Digestion and absorption of proteins

2.  Proteins, the primary constituents of the body.
 A regular & adequate supply of protein in the diet is
essential for cell integrity & function.
 Dietary proteins are the primary sources of the nitrogen.
 Adult man requires 70 to 100 gm protein/day.
 Dietary proteins serve three broad functions:
 Their constituent AAs are used for synthesis of body
proteins.
 The carbon skeletons of AAs can be oxidized to yield energy
 Their “C” & “N” atoms may be used to synthesize other
nitrogenous and non-nitrogenous metabolites.

3. Digestion is the disintegration of complex
nutrients into simple, soluble and assimilable
form.
Proteins are too large to be absorbed.
The dietary proteins are hydrolyzed to amino
acids by proteolytic enzymes, which can be
easily absorbed.
Proteolytic enzymes responsible for
degrading proteins are produced by three
different organs; The stomach, pancreas and
the small intestine.

4. AA1 AA2 AA3 AA4 AA5
Peptide bonds

5. AA1 AA2 AA3 AA4 AA5
Peptide bonds
AA1
AA2 AA3 AA4
AA5

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

7. Digestion of
Proteins Salivary gland:
Salivary juice
Stomach :
Gastric juice
Intestine:
Intestinal juice
Liver:
Hepatic juice
Pancreas:
Pancreatic juice Intestine:
Absorption

8. Mouth
No
enzymes
Stomach
Pancreas
Intestine

9. No digestion of protein in mouth.
No proteolytic enzymes present in the saliva.
Function of the saliva - lubricate the food, this
helps in making food soluble for the action of
proteolytic enzymes.
After mastication and chewing, the bolus of food
enters stomach where it is acted upon by gastric
juice.
Digestion in Mouth

10. Digestion of protein starts in stomach.
When proteins enters the stomach, it stimulates
the secretion of the hormone gastrin, from
gastric mucosal cells.
This gastrin, in turn, stimulates the release of
gastric juice, which contains...
Hydrochloric acid (HCL).
Pepsinogen (zymogen)
Rennin (in infants).
The pH of gastric juice is 1.5-2.5.
Digestion in Stomach

11. Digestion of protein begins in the stomach
Pepsinogen
Pepsin
Pepsin Autocatalysis
Pepsinogen
Strong acid
(HCl)
Proenzyme -
pepsinogen
• Denatures proteins
• Decreases pH (2-3)
• Activates pepsinogen
• Kills some bacteria
• Helps in the
absorption of
Vitamin B12
Secreted by Chief
cells. Activated to
pepsin by HCl.
And autocatalysis.
HCL

14. As the acidic stomach contents pass into duodenum,
the low pH triggers the secretion of the hormones…
Secretin – stimulate pancreas to secrete
bicarbonate. It neutralizes HCL and rises the pH
from 1.5-2.5 to 7.0
Cholecystokinin - stimulate secretion of
pancreatic
Endopeptidase – Trypsin, Chymotrypsin,
Elastase
Exopeptidase – Carboxypeptidases,
Aminopeptidases
Digestion of proteins in the Intestine By Pancreatic Enzymes

15. Endopeptidase

16. Exopeptidase

18. ChymotrypsinChymotrypsingen
pro-elastase Elastase
Procarboxypeptidase Carboxypeptidase
Trypsingen Trypsin
Enterokinase
Trypsin
Trypsin
Trypsin
Trypsin
• Secreted in the zymogen form (Trypsinogen)
• activated by Enterokinase and trypsin itself.
• specific for cleaving peptide bonds contributed
by lysine, arginine (basic amino acids)
• Acts on the zymogen forms of other pancreatic
enzymes and activates them
• It has weak action on casein
Trypsin

19. 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
Chymotrypsin and elastase

20. Carboxypeptidase
- Zinc containing
- Exopeptidase
- Zymogen form is
procarboxypeptidase
- Activated by trypsin
- Cannot act on
dipeptides
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
CARBOXYPEPTIDASE

21. Pepsin
Trypsin
Chymotrypsin Elastase Carboxypeptidase A
Carboxypeptidase B
AA
1
AA
2
AA
3
AA
4
AA
5
AA
6
AA
7
AA
8
NH3+
COO-
Pepsinogen
Trypsinogen
Chymotrypsinogen
Pro-elastase procarboxypeptidase A
Procarboxypeptidase B
Try
Phe
Tyr
Met
Leu
A
Ala
lle
Leu
Val
Ala
Gly
Ser
B
ArgL
ysArgL
ys
Try
Phe
Leu
Entrokinase
HCL
Overview of Digestion of proteins

22. The digestion products of hydrolysis by pepsin, trypsin,
elastase, chymotrypsin & carboxypeptidase is completed
by the intestinal peptidases, secreted by the mucosa of
the small intestine.
Some of these peptidases are…
Aminopeptidases
Dipeptidases

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

24. •Absorption of most of the AAs takes
place by active transport mechanism.
•By Na+ dependent active transport
system (Na+ amino acid cotransport)
• An energy requiring process
Absorption of amino acids

25. ATP
ADP + Pi
Na+ - K+
Pump
Sodium -
Potassium -
Amino Acids -
Intestinal
epithelium
Brush
Border
To capillaries
Sodium-amino acid
cotransport

26. Transport
System
• Small neutral amino
acids
• Large neutral amino
acids
• Basic amino acids
• Acidic amino acids
• imino acid and
Glycine
Amino acids
Transported
• Alanine, Serine &
Threonine
• Isoleucine, Leucine,
Valine, Tyrosine,
tryptophane, PA
• Arg, Lys, Ornithine &
Cystine
• Glu acid & Asp acid
• Proline, OH-proline &
Glycine
Disorder
Associated
• Hartnup
disease
• Cystinuria
• Glycinuria

27. L- Amino acid
Glutamyl-amino acid
Amino acid
Oxoproline
GlutamateGlutamyl-cysteine
Glutathione
Inside membrane
Outer membraneγ-glutamyl transferase
Cysteinyl glycine
Glycine Cysteine
ATP
ATP
ATP
γ – glutamyl cycle (Meister cycle)

28. Absorption of
intact proteins & 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.

29. Acute pancreatitis:
Premature activation of trypsinogen inside the pancreas itself will
result in the autodigestion of pancreatic cells
Defects in the intestinal amino acid transport systems
are seen in inborn errors of metabolism
• Hartnup’s disease
• Cystinuria : [dibasic amino acids, ornithine, arginine, and
lysine (represented as “COAL”)

30. Cystinuria- Common transporter for cystine,
ornithine, arginine and lysine(COAL) is present in
gut and renal tubules. Deficiency of transporter
results in loss of these amino acids in the feces
and urine.
Hartnup’s Disease- There is deficiency of
transporter for tryptophan and neutral amino
acid. no absorption of tryptophan takes place
,tryptophan deficiency produce neurological and
skin manifestation (pellagra-like rashes).

31. “THANK U”
Contact no. – 07418831766
E mail – ashokktt@gmail.com
Ashok Katta
Dept. of Biochemistry,
Dhanalakshmi Srinivasan Medical College,
Perambalur