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Protein Digestion

Notes

Protein Digestion

Now, let's draw out the pathways of protein digestion and absorption.

  • Draw a section of small intestine.
  • From inside to out, specify the intestinal lumen and the intestinal villus.
  • Within the villus, draw an enterocyte with its characteristic brush border (the absorptive microvilli) on the apical surface and then adjacent to the basolateral surface of the enterocyte, indicate interstitial fluid and draw a blood vessel.
  • Within the intestinal lumen, draw a protein – a polypeptide chain of > 50 amino acids.
  • Then, show a prototypical pancreatic peptidase, which cleaves (digests the protein) into oligopeptides (chains of several amino acids) and tri- and dipeptides or single amino acids.
  • Next, show that brush border peptidases line the apical surface.
  • Indicate that oligopeptides are cleaved into smaller oligopeptides, which can:
    • Enter the enterocyte via oligopeptide transporters OR
    • Be further cleaved into tri- or dipeptides or amino acids, which enter via specific transporters.
  • Next, show that cytosolic peptidases (proteases inside the enterocytes) further digest the peptides into amino acids.
  • Finally show that all the aforementioned products pass through transporters on the basolateral aspect of the enterocyte, through the interstitial fluid, and into blood vessels for systemic distribution.
    • Again, mostly it's amino acids (rather than peptide chains) that enter the blood stream.

Finally, let's create some general rules to consolidate the numerous peptide and amino acid transporters.

  • Indicate that, generally, oligopeptides enter the apical aspect of enterocytes via proton-dependent transporters (referred to as PepT1), whereas, amino acids enter via sodium-dependent transporters (eg, SLCA19).
  • Then, indicate that amino acids exit the basolateral surface of enterocytes in a wide-range of mechanisms: simple and facilitated diffusion or active transport whereas, generally, amino acids enter enterocytes via sodium-dependent transporters (just like on the apical side).
  • Finally, consider the purpose of the basolateral uptake of amino acids – it's important for intracellular protein synthesis; the synthesis of the protein machinery that runs the cell.