Biochemistry: A Short Course Second Edition Tymoczko Berg Stryer © 2013 W. H. Freeman and Company CHAPTER 14 Digestion: Turning a Meal into Cellular Biochemicals

The acid environment of the stomach denatures proteins, rendering them more susceptible to proteolytic digestion in the intestine. The stomach protease pepsin begins the digestion of proteins. As the partially digested food moves into the small intestine, the hormone secretin causes the pancreas to secrete sodium bicarbonate to neutralize the stomach acid. Cholecystokinin (CCK) stimulates the release of digestive enzymes from the pancreas as well as the secretion of bile salts from the gall bladder. Bile salts facilitate the digestion of fats.

The enzymes of the pancreas are secreted as precursors called proenzymes or zymogens. Enteropeptidase, secreted by intestinal cells, converts inactive trypsinogen into active trypsin. Trypsin, in turn, activates the other proenzymes. Proteins are digested into amino acids and small oligopeptides. The amino acids are absorbed by transporters. Peptidases on the surface of intestinal cells cleave the oligopeptides into di- and tripeptides, which are transported into the intestinal cells and degraded into amino acids. The amino acids are subsequently released into the blood.

Our primary source of carbohydrates is starch. α-Amylase initiates digestion by cleaving α-1,4 bonds, but not α-1,6 bonds. Other enzymes, including maltase, α-glucosidase, and α-dextrinase complete the digestion. Sucrose and lactose, two common disacharides, are digested by sucrase and lactase, respectively.

Lipases, secreted by the pancreas, convert the triacylglycerols into 2 fatty acids and monoacylglycerol. The digestion products are carried as micelles to the intestinal epithelium cells for absorption. In the intestine, triacylglycerols are reformed from free fatty acids and monoacylglycerol and packaged into lipoprotein particles called chylomicrons. The chylomicrons eventually enter the blood so that the triacylglycerols can be absorbed by tissues.

Triacylglycerols from the diet form lipid droplets in the stomach. Bile salts, secreted by the gall bladder, insert into the lipid droplets, rendering them more accessible to digestion by lipases.

Lipases, secreted by the pancreas, convert triacylglycerols into 2 fatty acids and monoacylglycerol. The digestion products are carried as micelles to the intestinal epithelium cells for absorption. In the intestine, triacylglycerols are reformed from free fatty acids and monoacylglycerol and packaged into lipoprotein particles called chylomicrons. The chylomicrons eventually enter the blood so that the triacylglycerols can be absorbed by tissues.

Signals secreted by the gastrointestinal tract induce short-term satiation. Cholecystokinin (CCK), in addition to facilitating digestion, binds to G-coupled receptor proteins in the nervous system to generate feelings of satiety. Glucagon-like peptide 1 (GLP-1), secreted by intestinal cells, also generates feelings of satiety as well as potentiating insulin action. Leptin is secreted by adipocytes in direct proportion to fat stores and acts in the brain. Insulin, secreted by the β cells of the pancreas, reports on the status of carbohydrate availability.

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