Digestion of proteins

• Proteins are large polymer molecules that can be hydrolysed by three enzymes:

  • Endopeptidases
  • Exopeptidases
  • Membrane-bound dipeptidases
• Protein digestion happens in the small intestine and the stomach
• Peptidases or proteases are enzymes which breakdown proteins in a series of hydrolysis reactions
• Endopeptidases - Hydrolyse polypeptide bonds within a large protein (polypeptide) to create smaller polypeptide chains
• Exopeptidases - Hydrolyse terminal peptide bonds (between amino acids on the ends of a polypeptide chain) to remove individual amino acids and create smaller polypeptide chains
• Dipeptidases:

  • A type of exopeptidase
  • These are often located on the membrane of epithelial cells lining the small intestine
• Dipeptidases:

  They hydrolyse peptide bonds between two amino acids

  • This creates two single amino acids which can then be transported through membrane into the epithelial cell
• There are three different types of protease enzyme that work on breaking bonds in different sections of proteins:

  • The internal bonds (endopeptidases)
  • The terminal bonds (exopeptidases)
  • Single peptide bonds holding dipeptides together (dipeptidases)
• Digestion of proteins: STOMACH

  1. Whole proteins are chewed and swallowed into the stomach
  2. Hydrochloric acid denatures proteins, unfolding their 3D structure to reveal the polypeptide chain
  3. Enzymatic digestion by pepsin forms shorter polypeptides
• Digestion of proteins: SMALL INTESTINE

  4. In the small intestine, trypsin, chymotrypsin, and proteases continue enzymatic digestion, forming tripeptides, dipeptides and amino acids

  5. In enterocytes, tripeptides and dipeptides are further broken down into amino acids, which are absorbed into the blood
• Digestion of proteins:

  1. Hydrochloric acid in the stomach creates the optimum pH for the enzyme pepsin which is secreted by cells which line the stomach
  2. Other peptidases are made and secreted by the pancreas into the small intestine e.g trypsin
  3. Other peptidases are membrane-bound and found within cells that line the small intestine e.g dipeptidases
  4. The single amino acids are then absorbed into the bloodstream through the ileum epithelium
• Protein digestion:

  1. Starts in the stomach
  2. Continues in the duodenum
  3. Fully digested in the ileum