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Digestion and Absorption Of
Proteins
Prof. Dr. Arzu SEVEN
• Total protein load received by the gut is
derived from 2 sources:
• 70-100 g dietary protein per day
• 35-200 g of endogenous protein secreted
into the gut (enzymes) or shed from the
epithelium as a result of cell turnover.
• The digestion and absorption of protein is
extremely efficient:
• only 1-2 g of nitrogen=equivalent to 6-12
of protein is lost into the feces daily.
• Proteins are hydrolyzed by peptidases
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Endopeptidases (cleave internal peptide bonds)
Exopeptidases
Carboxypeptidases
Aminopeptidases
• Endopeptidases break down large
polypeptides to smaller oligopeptides
which can be acted upon by
exopeptidases to produce amino acids
and di- and tripeptides which are absorbed
by enterocytes.
• Depending on the source of peptidases,
the protein digestive process can be
divided into 3 phases:
• 1-Gastric
• 2-Pancreatic
• 3-Intestinal
• Protein digestion begins in the stomach.
• Entry of dietary protein into the stomach
stimulates the gastric mucosa to secrete gastrin.
• HCI secreted by the parietal cells reduces the
pH of stomach to 1-2
• The acidic gastric juice is both an antiseptic
agent, killing most bacteria and other foreign
cells, and a denaturating agent, unfolding
globular proteins.
• Denaturation unfolds polypeptide chains,
making proteins more accesible to
protease activity.
• Pepsins are secreted by the chief cells of
the gastric mucosa, as inactive precursors,
pepsinogen I and II, and are activated
either by autoactivation at ph<5 or by
autocatalysis.
(a cleavage mediated by pepsinogen itself)
• At ph>2.0, the liberated peptide remains bound
to pepsin and acts as an inhibitor of pepsin
activity.
• This inhibition is removed either by a drop in pH
below 2.0 or by further pepsin action.
• Pepsin hydrolyzes ingested proteins at peptide
bonds on the amino-terminal side of aromatic
amino acid residues-Phe, Trp and Tyr
• The end product of protein digestion in the
stomach is peptide.
• As the acidic stomach contents pass into
the small intestine, the low pH triggers
secretion of the hormone secretin into the
blood.
• Secretin stimulates the pancreas to
secrete bicarbonate into the small intestine
to neutralize gastric HCI, abruptly
increasing pH to about 7.
• Gastric protein digests stimulate
cholecytokinin release in the duodenum,
triggering the release of main digestive
enzymes by the pancreas.
• Proteolytic enzymes ,released from the
pancreas,are inactive zymogens.
• Duodenal enteropeptidase converts trypsinogen
to active trypsin.
• This enzyme is capable of autoactivation and
activation of all other pancreatic zymogens chymo-trypsinogen, proelastase, procarboxypeptidases (A and B)
• Because of this prime role of trypsin in
activating other pancreatic enzymes, its
activity is controlled within the pancreas
and pancreatic ducts by a small molecular
weight inhibitory peptide.(pancreatic
trypsin inhibitor)
• Synthesis of enzymes as inactive
precursors protects the exocrine cells from
destructive proteolytic attack.
• Pancreatic proteases have different
substrate specificity with respect to
peptide bond cleavage.
• Trypsin
arginine and Iysine residues
• Chymotrypsin
aromatic amino acids
• Elastase
hydrophobic amino acids
• Carboxypeptidases are zinc containing
enzymes that remove successive
carboxyl-terminal residues from peptides.
• The combined effect of these pancreatic
enzymes produces free amino acids and
peptides of 2-8 residues.
• .Sodium bicarbonate, produced by the
pancreas, neutralizes the acid contents of
the stomach as they pass into duodenum,
thus promoting pancreatic protease
activity.
• The final digestion of di- and oligopeptides
is carried by small intestinal membranebound endopeptidases, dipeptidases and
aminopeptidases.
• The end products of this surface enzyme activity
are free amino acids, and di-and tripeptides
which are absorbed across the enterocyte
membrane 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
the enterocyte into portal blood.
• In humans, most globular proteins from
animal sources are almost completely
hydrolyzed to amino acids in the GI tract,
some fibrous proteins, such as keratin, are
only partly digested.
• Acute pancreatitis is a disease caused by
obstruction of the normal pathway by
which pancreatic enzymes enter the
intestine.
• The zymogens of the proteolytic enzymes
are converted to their catalytically active
forms,prematurely, inside the pancreatic
cells and attact the pancreatic tissue itself.