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

1. Digestion and Absorption of
Proteins
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2. 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
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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. 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.
H3N+
C
H
C
R
O
N
H
C
H
C
O
R
N
H
C
H
C
R
O
O–

6. Amino
acid
Amino
acid
Amino
acid
Amino
acid
Amino
acid
Peptide bonds
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7. Amino
acid
Amino
acid
Amino
acid
Amino
acid
Amino
acid
Peptide bonds
Amino
acid
Amino
acid
Amino
acid
Amino
acid
Amino
acid
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8. 8
• Endopeptidase : acts inside the core of protein,
forms small peptide fragments
• Exopeptidase : acts from the amino terminal or
carboxyl terminal ends of protein
• Aminopeptidase - e.g. : amino peptidase
• Carboxypeptidase - e.g. : carboxypeptidase
Peptidases
(Hydrolases;
Class 3)
• Peptidases are secreted in the inactive form
• When zymogens reach the site of action they are
activated
Proenzymes
(zymogens)

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
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STOMACH
Pancreas
Intestine

11. Digestion of protein begins in the stomach
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• 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
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

12. 12
Peptide bond formed by
carboxyl group of
Phenylalanine, Tyrosine,
Tryptophan, methionine
Converts proteins into
proteoses and peptones
Also called chymosin
Active in infants
curdling of milk
Absent in adult stomach
Converts Casein of milk into
paracasein
It combines with calcium to form
calcium paracaseinate

13. 13

14. 14
Secreted as
pro-enzymes
Optimum pH is 8
Pancreatic Peptidases
Carboxypeptidase
A
Carboxypeptidase
B
(Exopeptidase)
Trypsin
Chymotrypsin
Elastase
(Endopeptidases)
(Serine proteases)

15. 15
Trypsin
(endopeptidase)
Trypsin
Trypsinoge
n
ENTEROPEPTIDASE
Present on the brush border surface
Trypsinogen
(Zymogen)
[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

17. 17
Carboxypeptidase
- Zinc containing
- Exopeptidase
- Zymogen form is
Procarboxypeptidase
- Activated by trypsin
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

18. 18
• Present on the luminal surface of the intestinal
mucosa
• Is an exopeptidase.
• Acts on the amino terminal peptide bond
• Release free amino acid
Amino
peptidase
• Present on the surface of the intestinal mucosal.
• They act on dipeptides and release free amino acids
• Enterocytes take up some di and tripeptides
• These peptides are hydrolyzed to amino acids by
intracellular Dipeptidase
Di and Tri
peptidases

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
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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.
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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
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24. Thank you
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