Digestion and Absorption of Proteins for Allied Health Sciences

The presentation covers, the digestion of proteins in mouth, stomach, and intestine including the role of the proteolytic enzymes in digestion of proteins and reactions of Miester Cycle (Gamma Glutamyl Cycle) in absorption of amino acids from the intestinal lumen.

1. Digestion and absorption of Proteins Karthik Kamath Assistant Professor Department of Biochemistry 1

2. Objectives Define digestion List the organs of the digestive tract Discuss the role of organs of digestive tract Describe digestion of proteins Explain enzymes of protein digestion Explain digestion of proteins in stomach 2

3. Digestion Process involving hydrolysis of large and complex organic molecules of foodstuff Into smaller and preferably water-soluble molecules Which can be easily absorbed by GI For utilization of the organisms 3

4. Parts of GI Mouth Stomach Small intestine Large intestine The accessory parts of digestion? 4

5. Digestion of proteins Obtained from two sources Dietary – 50-100 g/day Endogenous 30-100 g/day Derived from digestive enzymes and worn out cells of digestive tract Very little protein is lost in feces 5

6. Enzymes of protein digestion Proteins are digested by hydrolases (peptidases) Cleave peptide bonds Can be two types Endopeptidases – act on internal peptide bonds and release peptides Pepsin, trypsin Exopeptidases – acts on terminal peptide bonds Can be divided into Carboxypeptidases – (C-Terminal) Aminopeptidases 6

7. The proteolytic enzymes are produced by stomach, pancreas and small intestine 7

8. Digestion of proteins Proteins remain undigested Absence of proteases in saliva 8

9. Digestion in stomach Digestion by gastric secretions Contains HCl and pepsinogen pH is <2 HCl, secreted by parietal cells Two important functions Denature of proteins Killing of microorganisms Denatured proteins are more susceptible for digestion through proteases 9

10. Digestion in stomach Pepsin is produced by serous cells As pepsinogen Converted to pepsin through 2 ways Autocatalysis – HCl Removal of fragment of polypeptide chain Pepsin digestion results in peptides And few amino acids These act as stimulant for release of cholecystokinin from duodenum 10

11. Digestion in stomach Chymosin is present in infants and children Renin involved in curdling of milk Converts casein to calcium paracaseinate Effectively digested by pepsin Renin is absent in adults 11

12. Digestion by pancreatic proteases Pancreatic juices are secreted as zymogens Activation is initiated by release of 2 polypeptide hormones Cholecystokinin and secretin 12

13. Activation of zymogens Enteropeptidase (key enzyme for activation) is secreted by intestinal mucosal cells Enteropeptidase cleaves off hexapeptide from N terminal end of trypsinogen - trypsin Trypsin then activates other through autocatalysis Trypsin is common activator for other active proteases 13

14. Digestion in small intestine Luminal surface of epithelium contains aminopeptidases and dipeptidases Aminopeptidases cleaves N terminal amino acids one by one to produce smaller peptides Dipeptidases act on different dipeptides to liberate amino acids 14

15. Overview of digestion 15 Proteins Unchanged MOUTH Polypeptides Amino acids STOMACH Pepsin Amino Acids Oligopeptides SMALL INTESTINE Trypsin Chymotrypsin Elastase Amino Acids Dipeptides Carboxypeptidases Aminopeptidases Dipeptidases

16. Absorption of proteins Mainly occurs in small intestine An energy requiring process Transport systems are carrier mediated &/or ATP sodium dependent transporters 5 different carriers Neutral amino acids Basic amino acids Imino acids Acidic amino acids Beta amino acids 16

17. Miester cycle (gamma glutamyl cycle) Meister proposed Participation of glutathione in an active group translocation of L-amino acids Into the cells from the intestine, kidneys, seminal vesicles, epididymis and brain A cyclic pathway in which glutathione is regenerated again is called as 17

18. Miester cycle (gamma glutamyl cycle) 18 Glutathione L-amino acids Cysteinyl-Glycine γ - glutamyl amino acid L-cysteine L-glycine 5-oxo-proline γ - glutamyl transferase Na + γ - glutamyl cyclo transferase L-glutamic acid H 2 O 5-oxo-prolinase ATP ADP+Pi L-glutamyl cysteine