Chem 1152: Ch. 17 Carbohydrates. Introduction Biomolecules: Organic compounds produced by living...

Chem 1152: Ch. 17

Carbohydrates

IntroductionIntroduction

Biomolecules: Organic compounds produced by living organisms•Carbohydrates•Lipids•Proteins•Nucleic acids

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Functions of carbohydrates1.Provide energy through their oxidation2.Supply carbon for synthesis of cell components3.Serve as stored form of chemical energy4.Form structural elements of some cells and tissues

Classes of Carbohydrates (saccharides)Classes of Carbohydrates (saccharides)

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/sugar.htm

Hydrates of carbon Cm(H2O)n characterized by having multiple functional groups

•Hydroxyl (alcohols)•Carbonyl (aldehydes or ketones)Mono- and disaccharides are simple sugars

glucosefructose

galactoseribose

deoxyribose

sucroselactose

Fructo-oligosaccharidesgalactooligosaccharides

starchglycogencellulose

chitin

Stereochemistry of CarbohydratesStereochemistry of Carbohydrates

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://web.fccj.org/~ethall/stereo/stereo.htm

Stereoisomerism: Isomeric molecules can have the same molecular formula and sequence of bonded atoms (constitution), but that differ in the three-dimensional orientations of their atoms in space.Enantiomers: Stereoisomers that are mirror images of each other.Chirality: “Handedness”. Refers to compounds that cannot be superimposed on mirror image.-Defined relative to central, chiral atom (carbon)

enantiomers

Stereochemistry of CarbohydratesStereochemistry of Carbohydrates


Chiral Carbon: Carbon in organic compound that has four different groups attached to it.Chirality: “Handedness”. Refers to compounds that cannot be superimposed on mirror image.

enantiomers

OHC

H

C OH

CH2OH

CHO

H

CHO

CH2OH

OHCC

OH

CH2OH

CHOC

HO

CH2OH

Chiral carbon

CHO indicates aldehyde

Identify Chiral CarbonsIdentify Chiral Carbons

OHC

H

C OH

CH2OH

OHCCHOH

CH2OH

CH3CCH2CH3

O

*

CH3CHCH3

OHCH3CHCH2CH3

OH

OH

O

Identify chiral carbons in glucoseIdentify chiral carbons in glucose

glucoseglucose

C

C

C

C

CH2

C

OH

OH

OH

OH

HO H

H

H

H

H O1

2

3

4

5

6

p. 528

chiral carbons in glucosechiral carbons in glucose


Max # of possible stereoisomers = 2n where n = number of chiral carbon atoms

Fischer ProjectionsFischer Projections


Chiral Carbons represented by intersection of two lines For fischer projections the prefixes (L-) and (D-) do not indicate Levorotatory

(L-) and Dextrorotatory (D-) related to rotation of polarized light, but represent orientation of functional groups when compared to glyceraldehyde

Fischer ProjectionsFischer Projections


When carbonyl is up: D-family: OH (or NH2) group of chiral C most distant from anomeric center projects to right

L-family: OH (or NH2) group of chiral C most distant from anomeric center projects to left Anomeric center: Carbonyl (aldehyde or ketone) carbon

Biological systems can only utilize the D- isomers.

*

* *

*

*

*

*

*

*

*

*

*

MonosaccharidesMonosaccharides


Contain single polyhydroxy aldehyde or ketone unit Further classified based on number of C atoms Aldehydes contain prefix aldo-, ketones have prefix keto-

Monosaccharides: D- aldosesMonosaccharides: D- aldoses


Chiral C’s?

1

2

3

4

stereoisomers

2

4

8

16

Monosaccharides: Chemical PropertiesMonosaccharides: Chemical Properties


Because aldehydes and ketones react with alcohols to form hemiacetals and hemiketals, all monosaccharides with at least 5 carbon atoms exist predominantly in cyclic forms.

These are depicted using Haworth structures Ring is drawn with oxygen to the back, and anomeric carbon to the right.

Furanose ring: A 5-member ring containing an oxygen atom. Pyranose ring: A 6-member ring containing an oxygen atom.

Glucose (Blood Sugar)Glucose (Blood Sugar)


Primary source of energy in cells Precursor for Vitamin C synthesis Modified subunits can form long polymer chains

starch, cellulose, glycogen

C

C

C

C

CH2

C

OH

OH

OH

OH

HO H

H

H

H

H O

1

2

3

4

5

6

Beta-hydroxy group: OH attached to anomeric carbon above ring. Alpha-hydroxy group: OH attached to anomeric carbon below ring.

Hemiacetal (on C1)

64%

0.02%

36%

FructoseFructose

http://upload.wikimedia.org/wikipedia/commons/1/1e/Fructose-isomers.jpg

Fruit sugar Absorbed directly into bloodstream during digestion (like glucose and galactose) Anomeric carbon is C2

70%

22%

Hemiketal (on C2)

1

23

456

Chemical Reactions of SugarsChemical Reactions of Sugars


Open chain forms of monosaccharides (aldehydes, hydroxyketones) can be readily oxidized. Ex. Use of Benedict’s reagent to oxidize aldehydes and ketones with hydroxy

group on adjacent carbon. At the same time, the cyclic forms are converted to open-chain forms and also react. Reducing sugars: Monosaccharides that can be oxidized Oxidation of carbohydrates to CO2 and H2O very important at cellular level, serves as

source of heat and energy.

Reducing sugar + Cu(II) oxidized cmpd + Cu2ODeep blue solution

Red-orange precipitate

Other Important MonosaccharidesOther Important Monosaccharides

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011: http://www.istpace.org/Web_Final_Report/WP_4_chem_subsystems/descr_chem_subsystems/pna_intro/pna_intro.html

Ribose and Deoxyribose Used to synthesize RNA and DNA Used in protein synthesis

Phosphodiester bond

Other Important MonosaccharidesOther Important Monosaccharides


Galactose Synthesized in mammary gland, incorporated into milk lactose Component of the antigens present on blood cells that determine blood type

Joining MonosaccharidesJoining Monosaccharides


Monosaccharide units can be joined together by glycosidic linkages. Glycosidic linkages are same as adding alcohol to hemi- intermediates

O

R H

+ H

R

O

RO

HO

R H

H+, RO-H

RO

RO

R H + H-O-H

Hemiacetal Hemiacetal intermediateintermediate

acetalacetal

O

R H

+ H

R

O

RO

HO

R H

H+, RO-H

RO

RO

R H + H-O-H

O

OH

CH2OH

OH

O

O

CH2OH

OHH

+O

CH2OH

OH

O

CH2OH

OHO

+ OH2

Polymerization of Monosaccharide SubunitsPolymerization of Monosaccharide Subunits

•Through glycosidic linkages at the hemiacetal carbons, many monosaccharide subunits can be put together to form long, branching chains via 1,4 or 1,6 linkages.•These can be between 2 α, 2 β, or between an α and a β.

DisaccharidesDisaccharidesMaltose

Lactose Sucrose


Properties of PolysaccharidesProperties of Polysaccharides


•Components of plant starch built from glucose residues.Plant starchesPlant starches

Amylose (10-20%)No branching

Amylopectin (80-90%)Branching 24-30 residues

Structure of glycogenStructure of glycogen

•Polymer built from glucose subunits.•Glucose in Glycogen are connected via α-1,4 or α-1,6 linkage.•α-1,4 linkage makes a linear chain, α-1,6 linkage makes a branch (~every 10 residues).•The end glucose residues without open 1’-OH is called nonreducing ends.•Branches provide more non-reducing ends for rapid degradation.

What is glycogen and why do we need it?What is glycogen and why do we need it?

•Long-term energy reservoir for glucose in animal and fungal cells. •Found primarily in muscle and liver cells.•Liver produces glycogen for needs of organism, while muscle takes care of only itself.•Glycogen not as energy rich as fatty acids, and is used differently. •Controlled breakdown of glycogen and glucose release maintain blood-glucose levels.•Glucose is the primary fuel used by the brain.•Unlike fatty acids, glucose from glycogen is readily mobilized and can provide energy for sudden, strenuous activity.

•Most important structural polysaccharide and single most abundant organic compound on earth.• Provides strength and rigidity to plant cell walls.•Wood is ~50% cellulose.•Contains 300-3000 glucose subunits.•Form extended straight chains that hydrogen bond with parallel chains, creating long, rigid fibers.•Undigestible by humans.

CelluloseCellulose

Intramolecular addition of alcohols to aldehydeIntramolecular addition of alcohols to aldehyde

C1 is hemiacetal carbon. Attached to it you will find: H, OH, OR and R, just like non-cyclical compounds.

Intramolecular Intramolecular hemiacetalhemiacetalglucoseglucose

C

C

C

C

CH2

C

OH

OH

OH

OH

HO H

H

H

H

H O1

2

3

4

5

6

*CH2OH

C

C

C C

O

C

6

5

4

3 2

1

OH

H

OH

OH

OH

H

H

H

H

Chem 1152: Ch. 17 Carbohydrates. Introduction Biomolecules: Organic compounds produced by living...

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