Tymoczko • Berg • Stryer © 2015 W. H. Freeman and Company Biochemistry: A Short Course Third Edition CHAPTER 14 Digestion: Turning a Meal into Cellular Biochemicals © 2015 W. H. Freeman and Company

Growing requires vast amounts of energy and biochemical building blocks. These needs do not disappear as we age but are required to maintain our bodies against the wear and tear of living. The energy and building blocks come in the form of food, which must be converted into biochemicals in the process of digestion. [Stuart Pearce/Agefotostock.]

Chapter 14 Outline

The components of a meal—proteins, lipids and polysaccharides—must be degraded into small molecules for absorption and transport. A diverse set of hydrolytic enzymes accomplish this degradation.

Figure 14.1 Pizza. Foods provide a pleasurable means of obtaining energy and building blocks for biological systems. [Mode/Ian O’Leary/age fotostock.] 

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.

Digestion begins in the mouth where food is mechanically degraded Digestion begins in the mouth where food is mechanically degraded. Chewing converts the meal into a slurry that is more readily attacked by hydrolytic enzymes.

The highly acidic environment of the stomach denatures proteins, making them more susceptible to digestion by proteolytic enzymes, such as the stomach enzyme pepsin. The acid environment of the stomach is generated by a K+/H+ ATPase. Excessive acid generation may result in gastroesophagel reflux disease (GERD). GERD can be treated with inhibitors of the K+/H+ ATPase such as omeprazole.

The movement of food from the stomach to the intestine stimulates the secretion of two key hormones by cells of the small intestine. Secretin causes the release of sodium bicarbonate, which neutralizes 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.

Proteins are digested into amino acids and small oligopeptides 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 by antiporters.

Figure 14. 3 The hormonal control of digestion Figure 14.3 The hormonal control of digestion. Cholecystokinin (CCK) is secreted by specialized intestinal cells and causes the secretion of bile salts from the gall bladder and digestive enzymes from the pancreas. Secretin stimulates sodium bicarbonate (NaHCO3) secretion, which neutralizes the stomach acid. [After D. Randall, W. Burggren, and K. French, Eckert Animal Physiology, 5th ed. (W. H. Freeman and Company, 2002), p. 658.]

Figure 14. 4 The digestion and absorption of proteins Figure 14.4 The digestion and absorption of proteins. Protein digestion is primarily a result of the activity of enzymes secreted by the pancreas. Peptidases associated with the intestinal epithelium further digest proteins. The amino acids and di- and tripeptides are absorbed into the intestinal cells by specific transporters. Free amino acids are then released into the blood for use by other tissues.

Celiac disease, or gluten enteropathy, is an intestinal inflammatory disorder that results because susceptible individuals are unable to digest certain proteins from wheat, rye and barley. These proteins, which are rich in glutamine and proline, are referred to as gluten. The gluten-derivded peptides generate an inflammatory response that damages the intestinal lining and impairs nutrient absorption.

Our primary source of carbohydrates is starch. Several enzymes participate in carbohydrate digestion. 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 disaccharides, are digested by sucrase and lactase, respectively. Glucose and galactose are transported into the intestine by the sodium-glucose linked transporter and the transporter GLUT5 allows entry of fructose.

Figure 14. 5 The digestion of starch by -amylase Figure 14.5 The digestion of starch by -amylase. Amylase hydrolyzes starch into simple sugars. The α-1,4 bonds are shown in green. The α-1,6 bonds are red. The sites of α-amylase digestion are indicated by the small green arrows.

Figure 14. 6 Uptake of monosaccharides Figure 14.6 Uptake of monosaccharides. The results of carbohydrate digestion, primarily glucose, galactose, and fructose, are transported into the intestinal cells by specific transport proteins. The carbohydrates also exit the cell with the assistance of transport proteins.

Triacylglycerols from the diet form lipid droplets in the stomach 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 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.

Figure 14.7 Glycocholate. Bile salts, such as glycocholate, facilitate lipid digestion in the intestine.

Figure 14. 8 The action of pancreatic lipases Figure 14.8 The action of pancreatic lipases. Lipases secreted by the pancreas convert triacylglycerols into fatty acids and monoacylglycerol for absorption into the intestine.

Figure 14. 9 A diagram of a section of a micelle Figure 14.9 A diagram of a section of a micelle. Ionized fatty acids generated by the action of lipases readily form micelles.

Figure 14. 10 Chylomicron formation Figure 14.10 Chylomicron formation. Free fatty acids and monoacylglycerols are absorbed by intestinal epithelial cells. Triacylglycerols are resynthesized and packaged with other lipids and proteins to form chylomicrons, which are then released into the lymph system.