Where is most of the digestion of proteins completed and which enzymes are involved?

• A. In the stomach; pepsin
• B. In the small intestine; pancreatic proteases and brush-border enzymes
• C. In the mouth; salivary amylase
• D. In the large intestine; bacterial enzymes

Answer: (B)

Proteins are mainly digested in the small intestine. After the stomach’s pepsin begins breaking some peptide bonds, most of the work happens once chyme reaches the small intestine, where pancreatic proteases—trypsin, chymotrypsin, and carboxypeptidases—are released into the lumen and break proteins into smaller peptides and amino acids. The final steps are completed by brush-border enzymes on the enterocytes, such as aminopeptidases and dipeptidases, which finish trimming peptides into free amino acids ready for absorption. The mouth’s salivary enzyme targets carbohydrates, not proteins, and the large intestine has little to no active proteolysis, so it’s not the primary site of protein digestion.