15. The Importance of Energy Changes and Electron Transfer...

16. CARBOHYDRATES

Carbohydrates

Types: Monosaccharides: Cn(H2O)n

Oligosaccharides: A few monosaccharides are linked. Polysaccharides: glycogen, starch, cellulose Roles in biochemistry 1) Major energy sources 2) Oligosaccharides: processes in cell surfaces cell-cell interaction, immune recognition 3) Polysaccharides: essential structural components

16.1 Sugars: Their Structures and Stereochemistry

◈ Monosaccharides

- Building blocks of all carbohydrates

- Simple sugars

- Polyhydroxy aldehyde (aldose) or polyhydroxy ketone (ketose)

- Aldose: aldotriose, aldotetrose, aldopentose, aldohexose, aldoheptose

- Ketose: ketotriose, ketotetrose, ketopentose, ketohexose, ketoheptose


◈ Optical isomers (stereoisomers)

- Enantiomers: mirror images

- Configuration: three dimensional arrangement of groups around a chiral carbon atom (D, L system and R, S system)


◈ Fischer-projection

- Convention of a two dimensional perspective of molecular structure

- By German chemist Emil Fischer

- Bonds written vertically represent bonds directed behind the paper.

- Bonds written horizontally represent bonds directed in front of the paper.


- Most highly oxidized carbon is written at the “top”.


- What are the possible stereoisomers for an aldotetrose ?


- Diastereomers:

nonsuperimposable, nonmirror-image stereoisomers

L-threose D- & L-erythrose

L-erythrose D- & L-threose

- Epimers: diastereomers differ from each other in configuration at only one chiral carbon

D-erythrose D-threose


◈ Some of possible isomers

- Aldopentose: three chiral carbons, then 23 stereoisomers (four D forms and four L forms).

- Aldohexose: four chiral carbons, then 24 stereoisomers (eight D forms and eight L forms).


Aldose

Ketose



Cyclic Structures: Anomers

◈ Anomers

- Five or six carbon atoms as cyclic molecules rather than as open-chain forms

- Result of interaction between functional groups on distant carbons

- Hemiacetal: interaction between C-1 and C-5, to cyclic form in aldose

- Hemiketal: interaction between C-2 and C-5, to cyclic form in ketose

- Anomeric carbon: a new chiral center carbon (α and β forms)


Cyclic Structures : Anomers



- Fischer projection of α-anomer and β-anomer of a D sugar



◈ Haworth projection formula

- Cyclic structures in perspective drawings

- Furanose: five-membered ring

- Pyranose: six-membered ring



- Pyranoses exist in solution in chair conformation. (cf. boat form)



- Useful shorthand

for structures

16.2 Reactions of Monosaccharides

Oxidation-Reduction Reactions

◈ Oxidation-Reduction Reactions

- Oxidation of sugars: provides energy when oxidized to CO2 and H2O in aerobic process

- Reverse of oxidation of sugars: reduction of CO2 and H2O in photosynthesis



◈ Oxidation reactions of sugars in lab

- Aldose: reducing sugars

- In cyclic form, compound produced by oxidation of aldose is a lactone.

- Vitamin C: an unsaturated lactone

Lactone



◈ Detecting presence of reducing sugars

- Tollens reagent: using Ag(NH3)2+, Silver is reduced.

- Glucose oxidase



◈ Deoxy sugars

- L-fucose: in ABO blood-group antigens

- D-2-deoxyribose: found in DNA



◈ Alditols

- When carbonyl group of a sugar is reduced to a hydroxyl group

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Esterification Reactions

◈ Hydroxyl groups of sugars

- Behave like all other alcohols

- React with acids and derivatives of acids to form esters

- Phosphate ester: intermediates in breakdown of carbohydrates


The Formation of Glycosides

◈ Glycosidic linkage (R’-O-R)

- Not an ether: Because glycosides can be hydrolyzed to alcohols.

- Glycosidic bond: Hemiacetal carbon can react with methyl alcohol to give a full acetal, or glycoside.

- Furanosides: glycosides from furanoses

- Pyranosides: glycosides from pyranoses



◈ Glycosidic bonds between monosaccharide unit

- α- or β-glycosidic linkage: Anomeric carbon of one sugar bonded to any of –OH groups on a second sugar.



Wrong configuration??



◈ Oligosaccharides and polysaccharides

- Chemical nature: which monosaccharides are linked, particular glycosidic bond formed.

- Some internal residues can form three glycosidic bonds to branched-chain structures.





◈ Test for a reducing sugar

- Only if the end residue is a free hemiacetal: can be positive

- Can be negative for not enough reducing ends to detect

Maltose


Other Derivatives of Sugars

◈ Amino sugars

- Compounds related to monosaccharides

- Amino groups (-NH2) or one of its derivatives is substituted for hydroxyl group of parent sugar.

Found in Bacterial walls

16.3 Some Important Oligosaccharides

◈ Disaccharides

- Linking two monosaccharide unit by glycosidic bonds

- Most important examples: sucrose, lactose, and maltose

- Other examples: Isomaltose and cellobiose


◈ Sucrose

- Common table sugar

- Extracted from sugarcane and sugar beets

- The α C-1 carbon of glucose is linked to β C-2 carbon of fructose.

- Not a reducing sugar

- Hydrolysed to glucose and fructose


◈ Other sweeteners

- Fructose: sweeter than sucrose, high-fructose corn syrup

- Saccharin: cause cancer in laboratory animals

- Aspartame (NutraSweet): cause neurological problems

Saccharin

http://gurugupta.files.wordpress.com/2007/07/diet-coke-english.jpg

http://www.licoriceinternational.com/store/images/fs/US40A.JPG


◈ Artificial sweetener: sucralose (Splenda: trade name)

- Derivative of sucrose

- Differences from sucrose

① Three of hydroxyl groups have been replaced with three chlorine atoms.

② Configuration at carbon atom number four of pyranose ring of glucose has been inverted.

- Not metabolized by the body

(no calories)

- Some report for its toxicity


◈ Lactose

- Made up of β-D-galactose and D-glucose

- Can be either the α or β form.

- a reducing sugar

http://www.dairy.com.au/consumers/images/dairyaustralia/agda_07_dairy_farmers_milk_.jpg


◈ Maltose

- From hydrolysis of starch

- Consist of two residues of D-glucose in an α(1 → 4) linkage

- Differ from cellobiose in the glycosidic linkage

- Mammals can digest maltose, but not cellobiose.

16.4 Structure and Functions of Polysaccharides

◈ Polysaccharide

- Many monosaccharides are linked together.

- Homopolysaccharide: only one type of monosaccharide

- Heteropolysaccharide: more than one type of monosaccharide

- Cellulose and chitin: β-glycosidic linkage, structural materials

- Starch and glycogen: α-glycosidic linkage, storage polymers

Potato starch

http://www.global-b2b-network.com/direct/dbimage/50353563/Potato.jpg


Cellulose and Starch

◈ Cellulose

- Major structural component of plants

- Linear homopolysaccharide of β-D-glucose, β(1→4) glycosidic bonds


Cellulose and Starch

◈ Starch

- Energy-storage polymers

- Polymer of α-D-glucose, α(1→4) glycosidic bonds

- In plant cells, as starch granules in cytosol


The Forms of Starch

◈ Types of starch

- Amylose: linear polymer of glucose, helix with 6 residues per turn

- Amylopectin: branched chain polymer, α(1→6) linkage


The Forms of Starch

- Test for starch:

starch-iodine complex

(dark-blue color)


The Forms of Starch

◈ Hydrolyzing starches

- α- and β-amylase: attack α(1 → 4) linkages

- β-amylase: exoglycosidase, cleave from nonreducing end, produce maltose

- α-amylase: endoglycosidase, hydrolyze anywhere, produce glucose and maltose

- Debranching enzymes: degrade α(1 → 6) linkages


Glycogen

◈ Glycogen

- Carbohydrate storage polymer in animals

- Similar to amylopectin

- Glycogen (every 10 residues) is more highly branched than amylopectin (every 25 residues).


Glycogen

- Glycogen granules in well-fed liver and muscle cells

- Glycogen phosphorylase: cleaves one glucose from nonreducing end of a branch to produce glucose-1-phosphate

- Significance of the number of branch points

1) More branched polysaccharide is more water soluble.

2) More potential targets for quicker mobilization of glucose.

Glycogen in animal


Chitin

◈ Chitin

- Similar to cellulose in both structure and function

- Monomer is N-acetyl-β-D-glucosamine.

- Exoskeletons of invertebrates

- Cell walls of algae, fungi, and yeasts

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The Role of Polysaccharides in the Structure of Cell Walls

◈ <Bacteria Cell Walls>

- Heteropolysaccharides are major components.

- Two monomers: N-acetylmuramic acid, N-acetyl-D-glucosamine

- Polysaccharides are cross-linked by peptides.



- Cross-links in bacterial cell walls:

Oligomer is bound to

N-acetylmuramic acid,

forming a side chain.



- Tetrapeptide forms

two cross-links to

pentapeptide, (Gly)5.



- Peptidoglycan:

Result from cross-linking of

polysaccharides by peptides



◈ <Plant Cell Walls>

- Consist of largely cellulose

- Pectin: heteropolysaccharide made up mostly of D-galacturonic acid,

a gelling agent in yogurt, fruit preserves, jams, and jellies



- Lignin: major nonpolysaccharide component in plant cell walls

a polymer of coniferyl alcohol


Glycosaminoglycans

◈ Glycosaminoglycans

- Repeating disaccharide

- A variety of cellular functions and tissues

- Heparin: natural anticoagulant preventing blood clots


Glycosaminoglycans

16.5 Glycoproteins

◈ Glycoproteins

- Carbohydrate residues in addition to polypeptide chain

- Important examples: antibodies, antigenic determinants in erythrocyte

- Human blood groups: A, B, AB, and O types

16.5 Glycoproteins

◈ Proteoglycans

- Glycoproteins with extremely high carbohydrate content (85%-95% by weight)

- Constantly being synthesized and broken down in eukaryotes

THE END!!

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