Tymoczko Berg Stryer Biochemistry: A Short Course Third
Tymoczko Berg Stryer Biochemistry: A Short Course
Third Edition CHAPTER 14
Digestion: Turning a Meal into Cellular Biochemicals
2015 W. H. Freeman and Company Chapter 14 Outline
The components of a mealproteins, lipids and polysaccharidesmust be degraded into small molecules for absorption and transport.
A diverse set of hydrolytic enzymes accomplish this degradation. 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. 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. 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.
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. -Amylase initiates digestion by cleaving -1,4 bonds, but not -1,6 bonds.
Other enzymes, including maltase, -glucosidase, and -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.
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.
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