peptide bonds in proteins. As a result, protein digestion starts in the stomach rather than inactivation or absorption. It doesn’t neutralize stomach acid, initiate absorption, or hydrolyze fats—that role belongs to other processes and enzymes.