Where does the breakdown of proteins by protease begin?

The breakdown of proteins by protease begins in the stomach. This is primarily due to the action of an enzyme called pepsin, which is secreted as a precursor molecule called pepsinogen by the stomach lining. Once pepsinogen is exposed to the acidic environment of the stomach, it is activated into pepsin. Pepsin is specifically designed to target peptide bonds in proteins, breaking them down into smaller peptides.

The acidity of the stomach, created by hydrochloric acid, not only activates pepsin but also helps to denature proteins, making them more accessible to enzymatic action. Although the small intestine is involved in further protein digestion and absorption, the initial breakdown initiated by proteases specifically occurs in the stomach, making it the correct answer.