Protein

• Gastric juice contains hydrochloric acid (HCl) which lowers pH to around 2-3, creating an environment that denatures proteins and activates enzymes such as pepsin.
• Pepsinogen is activated into pepsin when it comes into contact with H+ ions from the gastric juices.
• Pepsin breaks down protein molecules into smaller peptides.
• Pepsinogen is activated into pepsin when it comes into contact with H+ ions from HCL.
• Pepsin breaks down proteins into peptides.
• Pepsin is made in the stomach and trypsin is made in the pancreas, however they both digest proteins.
• Peptidases/proteases are enzymes which break down proteins in a series of hydrolysis reactions.
• Hydrochloric acid in the stomach creates the optimum pH conditions for the protein-digesting enzyme pepsin, which is secreted by cells in the stomach lining.
• Other peptidases than pepsin are made and secreted by the pancreas into the duodenum (e.g. trypsin).
• Other peptidases than those found in the stomach and duodenum are membrane-bound and found within cells that line the ileum (e.g. dipeptidases).
• When proteins are digested the singular amino acids are absorbed into the bloodstream through the ileum epithelium.
• Excess amino acids can be used to make new proteins or stored as glycogen.
• Digestion of proteins:

  • Takes place in the stomach and duodenum of the small intestine
  • HCl in the stomach maintains an acidic pH for pepsin action
  • Endopeptidases break the peptide bond in the middle of the peptide chain
  • Exopeptidases act at the end of the peptide chain and help in releasing the last amino acid
• Protein Absorption:

  • Active transport in the small intestine, mostly at the duodenum and jejunum
  • Co-transport of di and tripeptides is possible with H+ ions via the PepT1 transporter
• Endopeptidases hydrolyse the peptide bonds between amino acids in the central region of a protein molecule, forming a series of peptide molecules.
• Exopeptidases hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. In this way they progressively release dipeptides and single amino acids.
• Dipeptidases hydrolyse the bond between the two amino acids on a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.
• Protein digestion begins in the stomach.
• Describe the action of membrane-bound dipeptidases and explain their importance.

  1. Hydrolyse peptide bonds to release amino acids;
  2. Amino acids can cross cell membrane;