Digestion and absorption of proteins


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Digestion and absorption of proteins

  1. 1. The proteins subjected to digestion and absorption are obtained from two sources.1.Exogenous2.Endogenous
  2. 2. The Fate of Dietary Protein The intake of dietary protein is in the range of 50-100g/day.Digestion and absorption .Maintenance of body protein stores.Net protein synthesis.Synthesis of non-protein compoundsOxidative deamination
  3. 3. PROTEINS in the BODYAmino Acid Pool – amino acids that are available throughout the body (tissues and fluids) for use when needed.Protein Turnover – of the ~ 300 grams of protein synthesized by the body each day, 200 grams are made from recycled amino acids.
  4. 4. Protein DigestionWhole proteins are not absorbed. Too large to pass through cell membranes intact. H3N+ H C C O H O R N C C HDigestive enzymes.Hydrolases R N H C C O H O– Break peptide bonds RSecreted as inactive pre-enzymes. Prevents self-digestion.
  5. 5. Protein Digestion Initiated in stomach HCl from parietal cells StomachpH 1.6 to 3.2 Denatures 40, 30, and 20 structures Pepsinogen from chief cells Pepsinogen HCl Pepsin Cleaves only when carbonyl group of the peptide bond is contributed by Aromatic amino acids. Protein leaves stomach as mix of insoluble protein, soluble protein, peptides and amino acids
  6. 6. Protein Digestion – Small IntestinePancreatic enzymes secreted Trypsinogen Chymotrypsinogen Procarboxypeptidase Proelastase Zymogens Collagenase
  7. 7. The release of pancreatic zymogens is mediated by the secreation of Cholecystokinin and secretin,two polypeptide hormones of digestive tract.
  8. 8. Digestion inSmall Intestine Zymogens must be converted to active form Trypsinogen Enteropeptidase/Trypsin Trypsin  Endopeptidase  Cleaves on carbonyl side of Lys & Arg Trypsin Chymotrypsinogen Chymotrypsin  Endopeptidase  Cleaves carboxy terminal Phe, Tyr and Trp Trypsin Procarboxypeptidase Carboxypeptidase  Exopeptidase  Removes carboxy terminal residues
  9. 9. Trypsin InhibitorsSmall proteins or peptidesPresent in plants, organs, and fluids Soybeans, peas, beans, wheat Pancreas, colostrumBlock digestion of specific proteinsInactivated by heat
  10. 10. Protein Digestion Proteins are broken down to Tripeptides Dipeptides Free amino acids
  11. 11. Peptide Absorption Form in which the majority of protein is absorbed More rapid than absorption of free amino acids Active transport  Energy required Metabolized into free amino acids in enterocyte Only free amino acids absorbed into blood
  12. 12. Free Amino Acid Absorption Free amino acids Carrier systems  Neutral AA  Basic AA Na+ Na+  Acidic AA  Imino acids Entrance of some AA is via active transport  Requires energy
  13. 13. Protein DigestionSmall intestine (brush border) Aminopeptidases  Cleave at N-terminal AA Dipeptidases  Cleave dipeptides into Aas. (Enterokinase or enteropeptidase)  Trypsinogen → trypsin  Trypsin then activates all the other enzymes
  14. 14. In the Enterocytes… First cells that can use the amino acids Transport into portal blood Protein synthesis Digestive enzymes Structure and growth Energy
  15. 15. Basolateral MembraneTransport of free amino acids only* Peptides are hydrolyzed within the enterocyteTransport mainly by diffusion and Groff & Gropper, 2000 Na-independent carriers *Whole proteins are nutritionally insignificant...
  16. 16. Absorption of Intact Proteins Newborns First 24 hours after birth Immunoglobulins  Passive immunity Adults Paracellular routes  Tight junctions between cells Intracellular routes  Endocytosis  Pinocytosis Of little nutritional significance... Affects health (allergies and passive immunity)
  17. 17. Abnormalities in the proteindigestion and amino acidabsorption.Defect in the pancreatic secreation.Cystic fibrosis,incomplete digestion of fat and protein,results in abnormal appearance of lipids (steatorrhea) and proteins in feces.Defective carrier system
  18. 18. HARTNUP’S DISEASEInability of itestinal and epithelial cells to absorb neutral amino acids.Tryptophan absorption is severely effected resulting in pellagra.CYSTINUREA.
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