lecture6_carbohydrates2011.pptx

OUTLINE

I. Carbohydrates Overview

II. Monosaccharides

III. Fisher Projections

IV. Cyclic Formations

V. Dissacharides

VI. Polysacharides

CARBOHYDRATES

•

_{carbohydrates are “carbon hydrates ”}



molecular formula (

CH

O)

= (C . H

O)n

where n ≥ 3 or C

(H

O)



_{visual clue for ‘normal sugars’ (e.g.,}

underivatized):



_{all carbons attached to an oxygen}



_{most abundant class of biological}

molecules



_{originate from solar-powered}

Functions



_{multiple biological functions in a cell:}

 _{energy source – component of DNA & RNA}

 _{structural roles – cellular interactions & recognition events}



_{structure material storage material for carbon and}

energy.



_{many proteins from eukaryotic cells are}

_glycosylated

(have sugars covalently bound to some of their

surface residues)



_{the surfaces of cells display a variety of sugar-based}

structures

Carbohydrates



_{About 80% of human caloric intake are}

carbohydrates



_{>90% of dry plant materials}



_{Functional Properties:}



_Sweetness



_{Chemical reactivity}

Nomenclature



_{monosaccharides (simple sugars) cannot be}

broken

down into simpler sugars under mild

conditions



_{Oligosaccharides}

_{– a covalently-linked chain of}

several monosaccharide - usually 2 to 10



_{polysaccharides}

_{are polymers of the simple}

sugars

Fischer Projections



_{Fischer projections/D,L system very common}

for sugars



_{– D,L designation refers to the configuration of the}

highest-numbered asymmetric center



_{– D,L only refers the stereocenter of interest back}

to D- and Lglyceraldehyde

Aldotriose Aldotetrose _Aldopentose

Carbohydrate Stereochemistry



_{Isomers -}

_{Two or more different compounds which}

contain the same number and types of atoms and the same

molecular weights.



_{stereoisomers that are mirror images of each other are}

enantiomers



_{pairs of isomers that have opposite configurations at one}

or more chiral centers but are NOT mirror images are

diastereomers



_{aldoses with 3C or more and ketoses with 4C or more}

CH₂OH OH CHO C

H

D-glyceraldehydes (hydroxyl group at the highest numbered asymmetric carbon atom is written to the right):

D-glyceraldehydes (hydroxyl group at the highest numbered asymmetric carbon atom is written to the right):

CH₂OH CHO C H HO

L-glyceraldehydes (hydroxyl group at the highest numbered asymmetric carbon atom is written to the left):

D-Erythrose is the mirror image of L-Erythrose.

L-glyceraldehydes (hydroxyl group at the highest numbered asymmetric carbon atom is written to the left):

Glyceraldehydes: Model compounds of

monosaccharides

Glyceraldehydes: Model compounds of

monosaccharides

D-Glyceraldehyde L-Glyceraldehyde

C O

CH₂OH OH C

H _{HO C H}

CH₂OH

H _{H O}

C

D-Erythrose L-Threose

C O

H _{H O}

C O

CH₂OH OH C H HO H H C OH OH C H HO

H C OH

OH

C H

H C OH C H H

HO H C

CH₂OH O

C C

O

CH₂OH C H HO H H C OH C H H C OH or

Anomers: Stereoisomers formed when ring is formed

(

a

,

b

).

Anomers: Stereoisomers formed when ring is formed

(

a

,

b

).

C O

CH₂OH OH C H HO H H C OH OH C H HO

HO C H

OH

C H H C OH

C H H

HO H C

CH₂OH O

C C

O

HAWORTH PROJECTION FORMULAS FOR SUGARS

OH

OH

C H

H C OH C H H

HO H C

CH₂OH

O C OH O OH OH HO

CH₂OH

1

2 3

4 5

6

Mutarotation:

A small amount of open

chain is in equilibrium with the cyclic forms.

Cyclic Forms of Glucose

The most

stable form of glucose is

β-D glucose.

The most

stable form of glucose is

β-D glucose.

HAWORTH PROJECTION FORMULAS FOR SUGARS HO C C H H OH HO OH H H C O C C O

CH 2OH

C H HO H H C OH C H H C OH OH C O

CH 2OH

OH C H HO H H C OH C H OH H C OH OH C H

H C OH C H H

HO H C

CH 2OH

Dissacharides

• _{2 monossacharides linked together by acetal formation} • _{Condensation of the hydroxyl functional groups of of}

monosacharides and another monosaccaharide

• _{The acetals and ketals formed are given the general name} glycosides and the C-O bond is called glycosidic bond

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DISSACHARIDES

β-MALTOSE

EXERCISE: Draw the structure of α-maltose? Linkage:

lactose

sucrose

50

Monossacharide: GLUCOSE-FRUCTOSE

trehalose

52

Sucrose

O CH₂OH

OH

OH

CH₂OH

o

CH₂OH OH

OH

OH O

α,

b

Polysaccharides

•

_{Most carbohydrates found in nature occur as}

polysaccharides, polymers of medium to high

molecular weight.

•

_{Polysaccharides, also called}

_{glycans, differ from each}

other in the identity of their:

– _{recurring monosaccharide units,} – _{in the length of their chains,}

– _{in the types of bonds} – _{linking the units,}

– _{and in the degree of branching.}

Types of polysaccharides

Homopolysaccharides

• _{contain only a single type of monomer}

• _{some homopolysaccharides serve as storage forms of} monosaccharides that are used as fuels

e.g. starch and glycogen

• _{Other homopolysaccharides serve as structural elements in} plant cell walls and animal exoskeletons.

e.g. cellulose and chitin

Heteropolysaccharides

• _{contain two or more different kinds}

• _{provide extracellular support for organisms of all kingdoms}

56

Polysaccharides generally do not have definite molecular weights. the program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the

Storage polysaccharides

• _{The most important storage polysaccharides are starch in} plant cells and glycogen in animal cells.

• _{Both polysaccharides occur intracellularly as large clusters or} Granules.

• _{Starch and glycogen molecules are heavily hydrated, because} they have many exposed hydroxyl groups available to

hydrogen-bond with water.

• _{Most plant cells have the ability to form starch, but it is}

58

•_{Two component forms: amylose and amylopectin}

Starch: amylose

Starch: amylopectin

60

Amylopectin has a high molecular weight (up to 100 million) but unlike amylose is highly branched.

62

A cluster of amylose and amylopectin like that believed to occur in starch

granules. Strands of amylopectin (red) form doublehelical structures with each other or with amylose strands (blue).

•_{Glycogen is stored energy for the organism}

•_{Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass}

•_{Glycogen is a polymer of glucose; similar to starch; also contains (α1→4)}

•

_{Glycogen and starch ingested in the diet are}

hydrolyzed by β-amylases, enzymes in saliva

and intestinal secretions that break (α1

→

4)

glycosidic bonds between glucose units.

•

_{Most animals cannot use cellulose as a fuel}

source, because they lack an enzyme to

hydrolyze the (α1

→

4) linkages.

•

_{Termites readily digest cellulose}

cellulose

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(β1→4) linkages make a big difference

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The structure of starch (amylose). (a) In the most stable conformation, with adjacent rigid chairs, the

polysaccharide chain is curved, rather than linear as in cellulose.

(b) Scale drawing of a segment of amylose.

The conformation of (α1→4)

linkages in amylose, amylopectin, and glycogen causes these polymers to assume tightly coiled helical

structures.

dextran

• _{Dextrans are bacterial and yeast polysaccharides} made up of (α1→6)linked poly-D-glucose; all have (α1→3) branches, and some also have (α1→2) or (α1→4) branches.

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