In this section, we describe digestion and absorption of proteins. Most of the slides are cited from: 1. Lippincott's Illustrated Review Biochemistry 2. U. Satyrana Biochemistry Dr. Haroon

1. Asst. Prof. Dr. Muhammad Haroon
MD, ECEA, MPH (JHSPH)
Head & Coordinator of MPH program
Former Biochemistry Guest lecturer at SMS
medical college, India.
Email:mstanik2@jhu.edu

2. Overview
⚫ Proteins are:
– Macromolecules
– Cannot be absorbed from the alimentary tract as such
⚫ For absorption:
– The have to be hydrolyzed into Amino-Acids
– They are Hydrolyzed by Proteolytic enzymes
⚫ Proteolytic enzymes are present in:
– Digestive juice
Digestion and
Absorption of
Proteins2

3. Overview Cont.….....
⚫ Proteolytic enzymes are:
– Group specific
– Specific for peptide bonds not actual substrate
⚫ They are classified into:
– Endopeptidase
– Exopeptidase
⚫ The proteolytic enzymes are generally secreted in
the form of:
– Inactive (Proenzymes) Digestion and
Absorption of
Proteins3

4. Overview Cont.….....
Digestion and
Absorption of
Proteins4

5. Overview Cont.….....
⚫ The catalytic site is concealed by:
– Portion of the polypeptide chain
⚫ Activation of pro-enzyme:
– Removal of the covering polypeptide
⚫ It occurs:
– Upon entry of food
⚫ This protects the structural proteins of mucosa
against:
– Proteolysis by digestive enzymes Digestion and
Absorption of
Proteins5

6. Overview Cont.….....
Digestion and
Absorption of
Proteins6

7. Digestive secretions taking part in
digestion of proteins
⚫ They include:
– Gastric juice
– Pancreatic juice
– Intestinal secretion (Succus Entericus)
Digestion and
Absorption of
Proteins7

8. Gastric Juice
⚫ The proteolytic enzymes present are:
– Pepsin
– Rennin
⚫ Pepsin:
– Secreted as Proenzyme (Pepsinogen)
– Gastric HCL splits a peptide off pepsinogen
– Converts Pepsinogen into pepsin
– Pepsin once formed, can convert further molecules of
pepsinogen into pepsin Digestion and
Absorption of
Proteins8

9. Gastric Juice Cont.….....
⚫ Pepsin:
– Endopeptidase
– Hydrolyses internal bonds of proteins to convert them
into peptides
⚫ It acts preferentially on peptide bonds formed by:
– Aromatic Amino acids
– Dicarboxylic Amino acids
Digestion and
Absorption of
Proteins9

10. Production and action of Pepsin
Digestion and
Absorption of
Proteins10

11. Gastric Juice Cont.….....
⚫ Rennin:
– Present in the gastric juice of infants
– Acts on casein of milk
– Converts it into calcium paracaseinate
– Calcium paracaseinate is hydrolyzed by pepsin into
peptides
Digestion and
Absorption of
Proteins11

12. Rennin
Digestion and
Absorption of
Proteins12

13. Pancreatic Juice
⚫ It contains the proenzymes:
– Trypsinogen
– Chymotrypsinogen
– Proelastase
– Procarboxypeptidase
⚫ Trypsinogen is converted into trypsin by:
– Enterokinase (Brunner glands, Duodenum)
– Trypsin can convert further molecules of trypsinogen into
trypsin Digestion and
Absorption of
Proteins13

14. Pancreatic Juice Cont.………
Digestion and
Absorption of
Proteins14

15. Pancreatic Juice Cont.….....
⚫ Trypsin also converts:
– Chymotrypsinogen into chymotrypsin
– Proelastase into elastase
– Procarboxypeptidase into carboxypeptidase
⚫ They hydrolyze proteins and polypeptides into:
– Small peptides
Digestion and
Absorption of
Proteins15

16. Trypsin & its function
Digestion and
Absorption of
Proteins16

17. Pancreatic Juice Cont.….....
⚫ Trypsin:
– Endopeptidase
– Acts preferentially on peptide bonds formed by basic
amino acids
⚫ Chymotrypsin:
– Endopeptidase
– Acts preferentially on peptide bonds formed by Aromatic
amino acids Digestion and
Absorption of
Proteins17

18. Pancreatic Juice Cont.….....
⚫ Elastase:
– Endopeptidase
– Acts preferentially on peptide bonds formed by Glycine,
Alanine, Serine
⚫ Carboxypeptidase:
– Exopeptidase
– Removes amino acids one by one from the carboxyl end
of proteins Digestion and
Absorption of
Proteins18

19. Intestinal Secretions
⚫ Intestinal secretion contains:
– Aminopeptidase
– Dipeptidases
⚫ Aminopeptidase:
– Removes amino acids one by one from the amino end
⚫ Dipeptidases:
– Hydrolyze dipeptides into amino acids
⚫ The final products of protein digestion are:
– L- Amino Acids Digestion and
Absorption of
Proteins19

20. Summary of Protein Digestion
Digestion and
Absorption of
Proteins20

21. Summary of Protein Digestion
Cont.…….
Digestion and
Absorption of
Proteins21

22. Absorption of Amino Acids
⚫ Site of absorption:
– Mucosal cells of the small intestine
⚫ The absorption is:
– Active and energy consuming
– Sodium Dependent Amino Acid Transporter
– Same as SGLT1
Digestion and
Absorption of
Proteins22

23. Na+ Dependent Carrier
⚫ There are at least six different Na+ dependent
carrier:
– Neutral Amino Acids
– Proline & Hydroxyproline
– Acidic Amino Acids
– Basic Amino Acids & Cysteine
Digestion and
Absorption of
Proteins23

24. Na+ Dependent Carrier Cont.….....
Digestion and
Absorption of
Proteins24

25. Na+ Dependent Carrier Cont.….....
Digestion and
Absorption of
Proteins25

26. Na+ Dependent Carrier Cont.….....
Digestion and
Absorption of
Proteins26

27. Hartnup Disease
⚫ First diagnosed in:
– Hartnup family
⚫ Defect:
– Transport defect involving all neutral amino acids
– Intestinal & Renal tubular absorption of neutral amino
acids is impaired
⚫ Neutral Amino Acids are:
– Tryptophan, Alanine
– Serine
– Threonine & Valine
Digestion and
Absorption of
Proteins27

28. Hartnup Disease Cont.….....
⚫ Decreased availability of tryptophan decreases:
– Synthesis of Niacin
⚫ Decreased Niacin produce:
– Pellagra-like picture
⚫ Treatment:
– High-protein diet
– Niacin supplements Digestion and
Absorption of
Proteins28

29. Pellagra
⚫ Cause:
– Niacin Deficiency
⚫ Clinical Features:
– 4Ds
– Dementia
– Diarrhea
– Dermatitis
– Death (If not treated)
⚫ Treatment:
– Niacin
Digestion and
Absorption of
Proteins29

30. Pellagra Cont.….....
Digestion and
Absorption of
Proteins30

31. References
⚫ Harvey, Richard A., Ph. D. (2017). Lippincott's illustrated reviews:
Biochemistry. 7th Edition. Philadelphia :Wolters Kluwer Health,
⚫ Murray, R. K. (2016). Harper's illustrated biochemistry. 31th Edition.
New York: McGraw-Hill.
⚫ U. Satyanarayana, Biochemistry, Books and Allied (P) Ltd., Calcutta,
Fourth Edition
⚫ The images are retrieved from: www.google.com/images
Digestion and
Absorption of
Proteins31

32. Thank You
Digestion and
Absorption of
Proteins32