BIOCHEMISTRY TRANS - CARBOHYDRATES - DIGESTION OF CARBOHYDRATES.pdf

1A BIOCHEMISTRY DIGESTION of CARBOHYDRATES DR. BRENDO JANDOC begins...

1A BIOCHEMISTRY DIGESTION of CARBOHYDRATES DR. BRENDO JANDOC begins in the mouth I. DIGESTION OF CARBOHYDRATES OVERVIEW  Starch becomes hydrated when heated that is why cooking is an important digestive and absorptive aid  Major dietary carbohydrates are Starch, Sucrose, and Lactose  Digestive tract enzymes metabolize carbohydrates to  High Acid Stomach inactivates alpha-amylase and it temporarily stops monosaccharides carbohydrate digestion  generally completed by the time stomach contents reach the duodenujejunal junction Further Carbohydrate Digestion by Pancreatic Enzymes Occurs in the  Principal Sites of Dietary Carbohydrate Digestion are Mouth Small Intestines and Intestinal lumen  Enzymes associated with the brush border of intestinal  Acidic stomach contents reach small intestines and are neutralized by epithelial cells digest sucrose, lactose, and the bicarbonate products generated from starch by α- amylase  Pancreatic alpha-amylase  Endoglycosidases break down oligosaccharides  continues digestion and polysaccharides  attacks alpha-1,4 linkages  Glycosidases hydrolyze glycosidic bonds  does not attack alpha-1,6 linkages  Glycosidases are specific for the structure and configuration of  Result of Action the glycosyl to be removed and the type of bond to be broken  Maltose  Final products of Carbohydrate digestion are D-glucose, D-  Maltotriose galactose & D-fructose  Limit Dextrins  Final products are absorbed by intestinal epithelial cells and enter the blood Digestion of Carbohydrates Begins in the Mouth Final Carbohydrate Digestion by Enzymes Synthesized by the Intestinal Mucosal Cells  occurs in the mucosal lining of the upper jejunum declining as they proceed down the small intestine  Disaccharidases  Isomaltase cleaves the α (1 6) bond in isomaltose producing glucose  Maltase cleaves maltose producing glucose  Sucrase cleaves sucrose producing glucose + fructose  Lactase (β-Galactosidase) cleaves lactose producing galactose + glucose  Oligosaccharidases  complete the hydrolysis of disaccharides and oligosaccharides on small intestinal epithelial cell surfaces  remove successive units from the nonreducing ends (ends  Glycogen and Starch are Major Dietary Polysaccharides opposite the aldehyde or ketone groups)  Salivary alpha-Amylase (Ptyalin) is used during mastication  α-Glucosidases occur in excess in human small intestines  Carbohydrates is the only dietary component which breakage  β-Glucosidases are required for lactose hydrolysis  1A BIOCHEMISTRY DIGESTION of CARBOHYDRATES DR. BRENDO JANDOC Monosaccharide Absorption by Intestinal Mucosal Cells  Duodenum, Upper Jejunum absorb the bulk of dietary sugars Abnormal Disaccharide Degradation  D-Galactose, D-Glucose, D-Fructose  are transported into mucosal cells by an active,  Specific disaccharidase deficiency energy-requiring process  causes disaccharides not degraded to monosaccharides  involves specific transport protein (carrier-mediated resulting to passage of undigested carbohydrates in the process) large intestine creating osmotic effect thus water is drawn  requires concurrent uptake of Na+ into the large intestinal lumen producing osmotic diarrhea  leave the mucosal cells by facilitated transport and  reinforced by bacterial fermentation of remaining simple diffusion carbohydrates to 2- to 3-carbon compounds (osmotically  Two Transport Systems active) 1. Na+-Dependent Monosaccharide Cotransport System  large volumes of CO2 and H2 gas produce flatulence  specific for D-galactose, D-glucose  Defects  Phlorhizin is a plant glycoside that inhibits this  Hereditary Defects system  severe diarrhea (of any etiology) causing rapid 2. Na+-Independent Monosaccharide Transport System loss of brush border enzymes  transports D-fructose by facilitated diffusion  Generalized Defects  Cytochalasin B is derived from molds that  intestinal diseases inhibits this system  malnutrition  Monosaccharide Transport Out of Epithelial Cells  drugs that injure small intestinal mucosa  by Na+-independent transport system specific for D-  Lactose Intolerance galactose, D-glucose  > ½ of adults are lactose intolerant mostly (Blacks & Asians)  lactase deficiency  treatment: remove lactose from the diet or take lactase in pill form  Isomaltase-Sucrase Deficiency  sucrose intolerance  tx: remove sucrose from the diet  Diagnosis  Oral Tolerance Test  with the specific disaccharides  H2 Gas Measurement in the Breath  determine the amount of ingested carbohydrate not absorbed by the body (metabolized by the intestinal flora)  1A BIOCHEMISTRY DIGESTION of CARBOHYDRATES DR. BRENDO JANDOC  primary transporter of fructose in the small intestines & testes  GLUT-7  mediates glucose flux across the endoplasmic reticular membrane TRANSPORT of GLUCOSE into CELLS  Na+-Monosaccharide Cotransport System  Na+-Independent Facilitated Diffusion Transport  family of at least 14 glucose transporters (GLUT-1 to GLUT-14)  exist in 2 conformational states  extracellular glucose binds to transporter causing altered transporter conformation & transport of glucose  from area of high glucose (plasma) to area of low glucose level (cells)  Tissue Specificity of GLUT Gene Expression  GLUT-1  abundant in RBCs & brain  low in muscles  GLUT-2  energy-requiring process  found in liver, kidney & β cells of the  transport glucose against concentration gradient pancreas  movement of glucose is coupled to the  GLUT-3 concentration gradient of Na+ (carrier-mediated  primary glucose transporter in neurons process)  GLUT-4  occurs in intestinal epithelial cells, renal tubules,  abundant in adipose tissues, skeletal choroid plexus muscles  number and activity are increased by insulin  GLUT-7  expressed in the liver & other gluconeogenic tissues  Specialized Functions of GLUT Isoforms  glucose movement from area of high glucose (plasma) to area of low glucose level (cells)  GLUT-1, 3, and 4  glucose uptake from the blood  GLUT-2  transport glucose from blood to the cells when blood glucose levels are high  transport glucose from cells to the blood when blood glucose levels are low  GLUT-5