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Sugars

Sugars. Alice Skoumalová. Repetition: Glucose, fructose and galactose (structure) Optical isomerism - definition (D- and L-glucose) Formation of a cyclic glucose (anomers) Reducing sugars - definition The most important disaccharides (composition)

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Sugars

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  1. Sugars Alice Skoumalová

  2. Repetition: • Glucose, fructose and galactose (structure) • Optical isomerism - definition (D- and L-glucose) • Formation of a cyclic glucose (anomers) • Reducing sugars - definition • The most important disaccharides (composition) • The most important polysaccharides (structure)

  3. Important sugars in an organism: 1. Glucose 2. Glycogen 3. Glycoproteins 4. Glycolipids 5. Proteoglycans

  4. An introduction to carbohydrates: • a large class of naturally occurring polyhydroxy aldehydes and ketones • monosaccharides (simple sugars): from 3 to 7 carbons + one aldehyde or ketone functional group • chiral compounds (having right- or left-handedness with two different mirror-image forms) because they contain carbons bonded to four different groups An aldose A ketose Monosaccharides, disaccharides, polysaccharides Modified saccharides

  5. The D and L families of sugars: Enantiomers - „mirror images“ (rotate polarized light in opposite directions → optical activity) Fischer projection: a mirror D sugar→ the OH group on the chiral carbon farthest from the carbonyl group pointing to the right in a Fischer projection L sugar→ the OH group on the chiral carbon farthest from the carbonyl group pointing to the right in a Fischer projection 2n of possible stereoisomers (half that many pairs of enantiomers) n - the number of chiral carbons (e.g. glucose - 16 stereoisomers)

  6. The structure of glucose: • Mutarotation • change in rotation of plane-polarized light resulting from the equilibrium between cyclic anomers and the open-chain form of a sugar • Anomers • cyclic sugars that differ only in positions of substituents at the anomeric carbon

  7. Reactions of the monosaccharides:

  8. Reaction with oxidizing agents: Reducing sugars react in basic solution with a mild oxidizing agent In basic solutions, all monosaccharides are reducing sugars

  9. Reaction with alcohols (glycoside formation): Glycosidic bond → bond between the anomeric carbon of a monosaccharide and an -OR group Hydrolysis of a disaccharide (during digestion of carbohydrates)

  10. Important monosaccharides Aldoses: Pentoses • in RNA and NADH • in polysaccharides in the walls of plant cells Hexoses • „blood“ sugar (energy) • in cellulose and starch • in glycogen (as a source of energy in an organism) • in glycolipids and glycoproteins • in lactose (milk), glycolipids and glycoproteins • converted to glucose • galactosemia

  11. Ketoses: D-Ribulose D-Fructose • an intermediate in the pentose phoshate pathway • in fruit juices and in honey • in sucrose • converted to glucose Deoxyaldoses: 2-deoxy-D-ribose • in DNA

  12. Acetylated amino sugars: N-Acetyl-D-glucosamine N-Acetyl-D-galactosamine • in glycoproteins Acidic sugars: D-Glucuronic acid N-Acetylneuraminic acid (sialic) • in glycosaminoglycans in connective tissue • conjugation of bile acids • in glycoproteins

  13. Important disaccharides Sucrose Maltose • a breakdown product of the starch • cane sugar, beet sugar • non-reducing sugar Lactose • milk sugar (4,5% - 7%) • lactose intolerance (reduced activity of lactase)

  14. Important polysaccharides • Cellulose • ß-D-Glucose, ß-1,4 link • the fibrous substance that provides structure in plants • humans cannot hydrolyze cellulose

  15. Starch • α-D-Glucose • source of energy in plants • fully digestible - an essential part of the human diet (the grains wheat, potatoes, rice) • 1. Amylose (20%, soluble in water) • α-1,4 link • 2. Amylopectin (80%, not water soluble) • α-1,6 branches (every 25 units)

  16. Glycogen • α-D-Glukose, α-1,4 and α-1,6 link • source of energy in animals (liver, muscles)

  17. Other important sugars 1. Glycosaminoglycans • Hyaluronic acid • 25,000 disaccharide units • form very viscous mixture • in connective tissue, synovial fluid, vitreous humour • Chondroitin sulfate • in tendons and cartilage Glucuronic acid N-Acetylgalactosamine sulfate N-Acetylglucosamine Glucuronic acid

  18. Heparin • contains sulfate groups (negative charges) • anticoagulant - prevents the clotting of blood (binds to the clotting factors)

  19. 2. Proteoglycans • aggregates of proteins and glycosaminoglycans in the extracellular matrix • highly hydrated and resilient (cartilage)

  20. 3. Glycoproteins • on the surfaces of cells (receptors, blood type) Forms:

  21. Blood type • cell-surface carbohydrates

  22. Summary: • Epimers, enantiomers, anomers (examples) • The most important reaction of monosaccharides • The importance of glucose • Important disaccharides • Important polysaccharides • Glycosaminoglycans and proteoglycans - structure, function • Glycoproteins - structure, function

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