Which enzyme catalyzes the first proteolytic step in protein digestion?

Which enzyme catalyzes the first proteolytic step in protein digestion?

• A. Pepsin
• B. Trypsin
• C. Chymotrypsin
• D. Lipase

Answer: (A)

Protein digestion begins in the stomach with an acid-activated protease that initiates breakdown of proteins by cleaving internal peptide bonds. This enzyme, pepsin, is secreted as pepsinogen and becomes active in the highly acidic gastric environment. Once active, it cleaves peptide bonds within proteins, especially near bulky hydrophobic amino acids, producing large peptide fragments to be further digested downstream. This early step sets the stage for continued proteolysis by pancreatic enzymes in the small intestine. The other enzymes listed don’t start proteolysis in the stomach: trypsin and chymotrypsin act later in the small intestine after activation, and lipase digests fats rather than proteins.

Protein digestion begins in the stomach with an acid-activated protease that initiates breakdown of proteins by cleaving internal peptide bonds. This enzyme, pepsin, is secreted as pepsinogen and becomes active in the highly acidic gastric environment. Once active, it cleaves peptide bonds within proteins, especially near bulky hydrophobic amino acids, producing large peptide fragments to be further digested downstream. This early step sets the stage for continued proteolysis by pancreatic enzymes in the small intestine. The other enzymes listed don’t start proteolysis in the stomach: trypsin and chymotrypsin act later in the small intestine after activation, and lipase digests fats rather than proteins.