2. The proteins subjected to digestion and
absorption are obtained from two sources.
1.Exogenous
2.Endogenous

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. 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.

5. 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.

6. 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

7. Protein Digestion – Small Intestine
Pancreatic enzymes secreted
Trypsinogen
Chymotrypsinogen
Procarboxypeptidase
Proelastase Zymogens
Collagenase

8. The release of pancreatic zymogens is
mediated by the secreation of Cholecystokinin
and secretin,two polypeptide hormones of
digestive tract.

9. Digestion in
Small 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

11. 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

12. Protein Digestion
Proteins are broken down to
Tripeptides
Dipeptides
Free amino acids

14. 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

15. 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

16. 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

17. In the Enterocytes…
First cells that can use
the amino acids
Transport into portal
blood
Protein synthesis
Digestive enzymes
Structure and
growth
Energy

18. 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...

19. 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)

20. Abnormalities in the protein
digestion and amino acid
absorption.
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

21. HARTNUP’S DISEASE
Inability of itestinal and epithelial cells to
absorb neutral amino acids.Tryptophan
absorption is severely effected resulting in
pellagra.
CYSTINUREA.