Digestion of Proteins
- Protein digestion begins in the stomach and continues in the small intestine.
Proteins in food are large, complex polymers that must first be broken down into amino acids and small peptides before absorption.
Stomach Phase (Initiation of Protein Digestion)
- Hydrochloric acid (HCl) from parietal cells:
- Denatures (unfolds) proteins → making peptide bonds more accessible.
- Converts inactive pepsinogen into active pepsin.
- Pepsin (an endopeptidase):
- Cleaves peptide bonds inside the protein chain.
- Preferentially hydrolyzes peptide bonds involving aromatic amino acids (Trp, Tyr, Phe).
- End result: formation of polypeptides + some free amino acids.
Small Intestine Phase (Major Digestion)
- When acidic chyme enters duodenum → Secretin and CCK are released.
- CCK stimulates pancreas to release inactive zymogens into intestine:
- Trypsinogen
- Chymotrypsinogen
- Proelastase
- Procarboxypeptidases A and B
- Enteropeptidase (brush border enzyme) converts trypsinogen → trypsin.
- Trypsin then activates all other proteolytic zymogens.
- Actions of Pancreatic Enzymes
- Trypsin and Chymotrypsin: Endopeptidases → cleave internal peptide bonds.
- Elastase: Breaks elastin and other small neutral amino acid-containing peptides.
- Carboxypeptidases A & B: Exopeptidases → remove amino acids from carboxyl (COOH) end of peptide chains.
- Brush Border Peptidases
- Present on intestinal epithelial cell membranes.
- Convert oligopeptides into dipeptides, tripeptides, and free amino acids.