Chapter 1 - Carbohydrate

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CHAPTER 1

CARBOHYDRATE


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Learning Objectives:

1. Define basic constituents of carbohydrates

2. Describe the classification, structure and

functions of carbohydrates.


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CARBOHYDRATE

1. MONOSACCHARIDE


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Key words

Carbohydrate- definition

Aldose, ketose

Classification

Stereochemistry: Enantiomer, Diastreomer,

Epimer

Fischer projection, Haworth projection


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Carbohydrates

Carbohydrate:most abundant biomolecules (other

biomoleculesprotein, lipid, nucleic acid)

Suffixose indicates the molecule is a

carbohydrate

- glucose, fructose, maltose, sucrose, cellulose

Major role in energy metabolism and structural

component


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Carbohydrates

Definition:a polyhydroxyaldehyde or

polyhydroxyketone, or a substance thatgives these compounds on hydrolysis


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Carbohydrate

Three levelnumber of sugar/saccharide unit

1. Monosaccharidesimplest2. Oligosaccharideconsist more than 1

monosaccharide> disaccharide has 2

monosaccharide

3. Polysaccharidehas large number of monosaccharide


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Monosaccharides are classified by theirnumber of carbon atoms

- 3 C = triose- 4 C = tetrose

- 5 C = pentose

- 6 C = hexose

- 7 C = heptose

Trioses are simplest carbohydrate

monosaccharides

Monosaccharides


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Fischer Projectionsand

Haworth Projections


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Fischer Projections Fischer projection:a two dimensional representation for showing the configuration of

tetrahedral stereocenters

verticallines represent bonds projecting forward

horizontallines represent bonds projecting to the rear

the carbon atom at the intersection of the lines is not shown

D-Glyceraldehyde

CHO

CH OH

CH2 OH

D-Glyceraldehyde

convert to

a Fischerprojection

H OH

CHO

CH2 OH


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Stereoisomer

1. Enantiomer

2. Diastreomer

3. Epimer

Chiral carbon

Chiral molecule

mirror


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Stereoisomer

Enantiomer =

Stereoisomer that are

nonsuperimposablemirror images of each

other

Chiral carbon/centre =carbon atom connected to 4

different groups

Chiral carbon

Chiral molecule

mirror

Isomer:

Each of two or more compounds with the same formula but a different

arrangement of atoms in the molecule and different properties.


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Glyceraldehyde contains a stereocenter andexists as a pair of enantiomers

Mirror-images stereoisomers are calledenantiomers

Enantiomer


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Stereoisomer

D,L Monosaccharides

What is D and L?

D = dextrorotary, L = levorotary

Are stereoisomers, due to the rotated

direction of plane polarized light of solution

Enantiomers:stereoisomers that are mirror

images

example: D-erythrose and L-erythrose are

enantiomers


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D,LMonosaccharides-

FischerProjections

According to the conventions

proposed by Fischer

D-monosaccharide:a

monosaccharide that, when

written as a Fischer projection,

has the -OHon its penultimate

carbonon the right

L-monosaccharide:a

monosaccharide that, when

written as a Fischer projection,

has the -OHon its penultimate

carbonon the left

REMEMBER: D,L= enantiomers


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D,L Monosaccharides-

Fischer Projections

According to theconventionsproposed by Fischer

D-monosaccharide:has the -OHon itspenultimatecarbon

on the right L-monosaccharide:

has the -OHon itspenultimatecarbon

on the left

REMEMBER: D,L= enantiomers


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Diastereomers

Diastereomers:stereoisomers that are notmirror

images,non-superimposable example: D-erythrose and D-threose are diastereomers

Epimers-diastereomer differ at only one chiral carbon


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Diastereomers- example


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Enantiomers:stereoisomers that are mirror images

example: D-erythrose and L-erythrose are enantiomers

Diastereomers:stereoisomers that are not mirror images

example: D-erythrose and D-threose are diastereomers

LO

CHO

CH2OH

OHH

OHH

CHO

CH2OH

HHO

HHO

CHO

CH2OH

HHO

OHH

CHO

CH2OH

OHH

HHO

D-Erythrose L-Erythrose D-Thre ose L-Thre ose

Mirrorplane

Mirrorplane


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D-mannose and D-galactose differ

stereochemically from D-glucose at only 1 chiral

center D-mannose and D-galactose areEPIMERS of glucose

D-galactose is a C-4

Epimer of D-glucose

D-mannose is a C-2

Epimer of D-glucose

Epimers - example


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STRUCTURE OF MONOSACCHARIDES

Isomer = compound that have same chemical formula

Epimers = differ in configuration around one specific

carbon atom


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Tutorial

Carbohydrate


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Fructose

Epimers of D-fructose

Enantiomers of D-fructose

Diastrereomer of D-fructose

D-Psicose, D-Tagatose

L-fructose

D-Psicose, D-Tagatose, D-Sorbose


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Fructose

Which structure represents D-Fructose?


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Comparison of the Fischer andHaworth Representations


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What Happens if a Sugar

Forms a Cyclic Molecule?

Cyclization of sugars takes place due to interactionbetween functional groups on distant carbons, C1 to C5,to make a cyclic hemiacetal - aldose

Cyclization using C2 to C5 results in hemiketalformation.- ketose

In both cases, the carbonyl carbon is new chiral centerand becomes an anomeric carbon

Hemiacetal a cyclic

is formed by the

aldehyde group with a


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Formation of a Cyclic

Hemiacetal


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Cyclic Structure

via Haworth Projections

Monosaccharides have -OH and C=O

groups in the same molecule and exist

almost entirely as five- and six-memberedcyclic hemiacetals

anomeric carbon:the new stereocenter

resulting from cyclic hemiacetal formation anomers:carbohydrates that differ in

configuration only at their anomeric carbons


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Haworth Projections

Haworth projections

five- and six-membered hemiacetals are represented

as planar pentagons or hexagons, as the case may be,

viewed through the edge

most commonly written with the anomeric carbon on

the right and the hemiacetal oxygen to the back right

the designation - means that -OH on the anomeric

carbon is cis to the terminal -CH2OH; - means that it is

trans


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Haworth Projections

A six-membered hemiacetal ring is shown by

the infix -pyran- (pyranose)

A five-membered hemiacetal ring is shown bythe infix -furan- (furanose)


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CONVERTING A FISCHER PROJECTION TO A HAWORTH

CHO

OH

HO

OH

OH

CH2OH

This -OH

determines

D- or L-

These groups

are down in

the Haworth

These groups

are up in

the Haworth

UP DOWN

This group, whichis carbon 6, will be

up in the Haworth if

D- and down if L-

Carbon 1 will be the

anomeric carbon

CH2OH

OH

OH

OH

OHO

1

23

4

5

6D

This is the

-anomer because

the anomeric OH is

is

to -CH2OH

down

up

down

is


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Haworth

Projections


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Reaction of Monosaccharides

1. Oxidation-reduction

i. Oxidation of aldoses to acid

ii. Reduction of ribose to deoxyribose

iii. Reduction of L-galactose to L-fucose (L-6deoxygalactose)

iv. Reduction of carbonyl group to hydroxyl groupformingsugar alcohol/alditols

2. Esterificationi. Formation of phosphate esters-intermediate in energy

metabolism


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Oxidation

Reducing sugar:a sugar that has a

free carbonyl group (anomeric

carbon) one that can reduces an

oxidizing agent

Tollens reagent oxidizing agent


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Reduction

ribose to deoxyriboseReduction to sugar alcohol/alditols


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Esterification -

Phosphoric Esters Phosphoric esters are particularly important in the

metabolism of sugars to provide energy

phosphoric esters are frequently formed by transfer of a

phosphate group from ATP


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Monosaccharide derivatives

Amino sugars


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CARBOHYDRATE

2. DISACCHARIDE


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Disaccharides Sucrose

Table sugar; obtained from the juice of sugar cane and sugar beet

One unit of D-glucose and one unit of D-fructose joined by an -1,2-glycosidic bond

LactoseMade up of D-galactose and one unit of D-glucose joined by a -1,4-glycosidic bond

Galactose is a C-4 epimer of glucose

Maltose

Two units of D-glucose joined by an -1,4-glycosidic bond

Formed from the hydrolysis of starch

Cellobiose

Two units of D-glucose joined by an b-1,4-glycosidic bond

Formed from the hydrolysis of cellulose


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Sucrose

Anomeric carbons of glucose carbon C1 in configuration is linked to the anomeric carbon offructose C2 in bconfiguration

anomeric carbon is tied into aglycosidic bond and none of it

able to form an open chain

containing free aldehyde and

ketonesucrose is NOT a

reducing sugar


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Lactose

Galactose in a (1-4) linkage with glucose

Principal sugar present in milk

Galactose is converted by the body to glucose and glucose used forenergy

Reducing sugar?


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Maltose

Products from hydrolytic

breakdown of starch

Can be easily digested by

humans because of thepresence of enzymes

catalyzes the hydrolysis of

(1-4) glycosidic bonds

Reducing sugar?


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Cellobiose

Cellobiosehydrolysis product of cellulosemajor

component of plants

Differs from maltose at glycosidic bond

Humans do not have the capacity to

digest cellobiose or celluloselack

the enzyme cellulase that break (1-4)

glycosidic linkages between glucose

monomers

Ruminants animals canhave

bacteria in the rumen in gastrointestine

tract and secrete cellulase


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HaworthProjections

Anomeric

carbon

If OH is freeat the

anomeric carbon =

the monosaccharide is

reducing sugar


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GlycosidicBondFormation

Glycosidic bond: form between the hemiacetal ofmonosaccharide (saccharide) and the hydroxyl group of

organic compound such as alcohol.

A substance containing a glycosidic bond is a glycoside

Disaccharide: glycosidic bond formation between thehemiacetal of monosaccharide and hydroxyl group from

another hemiacetal

Glycosidic bonddue to dehydration/condensation


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GlycosidicBondFormation

Type of bond: O-glycosidic bond


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Two Different Disaccharides of

-D-Glucose

Glycosidic linkagescan take variousforms; the anomeric

carbon of one sugarto any of the -OHgroups of anothersugar to form an - or

b-glycosidic linkage

Type of bond: O-glycosidic bond


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N-glycosidic bond

The -NH group of amine

substitute for hydroxyl groups

and react at the anomeric

carbon center of

carbohydrates.

New linkage is called N-

glycosidic bond Importance in the construction

ATP and in nucleic acids RNA

and DNA ATP


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Disaccharides

Formation of glycosidic bondcondensation (-H2O)

Disaccharides = O-glycoside


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CARBOHYDRATE

3. POLYSACCHARIDE


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Key words

Definition

Function

Homopolysaccharide and heteropolysaccharide

Cellulose, Chitin, Pectin, Peptidoglycan

Starch, glycogen

Glycoaminoglycan, glycoprotein


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Polysaccharides

Polysaccharide-When many monosaccharides

are linked together

Divided into 2type of monosaccharide orfunction

Two main function: energy storage and structure

Type of monosaccharidehomopolysaccharideand heteropolysaccharide


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Cellulose

the major structural component of plants,

especially wood and plant fibers

a linear / unbranched polymerof approximately 2800b-D-glucoseunits joined by b-1,4-glycosidic bonds

extensive intra- and intermolecular hydrogen bonding

between chains-strength

Cellobiose is the repeating disaccharide

hydrolysis by cellulase

l f ll l


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Polymeric Structure of Cellulose


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Pectin

Important component of

plant cell walls

MonomerD-Galacturonicacid, derivative of galactose

Commercially important-

food processing industry- as gelling agent in jams

and jellies


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Chitin

the major structural component of the

exoskeletons of invertebrates and crustaceans;

and in cell walls of algae, fungi, and yeasts b-1,4-glycosidic bondssimilar with cellulose

linear polymer, each chain held together by

hydrogen bonds composed of units of N-acetyl- -D-

glucosaminediffer with cellulose


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Chitin

Differ with glucose


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Peptidoglycan

Bacterial cell walls:

prokaryotic cell walls

are constructed onthe framework of

the repeating unit

NAM-NAGjoined by

b-1,4-glycosidicbonds

Presence ofpeptide

bond


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StoragePolysaccharides

Starch

Glycogen


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Starch

Starch is used for energy storage in plants

amylose:continuous, unbranched chainsof up to 4000 a-D-glucose units joined by -1,4-glycosidic bonds

amylopectin:a highly branched polymerconsisting of 24-30 units

of D-glucose joined by -1,4-glycosidic bondsand branchescreated by -1,6-glycosidic bonds

amylases catalyze hydrolysis of -1,4-glycosidic bonds

* b-amylase is an exoglycosidase and cleaves from the nonreducing

end of the polymer* -amylase is an endoglycosidase and hydrolyzes glycosidic

linkages anywhere along the chain to produce glucose andmaltose

debranching enzymes catalyze the hydrolysis of -1,6-glycosidicbonds

Starch


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Starch


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AmyloseandAmylopectin


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Amyloseoccurs as a helixwith 6 residues per turn- able to give the dark-blue colorcomplex with iodine.


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Glycogen

Glycogen is used for energy storage in animals

Has amylose and amylopectinsimilar to starch

Differ with starchthe amylopectin is highly branched

- branch point: occur about every10 residues

instarch, about

every 25 residues


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Polysaccharides

Homopolysaccharide:consists of one type of

monosaccharide

Heteropolysaccharide:consists of more than

one type of monosaccharide


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H t l h id


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Heteropolysaccharide:

Glycosaminoglycans

Glycosaminoglycans:polysaccharides based on a

repeating disaccharide where one of the

monomers is an amino sugar and the other has a

negative charge due to a sulfate or carboxylate

group

Heparin:natural anticoagulant

Hyaluronic acid:a component of the vitreous humor ofthe eye and the lubricating fluid of joints

Chondroitin sulfateand keratan sulfate:components

of connective tissue

H t l h id


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Heteropolysaccharide:

Glycosaminoglycans


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Glycoproteins

Glycoproteinscontain carbohydrate units covalentlybonded to a polypeptide chain

antibodiesare glycoproteins

Oligosaccharide portion of glycoproteins act as antigenicdeterminants

Among the first antigenic determinants discovered were theblood group substances

In the ABO system, individuals are classified according to fourblood types: A, B, AB, and O

At the cellular level, the biochemical basis for this classification isa group of relatively small membrane-bound carbohydrates


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Tutorial 1

1. Give two important functions of polysaccharide.

2. Name 2 structural polysaccharide and 2 storagepolysaccharide.

3. Give one example of heteropolysaccharide.

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Chapter 1 - Carbohydrate