Principles of Biochemistry (Carbohydrates)

7/27/2019 Principles of Biochemistry (Carbohydrates)

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Carbohydrates

Structure & Function of

Biomolecules


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Carbohydrates Most abundant biomolecule

Many functions

Energy/energy storage

Structural elements

Mediator of interactions


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Monosaccharides Simple sugars

Polyhydroxy aldehydes or ketones


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Monosaccharides General formula: (CH2O)n

n = 3 - 7

Most highly oxidized

carbon at top (C1)


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Important Monosaccharides

O

H

HO

H

HO

H

H

OHHOH

OH

O

OH

H

H

HO

H

H

OHH

OH

OH

HO

HO OH

O

OHHO

Glucose Fructose Galactose

Must be able to draw Fischer projections

and Haworth projections of these.


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Stereoisomers vant Hoffs rule

# of stereoisomers = 2n

where n = # of

assymmetric carbons.

C

C

O

H

OH

C

C

C

CH2OH

H

HO

OHH

OHH

H

n = 4 # stereoisomers = 16


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Types of Stereoisomers

Enantiomers

Mirror images

Diastereomers

Not mirror images

Epimers

Differ at a single chiral carbon


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Examples

C

C

O

H

OH

C

C

C

CH2OH

H

H

OHH

OHH

OH

C

C

O

H

OH

C

C

C

CH2OH

H

HO

OHH

OHH

H

C

C

O

H

HO

C

C

C

CH2OH

H

OH

HO H

HO H

H

L-glucose

(enantiomer)

D-allose

(epimer at C3)

D-talose

(diastereomer)D-glucose

C

C

O

H

H

C

C

C

CH2OH

HO

HO

HHO

OHH

H


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Configuration Look at chiral carbon farthest from carbonyl

carbon to determine D or L configuration.

C

C

O

H

OH

C

C

C

CH2OH

H

HO

OHH

OHH

H

D-glucose

C

C

O

H

HO

C

C

C

CH2OH

H

OH

HO H

HO H

H

L-glucose

OH on right, then

D configuration

OH on left, then

L configuration


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Reactions with Alcohols


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Cyclization

C

C

O

OH

C

C

C

CH2OH

H

HO

OHH

OHH

H

HC

C

OH

OH

C

C

C

CH2OH

H

HO

OHH

H

H

H

O


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Furanoses/Pyranoses

O O

Furan Pyran


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Haworth ProjectionsAnomers


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Mutarotation a and b forms

spontaneously

interconvert in anaqueous environment.

Proportion of eachanomer differs for each

sugar.


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Reducing Sugars

Sugars that are susceptible

to oxidation.

Look at C1 on aldoses or

C2 on ketoses. If a free OHis present, then the sugar is

a reducing sugar.

(reducing)

(not reducing)


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Oxidation ReactionsOxidation of aldehyde =

aldonic acid

D-Gluconic acid

C

C

O

OH

OH

C

C

C

CH2OH

H

HO

OHH

OHH

H

Oxidation of terminal

CH2OH = uronic acid

C

C

O

H

OH

C

C

C

C

H

HO

OHH

OHH

H

O

OH

D-Glucuronic acid

Oxidation of both =

aldaric acid

C

C

O

OH

OH

C

C

C

C

H

HO

OHH

OHH

H

O

OH

D-Glucaric acid


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Reduction Reactions

C

C

O

OH

OH

C

C

C

CH2OH

H

HO

OHH

OHH

H

CH2OH

C OH

C

C

C

CH2OH

H

HO

OHH

OHH

HH2

Catalyst

D-Glucose D-Glucitol (Sorbitol)

Sugar alcohol


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Isomerization

Generally enzyme catalyzed reactions.

Important in carbohydrate metabolism.


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Modifications of

Monosaccharides

N-acetyl-a-D-glucosamine

O

H

HO

H

HO

H

H

OHHOH

O

P O-O

O-

b-D-Glucose-6-phosphate

OHCH2OH

OH

O

2-deoxy-b-D-ribose


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Glycosidic Linkages


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Glycosidic Linkages

Glycosidic

linkage

Aglycone


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Disaccharides

Maltose (a-D glucose (a 1-4) b-D glucose)

Bond type

Look at first subunit to determine ifa orb

Number according to 1st subunit then 2nd subunit

O

OH

OH

OH

CH2OH

O OH

OH

OH

CH2OH

O

a 1-4

linkage


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Sucrose (must be able to draw) Table sugar

Is sucrose a reducing sugar?

No, because glycosidic linkage

joins the 2 anomeric carbons.

a-D glucose (a 1-2) b-D fructose


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Oligosaccharides 3 - 10 or more subunits

Often attached to proteins

or lipids to formglycoproteins or glycolipids,

respectively.

N-linked

Attached via asparagine.

O-linked

Attached via threonine or

serine or OH on lipid


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Polysaccharides Composed of many subunits.

Homopolysaccharides or heteropolysaccharides


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Storage Polysaccharides: Plants Starch

Glucose monomers

Two forms Amylose

Unbranched a1-4 linkages

Amylopectin

a1-4 backbone with a1-6branches.

Branches occur ~12-25residues & contain ~20-25glucoses

Use a amylase to digest.


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Storage Polysaccharides: Animals Glycogen

Stored in liver & inmuscles.

a1-4 backbone with a1-6branches.

Branches occur ~10residues & contain ~ 8-

12 glucoses.

Glycogen phosphorylasebreaks a1-4 linkages.

Debranching enzyme

breaks a1-6 linkages.


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Framework of Nucleic Acids

Deoxyribose (DNA)

Ribose (RNA)


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Structural Elements: Plants Cellulose

Homopolysaccharide Glucose subunits joined

by b1-4 linkages.

Found in plant cell wall.

Mammals lackcellulose degradingenzyme.


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Structural Elements:

Arthropods and Some Fungi Chitin

Homopolysaccharide N-acetylglucosamine

monomers joined by b1-4

linkages.

Makes up exoskeleton ofarthropods and cell wall of

some fungi


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Structural Elements: Animals Glycosaminoglycans

(GAGs)

Heteropolysaccharides

Disaccharide repeating

units

Negatively charged at

physiological pH.

Bind a lot of H2O.


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Cell-Cell Recognition:

Blood Group AntigensCarbohydrate portion of blood group glycolipids

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Principles of Biochemistry (Carbohydrates)