Digestion and Absorption of Proteins

Protein digestion begins in the stomach with the action of pepsin. Some amino acids are liberated in the stomach, but the major products of pepsin digestion are short-chain polypeptides.

CH^MD

Short oligosaccharide o-o-o

Maltriose

Starch Maltose

■ Figure 18.32 The action of pancreatic amylase. Pancreatic amylase digests starch into maltose, maltriose, and short oligosaccharides containing branch points in the chain of glucose molecules.

Pepsin digestion helps to produce a more homogenous chyme, but it is not essential for the complete digestion of protein that occurs—even in people with total gastrectomies—in the small intestine.

Most protein digestion occurs in the duodenum and jejunum. The pancreatic juice enzymes trypsin, chymotrypsin, and elastase cleave peptide bonds in the interior of the polypep-tide chains. These enzymes are thus grouped together as endopep-tidases. Enzymes that remove amino acids from the ends of polypeptide chains, by contrast, are exopeptidases. These include the pancreatic juice enzyme carboxypeptidase, which removes amino acids from the carboxyl-terminal end of polypeptide chains, and the brush border enzyme aminopeptidase. Aminopeptidase cleaves amino acids from the amino-terminal end of polypeptide chains.

Brush Border Enzymes

The Digestive System

Polypeptide Chain Digestion

■ Figure 18.33 The digestion and absorption of proteins.

Polypeptide chains of proteins are digested into free amino acids, peptides, and tripeptides by the action of pancreatic juice enzymes and brush border enzymes. The amino acids, dipeptides, and tripeptides enter duodenal epithelial cells. Dipeptides and tripeptides are hydrolyzed into free amino acids within the epithelial cells, and these products are secreted into capillaries that carry them to the hepatic portal vein.

■ Figure 18.33 The digestion and absorption of proteins.

Polypeptide chains of proteins are digested into free amino acids, peptides, and tripeptides by the action of pancreatic juice enzymes and brush border enzymes. The amino acids, dipeptides, and tripeptides enter duodenal epithelial cells. Dipeptides and tripeptides are hydrolyzed into free amino acids within the epithelial cells, and these products are secreted into capillaries that carry them to the hepatic portal vein.

As a result of the action of these enzymes, polypeptide chains are digested into free amino acids, dipeptides, and tripep-tides. The free amino acids are absorbed by cotransport with Na+ into the epithelial cells and secreted into blood capillaries. The dipeptides and tripeptides enter epithelial cells by the action of a single membrane carrier that has recently been characterized. This carrier functions in secondary active transport using a H+ gradient to transport dipeptides and tripeptides into the cell cytoplasm. Within the cytoplasm, the dipeptides and tripeptides are hydrolyzed into free amino acids, which are then secreted into the blood (fig. 18.33).

Newborn babies appear to be capable of absorbing a substantial amount of undigested proteins (hence they can absorb some antibodies from their mother's first milk); in adults, however, only the free amino acids enter the portal vein. Foreign food protein, which would be very antigenic, does not normally enter the blood. An interesting exception is the protein toxin that causes botulism, produced by the bacterium Clostridium botu-linum. This protein is resistant to digestion and is thus intact when it is absorbed into the blood.

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Responses

  • semret fethawi
    Which enzymes remove the terminal amino acid from a polypeptide chain during digestion?
    8 years ago
  • Ursula
    Where are amino acids dipeptides and tripeptides absorbed?
    8 years ago
  • Meneaduc
    Are dipeptides absorbed into bloodstream after digestion?
    8 years ago
  • milford
    How amino acids are absorbed from protein digestion?
    8 years ago
  • karolin
    Why are amino acids absorbed into blood capillaries?
    8 years ago
  • torsten eberhardt
    What are the steps to protein digestion and absorption?
    8 years ago
  • salvia
    Do dipeptides enter the blood?
    8 years ago
  • Renate
    Are brush border enzymes produced in the pancreas?
    8 years ago
  • fre-weini
    How is protein absorbed hepatic portal system?
    8 years ago
  • amanuel
    Are proteins absorbed as amino acids, dipeptides and tripeptides?
    8 years ago
  • marcus isokoski
    Can a peptide chain be absorbed by the digestive system?
    8 years ago
  • neftalem
    How in oredr steps physiology iron absorption?
    7 years ago
  • tewelde
    Where are most amino acids absorbed in the digestive tract?
    7 years ago
  • courtney
    Are branch chained starches digested and absorbed?
    7 years ago
  • ANKE
    How is protein digestion completed in cells?
    7 years ago
  • juuso
    What are the products of digestion of proteins?
    7 years ago
  • Bo
    Where are most of the protein digesting enzymes produced?
    7 years ago
  • hanno
    Where is proteins digested and then absorbed into the blood within the digestive system?
    6 years ago
  • Millie
    Why do blood capillaries absorb amino acids?
    6 years ago
  • harding
    Where is protein digestion completed and absorbed?
    6 years ago
  • david
    What are the products of protein breakdown from the GIT that enter portal circulation.?
    3 years ago
  • Diamanda
    How does absorption of amino acids,dipepetides and tripeptides absorbed in the body?
    3 years ago
  • JAMES
    Does amino acids absorbed into portal circulation?
    3 years ago
  • Leon
    Why are most proteins resistant to digestion?
    3 years ago
  • albert
    Why do blood capilaries absorb amino acids?
    2 years ago
  • NICOLE
    How are proteins absorbed in the body?
    2 years ago
  • Edoardo
    How the proteins digestin the human body?
    2 years ago
  • lempi
    How is protein digested?
    2 years ago
  • TYYNE
    What happens to proteins within the hepatic portal system?
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  • kay
    How protein is being absorbed in the body?
    1 year ago
  • samuel craig
    How absorption of protein occurs in human body after digestio?
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  • edward
    Where does amino acid absorb in the stomach?
    1 year ago
  • luca
    How protien are digested and absorb in git?
    8 months ago
  • berylla chubb-baggins
    How are amino acids dipeptides and tripeptides absorbed into the blood from epithelial cells?
    6 months ago
  • awet
    How amino acids, dipeptides and tripeptides enter blood?
    3 months ago

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