Carbohydrates 2 Updated

7/30/2019 Carbohydrates 2 Updated

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CARBOHYDRATES

Classification

A. Monosaccharides consist of a single polyhydroxyaldehyde or ketone unit;may be aldoses or ketoses

B. Oligosaccharides compounds formed when a few (2 to 10) monosaccharideunits are linked together

C. Polysaccharides formed when many monosaccharide units are bondedtogether in linear or branched chains

A. Monosaccharides

StructureFischer projection structuresHaworth projection formulas

Stereochemistry

All monosaccharides (except dihydroxyacetone) contain one or more chiralcenters and therefore can exist in two different isomeric forms(stereoisomers) that have different configurations in space.

Configuration denotes the spatial arrangement of an organic molecule that is

conferred by the presence of either double bonds or a chiral center.

A molecule with n chiral centers can have 2n stereoisomers.

Enantiomers are stereoisomers that are nonsuperimposable mirror images of

each other. Enantiomers differ in their ability to rotate the plane of polarizedlight (optical activity).

Diastereomers are nonsuperimposable, non-mirror image stereoisomers.

Diastereomers that differ from each other in the configuration at only one

chiral carbon are called epimers. Most of the important sugars found in nature have the D configuration.

Reactions

1. Oxidation-reduction(a) Oxidation

C

OO

C OH

aldose aldonic acid (mild oxidizing agent)

aldose aldaric acid (strong oxidizing agent)

aldose alduronic acid

Reducing sugars undergo oxidation reactions.


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(b) Reduction

C

O

C

H

H

OH

aldose alditol

C OH C H

aldose deoxysugar

2. Formation of glycosides

Sugar OH + ROH Sugar O R + H2O

Hemiacetal + alcohol acetal or glycoside + H2O

(anomeric C)The newly formed bond is called a glycosidic bond.

Examples:

-D-glucopyranose + CH3OH methyl--D-glucopyranose + H2O

-D-glucopyranose + -D-glucopyranose

-D-glucopyranosyl-(14)--D-glucopyranose + H2O

3. Esterification

Sugar OH + acid (or acid derivative) esters

Example:

-D-glucose + ATP -D-glucose phosphate + ADP

4. Other derivatives of sugars: amino sugars

Example: N-acetyl--D-glucosamine


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B. Oligosaccharides

- compounds formed by linking 2 to 10 monosaccharide units byglycosidic bonds

Examples:1. sucrose

-D-fructofuranosyl-(21)--D-glucopyranose or-D-glucopyranosyl-(12)--D-fructofuranose

(12) linkage or (11) linkage

table sugar

non-reducing sugar

2. lactose

-D-galactopyranosyl-(14)--D-glucopyranose

may be or form

milk sugar reducing sugar

3. maltose

-D-glucopyranosyl-(14)--D-glucopyranose

may be or form

disaccharide obtained from hydrolysis of starch

reducing sugar

4. cellobiose

-D-glucopyranosyl-(14)--D-glucopyranose

may be or form disaccharide obtained from hydrolysis of cellulose

reducing sugar

5. Others

Trehalose: -D-glucopyranosyl-(11)--D-glucopyranose

Raffinose: -D-galactopyranosyl-(16)- -D-glucopyranosyl-(12)--D-

fructofuranose

Melizitose: -D-glucopyranosyl-(13)--D-fructofuranosyl-(21)--D-

glucopyranose

C. Polysaccharides

- formed when many monosaccharide units are bonded together byglycosidic linkages in long, linear or branched chains.

- a polymer that consists of one type of monosaccharide is ahomopolysaccharide; one that consists of more than one type ofmonosaccharide is a heteropolysaccharide


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- some serve as structural materials; others as carbohydrate storagepolymers

Examples:

1. Cellulose major structural component of plants

homopolysaccharide of-D-glucose

linkage (14)

individual chains are hydrogen-bonded together giving plant fibers their

mechanical strength

2. Starch

serve as vehicle for storage of glucose in plants

homopolysaccharide of-D-glucose

consists of 2 types:

amylose is a linear polymer of glucose, with all residues linked

together by -(14) bonds;

its most usual conformation is a helix with 6 residues per turnamylopectin is a branched chain polymer, with the branches starting at

-(16) linkages along the chain of-(14)

linkages; branch points occur about every 25 residues

amylose


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amylopectin

Enzymatic hydrolysis of starch (to glucose)

-amylase is an endoglycosidase which can hydrolyze -(14) linkages

anywhere along the chain to produce glucose and maltose

-amylase is an exoglycosidase that cleaves -(14) linkages from the

nonreducing end of the polymer.

Debranching enzymes degrade the -(16) linkages

3. Glycogen

carbohydrate storage polymer in animals (found in liver and muscle cells)

branched-chain polymer of-D-glucose

like amylopectin, consists of-(14) linkages with -(16) linkages at the

branch points

branch points occur about every 10 residues (glycogen is more highly

branched than amylopectin); average chain length is about 13 glucoseresidues and there are 12 layers of branching

a more branched polysaccharide is more water-soluble and will have more

potential targets for attack by hydrolytic enzymes

4. Chitin

structural component of exoskeletons of invertebrates (also in cell walls of

algae, fungi and yeasts)


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homopolysaccharide of N-acetyl--D-glucosamine

residues are linked by -(14) glycosidic bonds

5. Glycosaminoglycans

polysaccharides based on a repeating disaccharide in which one of the sugars

is an amino sugar and at least one of them has a negative charge

these molecules function as components in connective tissue (chondroitin

and keratan sulfates), lubricating fluid of joints (hyaluronic acid) etc.

Blood group antigenic determinants

-N-acetylgalactosamine-(13)--galactose-(13)- -N-acetylgalactosamine

2

1

L-fucose

A blood group antigen

-galactose-(13)--galactose-(13)- -N-acetylgalactosamine2

1

L-fucose


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B blood group antigen

Glycosaminoglycan Monsaccharidecomponents

(linkage)

Function

Chondroitin-4-sulfate D-glucuronic acidN-acetyl-D-galactosamine-4-sulfate

-(13)

Connective tissuecomponentSold as over-the-counterdrug to help replacedamaged cartilage (esp. inknees)

Dermatan sulfate D-glucuronic acid-2-sulfateN-acetyl-D-galactosamine-4-sulfate

-(13)

Component of extracellularmatrix of skin

Heparin D-glucuronic acid-2-sulfateN-sulfo-D-glucosamine-6-

sulfate -(14)

anticoagulant

Hyaluronic acid D-glucuronic acidN-acetyl-D-glucosamine

-(13)

Component of vitreoushumor of the eye and oflubricating fluid of joints

Keratan sulfate D-galactoseN-acetyl-D-glucosamine-6-sulfate

-(14)

Component of connectivetissue

Carbohydrates 2 Updated

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