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CarbohydratesStructure and Biological Function
CarbohydratesStructure and Biological Function
Monosaccharides
Carbohydrates in Cyclic Structures
Reactions of Glucose and Other Monosaccharides
Polysaccharides
Glycoproteins
Monosaccharides
Carbohydrates in Cyclic Structures
Reactions of Glucose and Other Monosaccharides
Polysaccharides
Glycoproteins
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CarbohydratesCarbohydrates
Compounds containing C, H and O
General formula : Cx(H2O)y
All have C=O and -OH functional groups.
Classified based on •Size of base carbon chain•Number of sugar units •Location of C=O•Stereochemistry
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Types of carbohydratesTypes of carbohydrates
Classifications based on number of sugar units in total chain.
MonosaccharidesMonosaccharides - single sugar unit
DisaccharidesDisaccharides - two sugar units
OligosaccharidesOligosaccharides - 2 to 10 sugar units
PolysaccharidesPolysaccharides - more than 10 units
Chaining relies on ‘bridging’ of oxygen atoms
glycoside bondsglycoside bonds
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MonosaccharidesMonosaccharides
Based on location of C=O H | C=O | H-C-OH | H-C-OH | H-C-OH | CH2OH
CH2OH | C=O |HO-C-H | H-C-OH | H-C-OH | CH2OH
AldoseAldose Ketose Ketose- aldehyde C=O - ketone C=O
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Monosaccharide classificationsMonosaccharide classifications
Number of carbon atoms in the chain
H | C=O | H-C-OH | H-C-OH | H-C-OH |
CH2OH
H | C=O | H-C-OH | H-C-OH | H-C-OH | H-C-OH |
CH2OH
H | C=O | H-C-OH | H-C-OH |
CH2OH
H | C=O | H-C-OH |
CH2OH
triose tetrose pentose hexose
Can be either aldose or ketose sugar.
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ExamplesExamples
H | C=O | H-C-OH |
CH2OH
CH2OH | C=O |HO-C-H | H-C-OH | H-C-OH |
CH2OHD-glyceraldehydeD-glyceraldehyde D-fructose D-fructose triose hexose aldose ketose aldotriose sugar ketohexose sugar
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ExamplesExamples
H | C=O | H-C-OH | H-C-OH | H-C-OH |
CH2OH
H | C=O | H-C-OH | H-C-OH |HO-C-H |HO-C-H |
CH2OHD-riboseD-ribose L-mannose L-mannose pentose, aldose hexose, aldose aldopentose sugar aldohexose sugar
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StereoisomersStereoisomers
StereochemistryStereochemistryStudy of the spatial arrangement of molecules.
Stereoisomers haveStereoisomers have•the same order and types of bonds.•different spatial arrangements.
•different properties.
Many biologically important chemicals, like sugars, exist as stereoisomers. Your body can tell the difference.
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EnantiomersEnantiomers
Pairs of stereoisomersPairs of stereoisomers
Designated by D- or L- at the start of the name.
They are mirror images that can’t be overlapped.
If you don’t believe it,give it a try!
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EnantiomersEnantiomers
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L- and D- glyceraldehydeL- and D- glyceraldehyde
CHO
HO H
C
CH OH2
CH OH2
H
CHO
HO
CHO
H
C
CH OH2
OH
CH OH2
H
CHO
OH
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EnantiomersEnantiomers
Chiral center.Chiral center.
Asymmetric carbon Asymmetric carbon - 4 different things are attached to it.
Cl |
I - C - F |
BrYou must have at least one asymmetric
carbon to have stereoisomers.
Chiral center
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ExamplesExamples
Is the ‘red’ carbon chiral?
H
ClCC=O
CC-OHH
H3C-
H
CC-OH
CH2CH3
H3C-
H
H | C=O | H-CC-OH |
CH2OH
CC=CCl
Br
I
F
H2N-C-C-CC-C-C-SH
H H H
HCl Cl
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Physical propertiesPhysical properties
Optical activityOptical activityability to rotate plane polarized light.
dextrorotatorydextrorotatory - rotate to right
- use + symbol
- usually D isomers
levorotatorylevorotatory - rotate to left
- use - symbol
- usually L isomers
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Plane polarized lightPlane polarized light
Light is passed through a polarized filter.A solution of an optical isomer will rotate the light one direction.
16
StereochemistryStereochemistry
Properly drawing enantiomers in 3-D is hard.
Use Fischer ProjectionsFischer ProjectionsSpecific type of formula that designatesthe orientation of groups.
H | C=O | H-C-OH | H-C-OH |
CH2OH
H | C=O | H-C-OH |
CH2OH
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With this system, a tetrahedral carbon atom is represented by two crossed lines.
A horizontal bond to an asymmetric carbon designates bonds in the front plane of the page.
Vertical bonds are behind the plane of the page.
Fischer projectionsFischer projections
H | C=O | H-C-OH | H-C-OH |
CH2OH
H O H OH
H OH
CH2OH
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Some important monosaccharidesSome important
monosaccharides
D-glyceraldehyde Simplest sugarD-glucose Most important in dietD-fructose Sweetest of all sugarsD-galactose Part of milk sugarD-ribose Used in RNA
note that each is a D- enantiomer
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D-glyceraldehydeD-glyceraldehyde
Three carbon sugarAldose sugarTriose sugar
aldotriose
H | C=O | H-C-OH |
CH2OH
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D-glucoseD-glucose
• Glucose is an aldohexose sugar.
• Common names include dextrose, grape sugar, blood sugar.
• Most important sugar in our diet.
• Most abundant organic compound found in nature.
• Level in blood can be as high as 0.1%
C
C
C
C
C
CH 2 OH
OH
OH
H
OHH
HO
H
H
OH
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D-fructoseD-fructose
Another common sugar.
It is a ketohexose.
Sweetest of all sugars.
CH2OH | C=O |HO-C-H | H-C-OH | H-C-OH |
CH2OH
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Carbohydrates in cyclic structures
Carbohydrates in cyclic structures
If optical isomers weren’t enough, sugars also form rings. For many sugars, its the most common form.
hemiacetalhemiacetal - forms from alcohol and aldehyde
hemiketalhemiketal - forms from alcohol and ketone
R OR’’ \ |
C=O + ROH R - C - OH / |
R’ R’
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Intramolecular cyclizationIntramolecular cyclization
Cyclization.Cyclization.Remember - chains can bend and rotate.
C
C
C
CH2OH
C
C
OH
O
H C
C
C
CH2OH
C
C
O
OH
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Intramolecular cyclizationIntramolecular cyclization
The -OH group that forms can be above or below the ring resulting in two forms - anomers anomers
and and are used to identify the two forms. are used to identify the two forms.
- OH group is down compared to CH2OH (trans).
- OH group is up compared to CH2OH (cis).
25
Intramolecular cyclizationIntramolecular cyclization
The and forms are in equilibrium so one form can convert to the other - mutarotation.mutarotation.
Haworth projections can be used to help see and orientations.
O O
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Cyclization of D-glucoseCyclization of D-glucose
-D - glucose-D - glucose
- D - glucose- D - glucose
H
OH
O H
OHOH
H
OHH
OH
CH 2 OH
H
C
C
C
C
C
CH 2 OH
OH
OH
H
OHH
HO
H
H
OH
OH
OH
OHOH
H
H
H
OH
CH 2 OH
H
OH
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Fischer vs. Haworth projectionsFischer vs. Haworth projections
H
OH
O H
OHOH
H
OHH
OH
CH2OH
H
-D-glucose-D-glucose
C
C
C
C
C
OH
H
OHH
HO
H
HO-CH2
H OH
H
O
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Cyclization of D-fructoseCyclization of D-fructose
This can also happen to ketose sugars. CH 2 OHO
OHOH
CH2OH
HOH H
H OH
OH
CH 2 OH O
CH2OH
OHOH
HH
H OH
CH 2 OH
C
C
C
C
CH 2OH
O
H
OH
OHH
H
HO
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D-galactoseD-galactose
• Not found in many biological systems• Common part of lactose - disaccharide
OOH
H
H
OHOHH
OHH
OH
CH2OH
H
C
C
C
C
C
CH2OH
OH
H
H
OHH
HO
HO
H
OH
OOH
H
OHOH
HH
H
OH
CH2OH
H
OH
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D-glucose vs. D-galactoseD-glucose vs. D-galactose
C
C
C
C
C
CH 2OH
OH
OH
H
OHH
HO
H
H
OH O
C
C
C
C
C
CH 2OH
OH
H
H
OHH
HO
HO
H
H
D-glucose D-galactose
Can you find a difference? Your body can!
You can’t digest galactose - it must be converted to glucose first.
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D-riboseD-ribose
An important sugar usedAn important sugar usedin genetic material.in genetic material.
This sugar is not used as an energy source but is a part of the backbone of RNA.
When the C-2 OH is removed,the sugar becomesdeoxyribosedeoxyribose which is usedin the backbone of DNA.
H | C=O | H-C-OH | H-C-OH | H-C-OH |
CH2OH
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Reactions of glucose and other monosaccharides
Reactions of glucose and other monosaccharides
Oxidation-Reduction.Oxidation-Reduction. Required for their complete metabolic breakdown.
Esterification.Esterification. Production of phosphate esters.
Amino derivatives.Amino derivatives. Used to produce structural components and glycoprotein.
Glycoside formation.Glycoside formation. Linkage of monosaccharides to form polysaccharides.
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Aldehyde sugars (reducing sugars) are readily oxidized and will react with Benedict’s reagent.
This provides a good test for presence of glucose in urine - forms a red precipitate.
Other testsOther tests - Tollen’s or Fehling’s solutions.
Oxidation-Reduction.Oxidation-Reduction.
H | C=O | + 2 Cu 2+ + 5 OH-
H-C-OH |
CH2OH
O- | C=O | + 2 Cu2O + 3H2O H-C-OH |
CH2OH
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Benedict’s reagentBenedict’s reagent
Benedict'sReagent
0.5% 2%
glucose
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Ketones are not easy to oxidize except ketoses.
Enediol reaction.Enediol reaction.
So all monosaccharides are reducing sugars.
Ketone sugarsKetone sugars
C
C
C
C
C
CH 2 OH
OH
OH
H
OHH
HO
H
H
OH
C
C
C
C
C
CH 2 OH
OH
OH
H
OH
HO
H
H
C
C
C
C
CH2OH
CH 2 OH
OH
OH
H
OH
HO
H
H
OHH
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EsterificationEsterification
Esters are formed by reaction of hydroxyl groups (alcohols) with acids.
The hydroxyl groups of carbohydrates react similarly to alcohols.
R OH C R'O
HOR O C
OR' + H2O+
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EsterificationEsterification
The most important biological esters of carbohydrates are phosphate esters.
Example.Example. Phosphoryl group from ATP forms an ester with D-glucose, catalyzed by kinases.
D-glucose + ATP D-glucose-6-phosphate + ADP
kinase
R OH P OHO
OH+ H2O+HO P OH
O
O - RHO
38
Amino derivativesAmino derivatives
The replacement of a hydroxyl group on a carbohydrate results in an amino sugar.
H O
OH
OH
H
H
OHH
OH
CH2OH
HH O
OH
OH
H
H
NH2H
OH
CH2OH
H
-D-glucose -D-2-aminoglucose (glucosamine)
39
Amino derivativesAmino derivatives
Uses for amino sugars.Uses for amino sugars.
Structural components of bacterial cell walls.
As a component of chitin, a polymer found in the exoskeleton of insects and crustaceans.
A major structural unit of chondroitin sulfate - a component of cartilage.
Component of glycoprotein and glycolipids.
40
Glycoside formationGlycoside formation
or -OH group of cyclic monosaccharide can form link with another one (or more).
glycosidic bond
sugar -O- sugar
oxygen bridge
OH
OH
OHOH
H
H
H
OH
CH2OH
HO H
OH
H
OHOH
H
OH H
OH
CH2OH
H
OH
OH
OH H
H
H
OH
CH2OH
H
O H
OH
H
H
OH H
OH
CH2OH
H
OH
oo
+ H2 O
41
Glycosidic bondsGlycosidic bonds
OO
Type is based on the position of the C-1 OH
glycosidic bondglycosidic bond - linkage between a C-1 OH and a C-4 OH
glycosidic bondglycosidic bond - linkage between a C-1 OH and a C-4 OH
bonds bonds
O O
C-4 end can be either up or down dependingon the orientation of the monosaccharide.
42
Glycosidic bondsGlycosidic bonds
O O
O O bonds bonds bonds bonds
43
Glycosidic bondsGlycosidic bonds
General format used to describe bond.
OH type carbon# of carbon# of ( or ) first sugar second sugar
As we work through the next few examples this will become clear.
For disaccharidesFor disaccharides - the sugar is either or based on form of the remaining C-1 OH.
( )
44
-Maltose-Maltose
Malt sugar. Malt sugar. Not common in nature except in germinating grains.
-D-glucose -D-glucose
OH
OH
H
H
H
OH
CH 2 OH
H
OH
OH OH
H
H
H
OH
CH 2 OH
H
OH
O
-D-glucose and -D-glucose, (1 4) linkage.
45
-Maltose-Maltose
It is referred to as -maltose because the unreacted C-1 on -D-glucose is in the position.
OH
OH
H
H
H
OH
CH 2 OH
H
OH
OH OHOH
H
H
H
OH
CH 2 OH
H
OH
O
46
-Maltose-Maltose
Uses for Uses for -maltose-maltoseIngredient in infant formulas.Production of beer.Flavoring - fresh baked aroma.
It is hydrolyzed the in body by:
maltose + H2O 2 glucosemaltase
47
CellobioseCellobiose
Like maltose, it is composed of two molecules of D-glucose - but with a (1 4) linkage.
H O
OH
H
OHH
OH
CH2OH
H
H
O
H O
H
OHH
OH
CH2OH
H
H
OH
48
CellobioseCellobiose
OH
OH
H
H
H
OH
CH 2 OH
H
OH
OH OH
HH
H
OH
CH 2 OH
H
OH
O
H O
OH
H
OHH
OH
CH2OH
H
H
O
H O
H
OHH
OH
CH2OH
H
H
OH
The difference inthe linkage resultsin cellobiose being unusable
We lack an enzymethat can hydrolyzecellobiose.
cellobiose (1 4)
maltose, (1 4)
49
LactoseLactose
Milk sugarMilk sugar - dimer of -D-galactose and either the or - D-glucose.
-Lactose-Lactose
OOH
H H
H
H
OH
CH 2 OH
H
OH
OH OH
H
H
H
OH
CH 2 OH
H
OH
O
-D-galactose -D-glucose
(1 4) linkage, disaccharide.
50
LactoseLactose
We can’t directly use galactose. It must be converted to a form of glucose.
GalactosemiaGalactosemia - absence of needed enzymes needed for conversion.
Build up of galactose or a metabolite like dulcitol (galactitol) dulcitol (galactitol) causes toxic effects.
Can lead to retardation, cataracts, death.
51
LactoseLactose
LactaseLactaseEnzyme required to hydrolyze lactose.
Lactose intoleranceLactose intoleranceLack or insufficient amount of the enzyme.
If lactase enters lowerGI, it can cause gasand cramps.
52
SucroseSucrose
Table sugar - most common sugar in all plants.
Sugar cane and beet, are up to 20% by mass sucrose.
Disaccharide of -glucose and -fructose.
(1 2) linkage
CH2OH O
CH2OHH
OH H
H OH
H O
OH
H
H
OHH
OH
CH2OH
H
O
53
SucroseSucrose
glucose
fructose
54
How sweet it is!How sweet it is!
Sweetness relative
Sugar to sucrose
lactose 0.16 galactose 0.32 maltose 0.33 sucrose 1.00 fructose 1.73 aspartame 180 saccharin 450
Sweetness relative
Sugar to sucrose
lactose 0.16 galactose 0.32 maltose 0.33 sucrose 1.00 fructose 1.73 aspartame 180 saccharin 450
55
PolysaccharidesPolysaccharides
Carbohydrate polymersCarbohydrate polymers
Storage PolysaccharidesStorage PolysaccharidesEnergy storage - starch and glycogen
Structural PolysaccharidesStructural PolysaccharidesUsed to provide protective walls or lubricative coating to cells - cellulose and mucopolysaccharides.
Structural PeptidoglycansStructural PeptidoglycansBacterial cell walls
56
StarchStarch
Energy storage used by plants
Long repeating chain of -D-glucose
Chains up to 4000 units
AmyloseAmylose straight chainmajor form of
starch
AmylopectinAmylopectin branched structure
57
Amylose starchAmylose starch
Straight chain that forms coils (1 4) linkage. Most common type of starch.
OH H
H
OHH
OH
CH2OH
H
OH H
H
OHH
OH
CH2OH
H
O O
OH H
H
OHH
OH
CH2OH
H
OH H
H
OHH
OH
CH2OH
H
O O
O
OO
OO
O
O
OO
OO
O
O
OO
OO
O
O
OO
OO
O
58
Amylose starchAmylose starch
Example showing coiled structure - 12 glucose units - hydrogens and side chains are omitted.
59
Amylopectin starchAmylopectin starch
Branched structure due to crosslinks.
(1 6) linkageat crosslink
OHH OHH OHH
OH H
HOH
CH2OH
H
OH H
HOH
CH2OH
H
O O
OH H
HOH
CH2OH
H
OH H
H
OHH
OH
CH2OH
H
O O
OH H
H
OHH
OH
CH2OH
H
OH H
H
OHH
OH
CH2OH
H
O O
OH H
H
OHH
OH
CH2OH
H
OH H
H
OHH
OH
CH2
H
O O
60
c
GlycogenGlycogen
• Energy storage of animals.• Stored in liver and muscles as granules.• Similar to amylopectin.
(1 6) linkageat crosslink
O
O
O
O
O
O
O
O
O
O
O
O
O
O
c
61
CelluloseCellulose
• Most abundant polysaccharide.• (1 4) glycosidic linkages.• Result in long fibers - for plant structure.
OH
H
H
OHH
OH
CH2OH
H
OH
H
H
OHH
OH
CH2OH
H
O
O
OH
H
H
OHH
OH
CH2OH
H
OH
H
H
OHH
OH
CH2OH
H
O
O
OH
H
H
OHH
OH
CH2OH
H O
62
H OO
HH
OHH
COO-
HO
H O
OH
O
HH
NH
CH2OH
H
C OCH3
H OO
HH
OHH
COO-
HO
H O
OH HH
NH
CH2OH
H
C OCH3
H OO
HH
OHH
COO-
HO
H O
OH
O
HH
NH
CH2OH
H
C OCH3O
MucopolysaccharidesMucopolysaccharides
These materials provide a thin, viscous, jelly-like coating to cells.
The most abundant form ishyaluronic acid.
Alternating units of N-acetylglucosamine and D-glucuronic acid.
(1 3)
(1 4)
63
Structural peptidoglycansStructural peptidoglycans
Bacterial cell walls are composed primarily of an unbranched polymer of alternating units of N-acetylglucosamine and N-acetylmuramic acid.
Peptide crosslinks between the polymer strands provide extra strength - varies based on bacterium.
H O
O H
H
NHH
OH
CH2OH
H
C
CH3
O
H O O
H
H
NHH
OR
CH2OH
H
C
CH3
O
O
CH3
CH
O L-Ala
D-Isoglu
L-Lys
D-Ala
(Gly)5
(Gly)5
R =
crosslink forcrosslink forStaphylococcusStaphylococcusaureusaureus
64
GlycoproteinsGlycoproteins
Proteins that carry covalently bound carbohydrate units.
They have many biological functions.
• immunological protection
• cell-cell recognition
• blood clotting
• host-pathogen interaction
65
Glycoprotein structureGlycoprotein structure
Carbohydrates only account for 1-30% of the total weight of a glycoprotein.
The most common monosaccharides are
glucoseglucose mannosemannose
galactose galactose fucosefucose
sialic acidsialic acid
N-acetylgalactosamineN-acetylgalactosamine
N-acetylglucosamineN-acetylglucosamine
66
Glycoprotein structureGlycoprotein structure
Linking sugars to Linking sugars to proteins.proteins.
O-glycosidic bonds using hydroxyl groups of serine
and threonine
N-glycosidic bonds using side chain amide nitrogen
of asparagine residue
C
C
C H
O
CH3
C H2
C
O
H N
OO
H H
H
NHCOCH3H
OH
CH2OH
H
H O
O H
H
NHCOCH3H
OH
CH2OH
H
poly
pep
tid
e c
hain
threonine
asparagine
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