Protein digestion and absorption

After go through this presentation you will be able to know about the digestion and absorption of Proteins

1. Digestion and Absorption of Proteins Aamir Ali Khan M.Phil. Biochemistry Head of pathology Northwest Institute of Health Sciences 1

2. Proteins Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins are assembled from amino acids using information encoded in genes. Each protein has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene encoding this protein. Polymers of amino acids Contain about 16% nitrogen in addition to Carbon, Hydrogen and Oxygen 2

3. 3 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

4. Digestion of proteins and absorption of amino acids 4 Intake of dietary protein is in the range of 50-100 g/day About 30-100 g/day of endogenous protein is derived form the digestive enzymes and worn out cells of the digestive tract The digestion and absorption of proteins is very efficient in healthy humans, hence very little protein ( about 5-10 g/day ) is lost through feces 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.

5. 5 Protein Digestion Whole proteins are not absorbed. Too large to pass through cell membranes intact. Digestive enzymes. Hydrolases Break peptide bonds Secreted as inactive pre-enzymes. Prevents self-digestion.

6. Amino acids are linked to each other by peptide linkage Amino acid Amino acid Amino acid Amino acid Amino acid Peptide bonds 6

7. The peptide bonds of the proteins must be hydrolyzed to give their constituent amino acids (Digestion of protein) Proteases (peptidases) Amino acid Amino acid Amino acid Amino acid Amino acid Peptide bonds Amino acid Amino acid Amino acid Amino acid Amino acid 7

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9. 9 Protein Digestion – Small Intestine Pancreatic enzymes secreted Trypsinogen Chymotrypsinogen Procarboxypeptidase Proelastase Collagenase Zymogens The release of pancreatic zymogens is mediated by the secretion of Cholecystokinin and secretin , two polypeptide hormones of digestive tract .

10. Proteolytic enzymes of GIT are secreted by three different organs 10

11. Digestion of protein begins in the stomach 11 Stomach secrets Strong mineral acid HCl Proenzyme pepsinogen Denatures proteins Decreases the pH (pH 2 to 3) Activates pepsinogen Kills some bacteria Helps in the absorption of Vitamin B12 Hydorchloric acid ( HCl ) Pepsinogen Activated to pepsin by HCl Acid Stable endopeptidase Pepsinogen Pepsinogen Pepsin Autocatalysis Pepsin Protein leaves stomach as mix of insoluble protein, soluble protein, peptides and amino acids

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13. 13 Digestion of proteins in the Intestine

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15. 15 Trypsin ( endopeptidase ) Trypsin Trypsinogen ENTEROPEPTIDASE Present on the brush border surface Trypsinogen (Zymogen) Hydrolyzes the peptide bond formed by the Basic amino acids (Arginine and Lysine ) Acts on the zymogen forms of other pancreatic enzymes and activates them [autocatalysis]

16. 16 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

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19. Absorption of amino acids By Na+ dependent active transport system ( Na + amino acid cotransport) An energy requiring process Transporter for acidic amino acids Transporter for basic amino acids Transporter for neutral amino acids Transporter for imino acid Transporter for β - amino acids There are 5 different carriers for amino acids 19

20. Absorption of intact proteins and 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 . 20

21. 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

22. Free Amino Acid Absorption Free amino acids Carrier systems Neutral AA Basic AA Acidic AA Imino acids Entrance of some AA is via active transport Requires energy Na + Na +

23. Celiac disease (celiac sprue) is a disease of malabsorption resulting from immune-mediated damage to the small intestine in response to ingestion of gluten (or gliadin produced from gluten), a protein found in wheat and barley. Acute pancreatitis: Premature activation of trypsinogen inside the pancreas itself will result in the auto digestion of pancreatic cells 23

24. Thank you 24