Digestion and absorption of carbohydrates

1. Digestion and absorption of carbohydrates

2. Definition: carbohydrates are polyhydroxi aldehydes or polyhydroxi ketones and their polymers monosaccharides: 3,4,5,6,7 C-atom containig sweet sugars oligosaccharides: contain 2-10 monosaccharides polysaccharides: contain more than 10 monosaccharides: same units – homopolysacch. different units – heteropolysacch. small molecules are hydrophilic, water soluble big molecules form colloid solutions, absorb water

3. Importance of carohydrates glucose is the preferable food for the creatures, formed during photosynthesis it is stored in polymer form it is degraded to yield energy plants synthesize many other sugars sugar and sugar derivative polymers form the cell wall carbohydrates bind to membrane lipids = sphingolipids to proteins: glycoproteins, proteoglycans found in nucleotides and nucleic acids: ribose carbohydrates change the characteristic of their complex: molecular weight increased stability increased, protecs from enzyme digestion viscosity increased take part in molecule interaction, receptors can recognize water solubility increased

4. Digestive enzymes salivary glands: α-amylase= ptyalin digest starch= amylum, glycogen pancreas between liver and stomach: α-amylase small intestine: oligo-1,6-glycosidase exo-1,4-glycosidase maltase saccharase lactase trehalse

5. Structure of glycogen G = glycogenin protein in middle it needs for beginning of synthesis glycogen has 12 layers (not 5). There are many free nonreducing ends, where snythesis and degradation occurs.

6. Homopolysaccharides from D-glucose glycogen: in animals and men storage of glucose molecular weight: 270 000-1 million main chain : α1,4-glycosidic bonds between D-glucose branching: α-1,6-glycosidic bonds in 8-12 glucose units starch: in plants storage of glucose straight chain amylose: α1,4-glycosidic bonds between D-glucose branched chain amylopectin: α-1,6-glycosidic bonds in 24-30 glucose units dextran: in bacteria and fungi storage of glucose main chain: α-1,6-glycosidic bonds branching:α1,2 α1,3 α1,4-glycosidic bonds Function of starch, glycogen, dextran: synthesis and degradation of glucose can be fast by many enzymes at the same time to maintain glucose level polymer form does not increase osmotic pressure, but the cell is capable to store high amount of glucose

7. digestible and usable poly- and oligosaccharides by men: glycogen, saccharose= cane sugar = sugar-beet sugar (maple syrup) lactose = milk sugar trehalose = disaccharide of mushroom plant originating polysaccharides that are partially digested by our enzymes: starch plant originating polysaccharides that are partially digested by our gut bacteria: not absorbed fructose galatoside oligosaccharide of legumes pectin: gelous material of riped fruits, used in jams hemocellulose: cell wall component inulin: fructose polymer plant polysaccharides not digested by man: cellulose: β1,4-D-glucose straigh chain homopolymer chitin: from fungi (and insectides) mannans (lignin: not carbohydrate cell wall componenet of woody parts)

8. Importance of not or partially digested dietary fibers All the carbohydrate fibers are hydrophilic, bind high amount of water, soften the content of gut, they are laxatives. They press the intestinal wall, increase the peristaltic movement, decrease the transit time in gut, acting against constipation. Fibers slow down the digestion, inhibit the absorption of glucose, which is favorable in diabetic patients, inhibit the absorption of cholesterol, adventiogous in hypercholesterolemic patients Fibers bind many compounds, including harmful ones, helping elimination of poisons. Because of above mentioned effects the fibers has antitumor effect, they protect from infection, inflammation, formation of diverticuli (enlargement with thin gut wall).

9. Absorption of monosaccharides 1.) SGLT1,2 = sodium-dependent glucose transporters: symporters help the secondary active transport of sugars against concentration gradient together to the same direction with Na+, that goes along its gradient Na+ conc. gradient is maintained by Na+/K+ATPase, that uses ATP found in intestine apical membrane and kidney tubules 2.) GLUT 1-12: glucose transporters: uniporters facilitated diffusion: protein helps the transport of hydrophilic molecule through the lipid bilayer along the concentration gradient does not need energy, increase the randomness different in localization, substrate specificity, kinetics several kinds are found on the same cell’s membrane, usually

10. jejunum lumen filtrate blood blood In ileum and colon GLUT-7 helps the absorption of glucose and fructose.

11. Secretion of insulin from pancreas β-cells GLUT-2 has the highest half saturation sugar concentration, work only after meal, can work by the same efficiency to both direction (the others only one way) found in pancreas, liver, gut, kidney, where the glucose concentration is higher than in other places

12. Skeletal muscle and adipocyte GLUT-4 is translocated to plasma membranes by the effect of insulin or contraction in muscle

13. Brain GLUT-1 is everywhere in endothel, in red blood cell GLUT-3 has the lowest KM, the highest affinity for glucose, found in neuron, spermatozoa, activated platelet

14. GLUT KM (glu) transport localization remark • 3 mM glu,gal,man vvt, endothel basal glu-uptake • 17 glu, gal, man, fru liver, pancreas β-sejt, biggest KM • glukosamin gut, kidney • 1.4 glu,gal,man,xyl neuron, testis, thromboc. lowest KM d.ascorbate • 5 glu, glucosamin muscle, adipoc., heart insulin-dependent d.ascorbate • 6 fru jejunum, kidney, muscle, only fructose adipoc., microglia • 5 glu brain, leukocyte, spleen (GLUT-9 volt) • high aff. glu, fru jejunum, colon • 2 glu, fru, gal testis, cerebellum, liver, (GLUTX1 volt) brown adip., spleen, lung mainly intracell. • ? glu, fru kidney, liver • ? glu,gal liver, pancreas, heart, lung brain, placenta, kidney • ? glu, fru most places (not in liver) • ? glu, fru heart, prostata