By F. Ressel. Ohio Dominican University.
TREHALASE complexes) Trehalase is only half as long as the other disaccharidases and has only one catalytic site floxin 200 mg fast delivery infection from root canal. It hydrolyzes the glycosidic bond in trehalose purchase floxin 400mg on-line antibiotics for uti augmentin, a disaccharide composed of two Absorptive cell glucosyl units linked by an -bond between their anomeric carbons (Fig. Tre- halose, which is found in insects, algae, mushrooms, and other fungi, is not cur- Basement rently a major dietary component in the United States. However, unwitting con- membrane sumption of trehalose can cause nausea, vomiting, and other symptoms of severe gastrointestinal distress if consumed by an individual deficient in the enzyme. Tre- Capillary halase deficiency was discovered when a woman became very sick after eating mushrooms and was initially thought to have -amanitin poisoning. Location of disaccharide complexes in intestinal villi. However, its pri- mary structure is very different from the other enzymes, and it is attached to the membrane through its carboxyl end by a phosphatidylglycan anchor (see Fig. The lactase catalytic site hydrolyzes the -bond connecting glucose and galactose in lactose (a -galactosidase activity; Fig. This polysaccharide is cellu- lose, which contains -1,4 glyco- alytic site in humans is the -bond between glucose or galactose and ceramide in sidic bonds. Pancreatic and sali- glycolipids (this catalytic site is sometimes called phlorizin hydrolase, named for its vary -amylase cleave only -1,4 ability to hydrolyze an artificial substrate). LOCATION WITHIN THE INTESTINE Bonds (1) and (3) would first be The production of maltose, maltotriose, and limit dextrins by pancreatic -amylase hydrolyzed by glucoamylase. Bonds (4) and (5) could then be hydrolyzed Sucrase–isomaltase activity is highest in the jejunum, where the enzymes can by the sucrase–isomaltase complex, or by hydrolyze sucrose and the products of starch digestion. Glucoamylase activity progressively increases along convert maltotriose and maltose to glucose. Thus, it CHAPTER 27 / DIGESTION, ABSORPTION, AND TRANSPORT OF CARBOHYDRATES 499 C sucrase Maltose α–1,4 bond C N O O HO O OH maltase isomaltase activity 1 2 O O O reducing HO O O end Maltotriose Connecting segment (stalk) Fig. Glucoamy- lase is an -1,4 exoglycosidase, which initiates Transmembrane cleavage at the nonreducing end of the sugar. The major portion of the sucrase-isomaltase complex, containing the catalytic sites, protrudes from the absorptive cells into the lumen of the intestine. Other domains of the protein form a connecting segment (stalk), and an anchoring segment that extends through the mem- brane into the cell. The complex is synthesized as a single polypeptide chain that is split into its α–1,6 bond two enzyme subunits extracellularly. Each subunit is a domain with a catalytic site (sucrase- O maltase) and isomaltase-maltase sites. In spite of their maltase activity, these catalytic sites are HO OH often called just sucrase and isomaltase. Trehalose 6 CH2OH H OH Lactose 5 O 2 3 H H H OH H H CH OH CH OH 4 1 1 6 4 2 β–1,4 2 HOH C O O HO OH H O 2 OH bond 3 2 5 HO OH O O H OH H OH OH Glucose Trehalase Glucose activity OH lactase OH Galactose Glucose Fig. This disaccharide con- tains two glucose moieties linked by an unusual Fig. It cleaves the -galactoside lactose, bond that joins their anomeric carbons. It is the major sugar in milk, forming galactose and glucose. Metabolism of Sugars by Colonic Bacteria Not all of the starch ingested as part of foods is normally digested in the small intestine (Fig. Dietary fiber and undigested sugars also enter the colon. Here colonic bacteria rapidly metabolize the saccharides, forming gases, short-chain fatty acids, and lactate. The major short-chain fatty acids formed are acetic acid (two carbon), propionic acid (three carbon), and butyric acid (four carbon). The short-chain fatty acids are absorbed by the colonic mucosal cells and can provide a sub- stantial source of energy for these cells. The major gases formed are hydrogen gas (H2), carbon dioxide (CO2), and methane (CH4). These gases are released through the colon, resulting in flatulence, or in the breath.
Hypoproteinemia may lead to edema because of a decrease in the protein-mediated osmotic pressure in the blood purchase floxin 200mg without prescription treatment for dogs back legs. This cheap floxin 400 mg visa bacteria found in urine, in turn, causes plasma water to leave the circulation and enter (and expand) the interstitial space, causing edema. Most circulating plasma proteins are synthesized by the liver. Therefore, the hepatocyte has a well-developed endoplasmic reticulum, Golgi system, and cellular cytoskeleton, all of which function in the synthesis, processing, and secretion of proteins. The most abundant plasma protein produced by the liver is albumin, which represents 55 to 60% of the total plasma protein pool. Albumin serves as a carrier for a large number of hydrophobic compounds, such as fatty acids, steroids, hydrophobic amino acids, vitamins, and pharmacologic agents. It is also an impor- tant osmotic regulator in the maintenance of normal plasma osmotic pressure. The other proteins synthesized by the liver are, for the most part, glycoproteins. They function in hemostasis, transport, protease inhibition, and ligand binding, as well as secretogogues for hormone release. The acute phase proteins that are part of the immune response and the body’s response to many forms of “injury” are also syn- thesized in the liver. Cirrhosis of the liver results in por- tal hypertension, which because of I. The Synthesis of Glycoproteins and Proteoglycans increasing back pressure into the esophageal veins promotes the develop- The liver, because it is the site of synthesis of most of the blood proteins (including ment of dilated thin-walled esophageal the glycoproteins), has a high requirement for the sugars that go into the oligosac- veins (varices). At the same time, synthesis charide portion of glycoproteins (The synthesis of glycoproteins is discussed in of blood coagulation proteins by the liver Chapter 30. These include mannose, fructose, galactose, and amino sugars. When the either dietary glucose or hepatic glucose to generate the precursor intermediates for esophageal varices rupture, massive bleed- these pathways. This is because the liver can generate carbohydrates from dietary ing into the thoracic or abdominal cavity as amino acids (which enter gluconeogenesis generally as pyruvate or an intermediate well as the stomach may occur. Much of the of the TCA cycle), lactate (generated from anaerobic glycolysis in other tissues), protein content of the blood entering the and glycerol (generated by the release of free fatty acids from the adipocyte). Of gastrointestinal tract is metabolized by intes- course, if dietary carbohydrate is available, the liver can use that source as well. Because hepa- attached to the protein at its anomeric carbon through a glucosidic link to the –OH tocellular function has been compromised, of a serine or a threonine residue. This is in contrast to the N-linked arrangement in the urea cycle capacity is inadequate, and which there is an N-glycosyl link to the amide nitrogen of an asparagine residue the ammonium ion enters the peripheral cir- (Fig. A particularly important O-linked sugar is N-acetylneuraminic acid culation, thereby contributing to hepatic encephalopathy (brain toxicity due to ele- (NANA or sialic acid), a nine-carbon sugar that is synthesized from fructose- vated ammonia levels). A Partial List of Proteins Synthesized in the Liver Type of Protein Examples Blood coagulation Blood coagulation factors: fibrinogen, prothrombin, Factors V, VII, IX and X. Metal-binding proteins Transferrin (iron), ceruloplasmin (copper), haptoglobin (heme), hemopexin (heme) Lipid transport Apoprotein B-100, apoprotein A-1 Protease inhibitor 1-Antitrypsin 852 SECTION EIGHT / TISSUE METABOLISM A. N-linked O CH2 HOCH2 O C O O O C O HO H H H H NH C CH2 CH O 2 CH NH H OH H O OH H H NH H NH H NH C O C O CH3 CH3 GalNAc Serine GlcNAc Asparagine Fig. The general configuration of O-linked and N-linked glycoproteins. NANA (sialic acid) residues are lost from the serum proteins. This change signals their removal from the circulation and their eventual degradation. An asialoglyco- protein receptor on the liver cell surface binds such proteins, and the receptor–- ligand complex is endocytosed and transported to the lysosomes. The amino acids from the degraded protein are then recycled within the liver.
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