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.

The Digestive System

■ 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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