Describe the digestion of protein by proteases in the digestive system: (a) pepsin breaks down protein in the acidic conditions of the stomach (b) trypsin breaks down protein in the alkaline conditions of the small intestine

The digestion of proteins in the digestive system is a complex process that involves several enzymes, primarily proteases. Here’s a detailed overview of how proteins are digested, focusing on the roles of pepsin and trypsin.

(a) Pepsin Breaks Down Protein in the Acidic Conditions of the Stomach

• Source: Pepsin is produced in the stomach as an inactive precursor called pepsinogen, which is secreted by the gastric glands. The presence of hydrochloric acid (HCl) in gastric juice activates pepsinogen into its active form, pepsin.
• Function:
  • Action in the Stomach: Pepsin functions optimally in the highly acidic environment of the stomach, where the pH ranges from 1.8 to 3.5. This acidity not only activates pepsin but also denatures proteins, unfolding their complex structures and making peptide bonds more accessible.
  • Mechanism: Pepsin cleaves peptide bonds, particularly those involving hydrophobic amino acids, breaking down proteins into smaller peptides and polypeptides. This initial digestion accounts for approximately 10-15% of total protein digestion and occurs primarily during the first hour after food enters the stomach.

(b) Trypsin Breaks Down Protein in the Alkaline Conditions of the Small Intestine

• Source: Trypsin is produced by the pancreas as an inactive precursor called trypsinogen. Once trypsinogen reaches the small intestine, it is activated by an enzyme called enterokinase, which is secreted by the intestinal mucosa.
• Function:
  • Action in the Small Intestine: Trypsin operates in the alkaline environment of the small intestine (pH around 7-8), where it continues the digestion of proteins that have been partially digested by pepsin.
  • Mechanism: Trypsin catalyzes the hydrolysis of peptide bonds, breaking down proteins into smaller peptides. It specifically cleaves at the carboxyl side of lysine and arginine residues, producing shorter peptide chains that can be further digested by other proteases present in pancreatic secretions, such as chymotrypsin and carboxypeptidase