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Digestion and absorption of proteins mani@
1. Digestion and Absorption Of Prot
eins
By
MANI BHASKAR KUMAR
ROLL NO- 1802101018
SUBJECT-FPT112
College of Fisheries Dholi
2. • Total protein load received by the gut is de
rived from 2 sources:
• 70-100 g dietary protein per day
• 35-200 g of endogenous protein secreted i
nto the gut (enzymes) or shed from the epi
thelium as a result of cell turnover.
3. • The digestion and absorption of protein is
extremely efficient:
• only 1-2 g of nitrogen=equivalent to 6-12 o
f protein is lost into the feces daily.
• Proteins are hydrolyzed by peptidases
5. • Endopeptidases break down large polypep
tides to smaller oligopeptides which can b
e acted upon by exopeptidases to produce
amino acids and di- and tripeptides which
are absorbed by enterocytes.
6. • Depending on the source of peptidases, th
e protein digestive process can be divided
into 3 phases:
• 1-Gastric
• 2-Pancreatic
• 3-Intestinal
7. • Protein digestion begins in the stomach.
• Entry of dietary protein into the stomach stimulat
es the gastric mucosa to secrete gastrin.
• HCI secreted by the parietal cells reduces the p
H of stomach to 1-2
• The acidic gastric juice is both an antiseptic age
nt, killing most bacteria and other foreign cells, a
nd a denaturating agent, unfolding globular prote
ins.
8. • Denaturation unfolds polypeptide chains,
making proteins more accesible to proteas
e activity.
• Pepsins are secreted by the chief cells of t
he gastric mucosa, as inactive precursors,
pepsinogen I and II, and are activated eith
er by autoactivation at ph<5 or by autocata
lysis.
(a cleavage mediated by pepsinogen itself)
9. • At ph>2.0, the liberated peptide remains bound
to pepsin and acts as an inhibitor of pepsin activi
ty.
• This inhibition is removed either by a drop in pH
below 2.0 or by further pepsin action.
• Pepsin hydrolyzes ingested proteins at peptide b
onds on the amino-terminal side of aromatic ami
no acid residues-Phe, Trp and Tyr
10. • The end product of protein digestion in the
stomach is peptide.
• As the acidic stomach contents pass into t
he small intestine, the low pH triggers secr
etion of the hormone secretin into the bloo
d.
11. • Secretin stimulates the pancreas to secret
e bicarbonate into the small intestine to ne
utralize gastric HCI, abruptly increasing pH
to about 7.
• Gastric protein digests stimulate cholecyto
kinin release in the duodenum, triggering t
he release of main digestive enzymes by t
he pancreas.
12. • Proteolytic enzymes ,released from the pancrea
s,are inactive zymogens.
• Duodenal enteropeptidase converts trypsinogen
to active trypsin.
• This enzyme is capable of autoactivation and act
ivation of all other pancreatic zymogens -chymo-
trypsinogen, proelastase, procarboxy-peptidases
(A and B)
13. • Because of this prime role of trypsin in acti
vating other pancreatic enzymes, its activit
y is controlled within the pancreas and pan
creatic ducts by a small molecular weight i
nhibitory peptide.(pancreatic trypsin inhibit
or)
• Synthesis of enzymes as inactive precurso
rs protects the exocrine cells from destruct
ive proteolytic attack.
14. • Pancreatic proteases have different substr
ate specificity with respect to peptide bond
cleavage.
• Trypsin arginine and Iysine residues
• Chymotrypsin aromatic amino acids
• Elastase hydrophobic amino acids
15. • Carboxypeptidases are zinc containing en
zymes that remove successive carboxyl-te
rminal residues from peptides.
• The combined effect of these pancreatic e
nzymes produces free amino acids and pe
ptides of 2-8 residues.
16. • .Sodium bicarbonate, produced by the pan
creas, neutralizes the acid contents of the
stomach as they pass into duodenum, thu
s promoting pancreatic protease activity.
• The final digestion of di- and oligopeptides
is carried by small intestinal membrane-b
ound endopeptidases, dipeptidases and a
minopeptidases.
17. • The end products of this surface enzyme activity
are free amino acids, and di-and tripeptides whic
h are absorbed across the enterocyte membran
e by specific carrier-mediated transport.
• Di-and tri-peptides are further hydrolyzed to their
constituent amino acids within the enterocyte.
• Final step is the transfer of amino acids out of th
e enterocyte into portal blood.
18. • In humans, most globular proteins from ani
mal sources are almost completely hydroly
zed to amino acids in the GI tract, some fib
rous proteins, such as keratin, are only par
tly digested.
19. • Acute pancreatitis is a disease caused by
obstruction of the normal pathway by whic
h pancreatic enzymes enter the intestine.
• The zymogens of the proteolytic enzymes
are converted to their catalytically active fo
rms,prematurely, inside the pancreatic cell
s and attact the pancreatic tissue itself.
