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Biochemistry

Delve into the world of carbohydrates, from monosaccharides to polysaccharides, exploring structures, isomers, and chemical reactions. Learn about Fischer projections, cyclic structures, and important compounds like starch and cellulose.

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Biochemistry

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  1. Biochemistry Chapter 20 Carbohydrates

  2. Problem Sets • PS#1 • Sections 20.1 – 20.2 • 6, 7, 11, 13, 15, 17, 19, 21, 22, 26, 27, 28 • PS#2 • Sections 20.3 – 20.6 • 31, 34, 37, 39, 40, 41, 42, 44, 45, 48, 49, 50

  3. 20.1 Introduction • Carbohydrate – a polyhydroxyaldehyde or polyhydroxyketone, or a substance that produces these compounds on hydrolysis • Literally “hydrate of carbon” • Cn(H2O)m • Not all fit this formula • Often called saccharides • Mono-, oligo-, or polysaccharides

  4. 20.1 Monosaccharides • Carbohydrate that cannot be hydrolyzed to a simpler compound • General formula: CnH2nOn • One carbon is an aldehyde or ketone • Nomenclature • -ose suffix • Prefix for number of C’s (tri-, tetra-, penta-, etc.) • Aldose – contains an aldehyde group • Ketose – contains a ketone group • A ketopentose is a 5 C ketone monosaccharide

  5. 20.1 Fischer Projections • 2D representation of carbohydrates • Carbon skeleton drawn as a vertical line • Horizontal lines show bonds attached to the carbons D-Glucose An aldohexose

  6. 20.1 Stereoisomerism in Carbohydrates Many carbohydrates contain a stereocenter and can exist as a pair of enantiomers D-Glyceraldehyde L-Glyceraldehyde A sugar’s penultimate (second last) carbon determines whether it is D or L

  7. 20.1 Amino Sugars • An –NH2 group replaces an –OH group • Example: D-Glucosamine

  8. 20.2 Cyclic Structure of Monosaccharides • Aldehyde/ketone group can react with –OH group at other end of molecule • Forms a hemiacetal ring

  9. 20.2 Haworth Projections A way of representing the cyclic structure of monosaccharides Anomeric carbon – new stereocenter created by forming the ring Furanose = five member ring Pyranose = six member ring Anomers: a = OH on anomeric carbon is opposite the terminal CH2OH b = OH on anomeric carbon is On same side as terminal CH2OH

  10. 20.2 Conformation Representations • Furanoses (5 member rings) are planar • Pyranoses (6 member rings) are not • Chair conformation b-D-Glucose

  11. 20.2 Mutarotation • A pure a or b enantiomer will revert to a racemic mixture in aqueous solution • Detected by change in polarized light rotation Equilibrium value = +52.7o

  12. 20.3 Formation of Glycosides Glycoside – carbohydrate in which the –OH group on its anomeric carbon is replaced by an –OR group Monosaccharide + alcohol  glycoside Name the R group attached to O, then the carbohydrate, final –e changed to –ide. Keep the enantiomeric notation! Ethyl a-D-glucoside

  13. 20.3 Reduction to Alditols Alditol – monosaccharide where the CH=O group is reduced to a CH2OH group Naming: change the –ose to –itol Often used as sugar substitutes for diabetics

  14. 20.3 Oxidation to Aldonic Acids Aldonic acid – monosaccharide where the CH=O group is oxidized to a COOH group (carboxylic acid) Naming: change the –ose to –onic acid (– ate for the salt form) A monosaccharide that does this is called a reducing sugar

  15. 20.3 Oxidation to Uronic Acids Uronic acid – monosaccharide where the OH group of carbon #6 is oxidized to a COOH group (carboxylic acid) Naming: change the –ose to –uronic acid Uronic acids are often used to detoxify and excrete toxins

  16. 20.3 Phosphoric Esters An OH group of a monosaccharide is replaced by a phosphate group (PO4) Naming: add “phosphate” prefixed by carbon number Important intermediates in metabolism of monosaccharides

  17. 20.4 Beyond Monosaccharides • Glycosidic bond – bond between the anomeric carbon of one monosaccharide and an –OH group of another • Disaccharide – two monosaccharide units joined by a glycosidic bond • Oligosaccharide – six to ten monosaccharide units • Polysaccharide – larger numbers of monosaccharide units

  18. 20.4 Sucrose

  19. 20.4 Sucrose • Table sugar • Glucose – fructose (a-1,2-glycosidic bond) • Nonreducing sugar

  20. 20.4 Lactose • Milk sugar • Galactose–glucose (b-1,4-glycosidic bond) • Reducing sugar (glucose can be oxidized)

  21. 20.4 Maltose • Malt sugar (barley juice) • 2 glucoses (a-1,4-glycosidic bond) • Reducing sugar (one glucose can be oxid)

  22. 20.5 Polysaccharides • Many monosaccharides bonded together • Glycosidic bonds • Can be linear or branched • 3 most important: • Starch • Glycogen • Cellulose

  23. 20.5 Starch • Used for energy storage by plants • Composed of D-glucose • Amylose • Continuous unbranched chains of up to 4000 units • Joined by a-1,4-glycosidic bonds • Amylopectin • Branched chains of up to 10 000 units • Joined by a-1,4-glycosidic bonds • Branch points started by a-1,6-glycosidic bonds

  24. 20.5 Glycogen • Used for energy storage by animals • Composed of D-glucose • Branched like amylopectin • Joined by a-1,4-glycosidic bonds • Branched by a-1,6-glycosidic bonds • Contains approximately 106 glucose units • Stored mainly in liver and muscle tissue

  25. 20.5 Cellulose • Used in plant cell walls • Composed of D-glucose • Linear chains of approximately 2200 units • Joined by b-1,4-glycosidic bonds • Molecules act like stiff rods, form insoluble fibers with high mechanical strength • Most animals can’t digest b-glycosidic bonds, but some bacteria can

  26. 20.6 Acidic Polysaccharides • Polysaccharides that contain carboxyl groups and/or sulfuric ester groups • Contain amino sugars • Also called glycosaminoglycans (GAGs) • Used to be called mucopolysaccharides • Play an important role in connective tissue • Examples: • Hyaluronic acid – found in synovial fluid in joints • Heparin – sulfonated; used as an anticoagulant

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