Question

Which of the following statements about the digestion of proteins is correct?

• A. Protein digestion begins in the small intestine
• B. Protein digestion begins in the stomach
• C. Protein digestion begins when the hydrochloric acid first hydrolyses the peptide bonds
• D. Protein digestion begins when trypsinogen has been activated to trypsin by hydrochloric acid

Answer 1

The correct option is B Protein digestion begins in the stomach

Dietary proteins are first denatured by the hydrochloric acid secreted by the parietal cells of gastric glands (the weak non-covalent interactions that hold proteins in their specific 3-D configuration are disrupted exposing many peptide bonds that were initially shielded inside the protein). The cells of the stomach epithelium secrete pepsinogen (stimulated by the hormone gastrin) which is activated to pepsin initially by the hydrochloric acid, then by autocatalysis by pepsin itself. Pepsin then begins the specific partial hydrolysis of the denatured proteins in the stomach where the pH of the environment is 1-2 (optimum pH of pepsin). Pepsin is an endopeptidase that only hydrolyses those peptide bonds in which an aromatic amino acid supplies the NH of the peptide bond.
Trypsin is synthesised in the pancreas as trypsinogen (together with other proenzymes) and released into the small intestine. It is not present in the stomach.