Digestion of Carbohydrates Biochemistry 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