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Carbohydrates Structure and Biological Function

Carbohydrates Structure and Biological Function. Monosaccharides Carbohydrates in Cyclic Structures Reactions of Glucose and Other Monosaccharides Polysaccharides Glycoproteins. Carbohydrates. Compounds containing C, H and O General formula : C x (H 2 O) y

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Carbohydrates Structure and Biological Function

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  1. CarbohydratesStructure and Biological Function • Monosaccharides • Carbohydrates in Cyclic Structures • Reactions of Glucose and Other Monosaccharides • Polysaccharides • Glycoproteins

  2. Carbohydrates • Compounds containing C, H and O • General formula : Cx(H2O)y • All have C=O and -OH functional groups. • Classified based on • Size of base carbon chain • Number of sugar units • Location of C=O • Stereochemistry

  3. Types of carbohydrates • Classifications based on number of sugar units in total chain. • Monosaccharides - single sugar unit • Disaccharides- two sugar units • Oligosaccharides - 2 to 10 sugar units • Polysaccharides - more than 10 units • Chaining relies on ‘bridging’ of oxygen atoms • glycoside bonds

  4. Monosaccharides • Based on location of C=O H | C=O | H-C-OH | H-C-OH | H-C-OH | CH2OH CH2OH | C=O | HO-C-H | H-C-OH | H-C-OH | CH2OH Aldose Ketose - aldehyde C=O - ketone C=O

  5. Monosaccharide classifications • Number of carbon atoms in the chain H | C=O | H-C-OH | H-C-OH | H-C-OH | H-C-OH | CH2OH H | C=O | H-C-OH | H-C-OH | H-C-OH | CH2OH H | C=O | H-C-OH | H-C-OH | CH2OH H | C=O | H-C-OH | CH2OH triose tetrose pentose hexose Can be either aldose or ketose sugar.

  6. Examples CH2OH | C=O | HO-C-H | H-C-OH | H-C-OH | CH2OH H | C=O | H-C-OH | CH2OH D-glyceraldehyde D-fructose triose hexose aldose ketose aldotriose sugar ketohexose sugar

  7. Examples H | C=O | H-C-OH | H-C-OH | HO-C-H | HO-C-H | CH2OH H | C=O | H-C-OH | H-C-OH | H-C-OH | CH2OH D-ribose L-mannose pentose, aldose hexose, aldose aldopentose sugar aldohexose sugar

  8. Stereoisomers • Stereochemistry • Study of the spatial arrangement of molecules. • Stereoisomers have • the same order and types of bonds. • different spatial arrangements. • different properties. • Many biologically important chemicals, like sugars, exist as stereoisomers. Your body can tell the difference.

  9. Enantiomers • Pairs of stereoisomers • Designated by D- or L- at the start of the name. • They are mirror images that can’t be overlapped. If you don’t believe it, give it a try!

  10. Enantiomers

  11. L- and D- glyceraldehyde

  12. Enantiomers • Chiral center. • Asymmetric carbon - 4 different things are attached to it. • Cl • | • I - C - F • | • Br • You must have at least one asymmetric carbon to have stereoisomers. Chiral center

  13. H H I Cl H3C- C-OH C=C C=O Br F Cl CH2CH3 H H H H H2N-C-C-C-SH H3C- C-OH H Cl Cl H Examples • Is the ‘red’ carbon chiral? H | C=O | H-C-OH | CH2OH

  14. Physical properties • Optical activity • ability to rotate plane polarized light. • dextrorotatory - rotate to right • - use + symbol • - usually D isomers • levorotatory - rotate to left • - use - symbol • - usually L isomers

  15. Plane polarized light Light is passed through a polarized filter. A solution of an optical isomer will rotate the light one direction.

  16. Stereochemistry • Properly drawing enantiomers in 3-D is hard. • UseFischer Projections • Specific type of formula that designates • the orientation of groups. H | C=O | H-C-OH | H-C-OH | CH2OH H | C=O | H-C-OH | CH2OH

  17. H O H OH H OH CH2OH Fischer projections • With this system, a tetrahedral carbon atom is represented by two crossed lines. • A horizontal bond to an asymmetric carbon designates bonds in the front plane of the page. • Vertical bonds are behind the plane of the page. H | C=O | H-C-OH | H-C-OH | CH2OH

  18. Some important monosaccharides • D-glyceraldehyde Simplest sugar • D-glucose Most important in diet • D-fructose Sweetest of all sugars • D-galactose Part of milk sugar • D-ribose Used in RNA • note that each is a D- enantiomer

  19. D-glyceraldehyde • Three carbon sugar • Aldose sugar • Triose sugar • aldotriose H | C=O | H-C-OH | CH2OH

  20. O H C H C OH HO C H H C OH H C OH CH OH 2 D-glucose • Glucose is an aldohexose sugar. • Common names include dextrose, grape sugar, blood sugar. • Most important sugar in our diet. • Most abundant organic compound found in nature. • Level in blood can be as high as 0.1%

  21. D-fructose • Another common sugar. • It is a ketohexose. • Sweetest of all sugars. CH2OH | C=O | HO-C-H | H-C-OH | H-C-OH | CH2OH

  22. Carbohydrates in cyclic structures • If optical isomers weren’t enough, sugars also form rings. For many sugars, its the most common form. • hemiacetal - forms from alcohol and aldehyde • hemiketal - forms from alcohol and ketone R OR’’ \ | C=O + ROH R - C - OH / | R’ R’

  23. CH2OH CH2OH H C C OH O O C C C C OH C C C C Intramolecular cyclization • Cyclization. • Remember - chains can bend and rotate.

  24. Intramolecular cyclization • The -OH group that forms can be above or below the ring resulting in two forms - anomers •  and  are used to identify the two forms. •  - OH group is down compared to CH2OH (trans). •  - OH group is up compared to CH2OH (cis).

  25. Intramolecular cyclization • The  and  forms are in equilibrium so one form can convert to the other - mutarotation. • Haworth projections can be used to help see  and  orientations.

  26. CH OH 2 H O H H H OH O H C OH OH H C OH OH H HO C H H C OH CH OH 2 H C OH O OH H H CH OH H OH 2 OH H H OH Cyclization of D-glucose -D - glucose - D - glucose

  27. Fischer vs. Haworth projections -D-glucose C OH H CH OH 2 H C OH H O H H O HO C H H OH H C OH OH OH HO-CH2 C H OH H

  28. Cyclization of D-fructose • This can also happen • to ketose sugars. CH2OH CH OH 2 O H OH  OH CH 2 H OH C O H OH HO C H H C OH C OH H OH CH OH 2 O CH OH H OH 2  H CH2OH H OH

  29. CH OH 2 CH OH 2 O OH OH H H OH CH OH 2 H H H OH D-galactose • Not found in many biological systems • Common part of lactose - disaccharide O H OH H H OH O H C OH H H C OH OH H HO C H HO C H H C OH

  30. O H H C C H C OH H C OH HO C H HO C H H C OH C H HO H C OH H C OH CH OH CH OH 2 2 D-glucose vs. D-galactose D-glucose D-galactose O Can you find a difference? Your body can! You can’t digest galactose - it must be converted to glucose first.

  31. D-ribose • An important sugar used • in genetic material. • This sugar is not used as • an energy source but is a part • of the backbone of RNA. • When the C-2 OH is removed, • the sugar becomes • deoxyribose which is used • in the backbone of DNA. H | C=O | H-C-OH | H-C-OH | H-C-OH | CH2OH

  32. Reactions of glucose and other monosaccharides • Oxidation-Reduction. Required for their complete metabolic breakdown. • Esterification. Production of phosphate esters. • Amino derivatives. Used to produce structural components and glycoprotein. • Glycoside formation. Linkage of monosaccharides to form polysaccharides.

  33. O- | C=O | + 2 Cu2O + 3H2O H-C-OH | CH2OH H | C=O | + 2 Cu 2+ + 5 OH- H-C-OH | CH2OH Oxidation-Reduction. • Aldehyde sugars (reducing sugars) are readily oxidized and will react with Benedict’s reagent. • This provides a good test for presence of glucose in urine - forms a red precipitate. • Other tests - Tollen’s or Fehling’s solutions.

  34. g l u c o s e 0 . 5 % 2 % B e n e d i c t ' s R e a g e n t Benedict’s reagent

  35. O H C CH2OH H C OH H C OH H C O C OH HO C H HO C H HO C H H C OH H C OH H C OH H C OH H C OH H C OH CH OH CH OH CH OH 2 2 2 Ketone sugars • Ketones are not easy to oxidize except ketoses. • Enediol reaction. • So all monosaccharides are reducing sugars.

  36. Esterification • Esters are formed by reaction of hydroxyl groups (alcohols) with acids. • The hydroxyl groups of carbohydrates react similarly to alcohols.

  37. Esterification • The most important biological esters of carbohydrates are phosphate esters. • Example. Phosphoryl group from ATP forms an ester with D-glucose, catalyzed by kinases. • D-glucose + ATP D-glucose-6-phosphate + ADP kinase

  38. Amino derivatives • The replacement of a hydroxyl group on a carbohydrate results in an amino sugar. -D-glucose -D-2-aminoglucose (glucosamine)

  39. Amino derivatives • Uses for amino sugars. • Structural components of bacterial cell walls. • As a component of chitin, a polymer found in the exoskeleton of insects and crustaceans. • A major structural unit of chondroitin sulfate - a component of cartilage. • Component of glycoprotein and glycolipids.

  40. CH OH CH OH 2 2 O O OH H H H H H H H OH OH H OH OH OH H OH H OH CH OH 2 CH OH O H 2 H H O H H OH H o H OH OH H OH OH H H OH + H O 2 Glycoside formation •  or  -OH group of cyclic monosaccharide can form link with another one (or more). • glycosidic bond • sugar -O- sugar • oxygen bridge

  41. O O O O Glycosidic bonds Type is based on the position of the C-1 OH  glycosidic bond - linkage between a C-1  OH and a C-4 OH  glycosidic bond - linkage between a C-1  OH and a C-4 OH  bonds  bonds C-4 end can be either up or down depending on the orientation of the monosaccharide.

  42. O O O O Glycosidic bonds  bonds  bonds

  43. Glycosidic bonds General format used to describe bond. OH type carbon# of carbon# of ( or ) first sugar second sugar As we work through the next few examples this will become clear. For disaccharides - the sugar is either  or  based on form of the remaining C-1 OH. ( )

  44. CH CH OH OH 2 2 O O OH H H H H H H H OH OH O H OH OH OH H H -D-glucose -D-glucose -Maltose • Malt sugar. Not common in nature except in germinating grains. -D-glucose and -D-glucose,  (14) linkage.

  45. CH CH OH OH 2 2 O O OH H H H H H H H OH OH O H OH OH OH H H -Maltose • It is referred to as -maltose because the unreacted C-1 on -D-glucose is in the  position.

  46. -Maltose • Uses for -maltose • Ingredient in infant formulas. • Production of beer. • Flavoring - fresh baked aroma. • It is hydrolyzed the in body by: • maltose + H2O 2 glucose maltase

  47. Cellobiose • Like maltose, it is composed of two molecules of D-glucose - but with a  (1 4) linkage.

  48. CH CH OH OH 2 2 O O H OH H H H H H H OH OH O H OH OH OH H H Cellobiose The difference in the linkage results in cellobiose being unusable We lack an enzyme that can hydrolyze cellobiose. maltose,  (1 4) cellobiose  (1 4)

  49. CH CH OH OH 2 2 O O OH H OH H H O H H OH OH H H H OH OH H H -D-galactose -D-glucose  (1 4) linkage,  disaccharide. Lactose • Milk sugar - dimer of -D-galactose and either the  or  - D-glucose. • -Lactose

  50. Lactose • We can’t directly use galactose. It must be converted to a form of glucose. • Galactosemia - absence of needed enzymes needed for conversion. • Build up of galactose or a metabolite like dulcitol (galactitol) causes toxic effects. • Can lead to retardation, cataracts, death.

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