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Digestion and absorption of proteins

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Digestion and absorption of proteins

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Digestion and absorption of proteins

  1. 1.  Proteins, the primary constituents of the body.  A regular & adequate supply of protein in the diet is essential for cell integrity & function.  Dietary proteins are the primary sources of the nitrogen.  Adult man requires 70 to 100 gm protein/day.  Dietary proteins serve three broad functions:  Their constituent AAs are used for synthesis of body proteins.  The carbon skeletons of AAs can be oxidized to yield energy  Their “C” & “N” atoms may be used to synthesize other nitrogenous and non-nitrogenous metabolites.
  2. 2. Digestion is the disintegration of complex nutrients into simple, soluble and assimilable form. Proteins are too large to be absorbed. The dietary proteins are hydrolyzed to amino acids by proteolytic enzymes, which can be easily absorbed. Proteolytic enzymes responsible for degrading proteins are produced by three different organs; The stomach, pancreas and the small intestine.
  3. 3. AA1 AA2 AA3 AA4 AA5 Peptide bonds
  4. 4. AA1 AA2 AA3 AA4 AA5 Peptide bonds AA1 AA2 AA3 AA4 AA5
  5. 5. • Endopeptidase : acts inside the core of protein, forms small peptide fragments • Exopeptidase : acts from the amino terminal or carboxyl terminal ends of protein • Aminopeptidase • Carboxypeptidase Peptidases (Hydrolases; Class 3) • Peptidases are secreted in the inactive form • When zymogens reach the site of action they are activated Proenzymes (zymogens)
  6. 6. Digestion of Proteins Salivary gland: Salivary juice Stomach : Gastric juice Intestine: Intestinal juice Liver: Hepatic juice Pancreas: Pancreatic juice Intestine: Absorption
  7. 7. Mouth No enzymes Stomach Pancreas Intestine
  8. 8. No digestion of protein in mouth. No proteolytic enzymes present in the saliva. Function of the saliva - lubricate the food, this helps in making food soluble for the action of proteolytic enzymes. After mastication and chewing, the bolus of food enters stomach where it is acted upon by gastric juice. Digestion in Mouth
  9. 9. Digestion of protein starts in stomach. When proteins enters the stomach, it stimulates the secretion of the hormone gastrin, from gastric mucosal cells. This gastrin, in turn, stimulates the release of gastric juice, which contains... Hydrochloric acid (HCL). Pepsinogen (zymogen) Rennin (in infants). The pH of gastric juice is 1.5-2.5. Digestion in Stomach
  10. 10. Digestion of protein begins in the stomach Pepsinogen Pepsin Pepsin Autocatalysis Pepsinogen Strong acid (HCl) Proenzyme - pepsinogen • Denatures proteins • Decreases pH (2-3) • Activates pepsinogen • Kills some bacteria • Helps in the absorption of Vitamin B12 Secreted by Chief cells. Activated to pepsin by HCl. And autocatalysis. HCL
  11. 11. As the acidic stomach contents pass into duodenum, the low pH triggers the secretion of the hormones… Secretin – stimulate pancreas to secrete bicarbonate. It neutralizes HCL and rises the pH from 1.5-2.5 to 7.0 Cholecystokinin - stimulate secretion of pancreatic Endopeptidase – Trypsin, Chymotrypsin, Elastase Exopeptidase – Carboxypeptidases, Aminopeptidases Digestion of proteins in the Intestine By Pancreatic Enzymes
  12. 12. Endopeptidase
  13. 13. Exopeptidase
  14. 14. ChymotrypsinChymotrypsingen pro-elastase Elastase Procarboxypeptidase Carboxypeptidase Trypsingen Trypsin Enterokinase Trypsin Trypsin Trypsin Trypsin • Secreted in the zymogen form (Trypsinogen) • activated by Enterokinase and trypsin itself. • specific for cleaving peptide bonds contributed by lysine, arginine (basic amino acids) • Acts on the zymogen forms of other pancreatic enzymes and activates them • It has weak action on casein Trypsin
  15. 15. CHYMOTRYPSIN (Endopeptidase)  Zymogen form – Chymotrypsinogen  activated to chymotrypsin by trypsin  Hydrolyzes the peptide bond formed by the carboxyl group of aromatic amino acids ELASTASE (Endopeptidase)  Zymogen form – Proelastase  activated to elastase by trypsin  Acts on peptide bonds formed by the amino acids like glycine, alanine, serine Chymotrypsin and elastase
  16. 16. Carboxypeptidase - Zinc containing - Exopeptidase - Zymogen form is procarboxypeptidase - Activated by trypsin - Cannot act on dipeptides Carboxypeptidase A acts on the Carboxy terminal peptide bond connected to Tyrosine, Phenylalanine or tryptophan Carboxypeptidase B acts on the carboxy terminal peptide bond connected to Arginine, lysine CARBOXYPEPTIDASE
  17. 17. Pepsin Trypsin Chymotrypsin Elastase Carboxypeptidase A Carboxypeptidase B AA 1 AA 2 AA 3 AA 4 AA 5 AA 6 AA 7 AA 8 NH3+ COO- Pepsinogen Trypsinogen Chymotrypsinogen Pro-elastase procarboxypeptidase A Procarboxypeptidase B Try Phe Tyr Met Leu A Ala lle Leu Val Ala Gly Ser B ArgL ysArgL ys Try Phe Leu Entrokinase HCL Overview of Digestion of proteins
  18. 18. The digestion products of hydrolysis by pepsin, trypsin, elastase, chymotrypsin & carboxypeptidase is completed by the intestinal peptidases, secreted by the mucosa of the small intestine. Some of these peptidases are… Aminopeptidases Dipeptidases
  19. 19. • Present on the luminal surface of the intestinal mucosa • Is an exopeptidase. • Acts on the N terminal peptide bond • Release free amino acid Amino peptidase • Present on the surface of the intestinal mucosal. • Act on dipeptides & release free AAs • Enterocytes take up some di & tripeptides • These peptides are hydrolyzed to amino acids by intracellular dipeptidase Di and Tri peptidases
  20. 20. •Absorption of most of the AAs takes place by active transport mechanism. •By Na+ dependent active transport system (Na+ amino acid cotransport) • An energy requiring process Absorption of amino acids
  21. 21. ATP ADP + Pi Na+ - K+ Pump Sodium - Potassium - Amino Acids - Intestinal epithelium Brush Border To capillaries Sodium-amino acid cotransport
  22. 22. Transport System • Small neutral amino acids • Large neutral amino acids • Basic amino acids • Acidic amino acids • imino acid and Glycine Amino acids Transported • Alanine, Serine & Threonine • Isoleucine, Leucine, Valine, Tyrosine, tryptophane, PA • Arg, Lys, Ornithine & Cystine • Glu acid & Asp acid • Proline, OH-proline & Glycine Disorder Associated • Hartnup disease • Cystinuria • Glycinuria
  23. 23. L- Amino acid Glutamyl-amino acid Amino acid Oxoproline GlutamateGlutamyl-cysteine Glutathione Inside membrane Outer membraneγ-glutamyl transferase Cysteinyl glycine Glycine Cysteine ATP ATP ATP γ – glutamyl cycle (Meister cycle)
  24. 24. Absorption of intact proteins & polypeptides • Short period, immediately after birth, the small intestine of infants can absorb intact proteins and polypeptide by endocytosis or pinocytosis • Intact proteins and polypeptides are not absorbed by the adult intestine • Macromolecular absorption in certain individuals appears to be responsible for antibody formation that often causes food allergy.
  25. 25. Acute pancreatitis: Premature activation of trypsinogen inside the pancreas itself will result in the autodigestion of pancreatic cells Defects in the intestinal amino acid transport systems are seen in inborn errors of metabolism • Hartnup’s disease • Cystinuria : [dibasic amino acids, ornithine, arginine, and lysine (represented as “COAL”)
  26. 26. Cystinuria- Common transporter for cystine, ornithine, arginine and lysine(COAL) is present in gut and renal tubules. Deficiency of transporter results in loss of these amino acids in the feces and urine. Hartnup’s Disease- There is deficiency of transporter for tryptophan and neutral amino acid. no absorption of tryptophan takes place ,tryptophan deficiency produce neurological and skin manifestation (pellagra-like rashes).
  27. 27. “THANK U” Contact no. – 07418831766 E mail – ashokktt@gmail.com Ashok Katta Dept. of Biochemistry, Dhanalakshmi Srinivasan Medical College, Perambalur

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