Carbohydrates

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Carbohydrates

• Dr. Martina Rezácová

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Carbohydrates

• Widely distributed in nature
• Function
• Structural
• Source of energy
• Storage of energy
• Chemical structure
• Polyhydroxyaldehydes - aldoses
• Polyhydroxyketones - ketoses
• Classification
• monosaccharides (1 unit)
• oligosaccharides (2-10 units)
• polysaccharides (gt 10 units)

SUGARS
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Monosaccharides

• Chemical structure
• Polyhydroxyaldehydes - aldoses
• Polyhydroxyketones - ketoses

• Number of carbon atoms
• trioses
• tetroses
• pentoses
• hexoses
• heptoses

• Contain asymmetric carbon atoms C gt optically
  active

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Asymmetric carbon
L-
D-
glyceradehyde
Most natural saccharides D-form
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Enantiomers

• Optical isomers rotate the beam of
  plane-polarized light for the same angle, but in
  opposite direction
• Equimolar mixture of optical isomers has no
  optical activity - racemic mixture
• Monosaccharides two series D- and L-
  according to orientation of -H and -OH on last
  C (reference carbon) , based on orientation of
  the simplest aldose glyceraldehyde

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D and L configuration
optical antipods enantiomers
D-ribose
Racemic mixture is opticaly inactivne
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Epimers

• Pairs of monosaccharides different only in
  configuration on C adjacent to aldo/oxogroup

D-mannose
D-glucose
CH2OH
CH2OH
D-fructose
D-psicose
CH2OH
CH2OH
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Cyclic structure

• Acyclic structure cannot account for some of the
  properties
• Cyclic hemiacetal forms exist

O
H
H
OH
OH
H


C
C
OH
HO
OH
O
O
OH
CH2OH
CH2OH
?-
?-
anomers
? hemiacetal hydroxyl of the same orientation
as reference C
? hemiacetal hydroxyl of the opposite
orientation then reference C
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Cyclic structure

• most stable are rings with 5 and 6 members

most common in nature

• in accordance to oxygen containing heterocycles
  monosaccharides are called

with 5 atoms in cycle furanoses
with 6 atoms in cycle pyranoses
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Mutarotation

• ??and ??anomers have different specific rotatory
  power gt reaching equilibrium is accompanied by
  changes in optical rotatory power of solution

? -D-glucopyranose
?-D-glucofuranose
OH
H
?-D-glucopyranose
?-D-glucofuranose
OH
H
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Haworth projection

• more accurate for cyclic forms
• transcription from Fisher projection
• What was on right goes down
• What was on left goes up
• O is always up and right
• D-forms CH2OH always up
• can be flipped and turned freely

H
OH
CH2OH
C
O
O
CH2OH
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Haworth projection of ?-D-glucopyranose
It is not planar!!!
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Saccharides isomerism - rewiev

• STRUCTURE isomerism (constitutional)
• same summary formula, different functional
  groups
• aldose ketose
• glucose fructose
• OPTICAL isomerism (antipods, enantiomers)
• D- and L- isomerism
• mirror images
• D-glucose L-glucose
• EPIMERISM
• different orientation of hydroxyl on one C
• ( not reference)
• glucose manose
• glucose galactose
• PYRANOSE-FURANOSE-ACYKLIC FORM
• ?-D-glucopyranose ?-D-glucofuranose
• acyclic glucose

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Derivates of monosaccharides

• Deoxysaccharides
• one hydroxyl substituted by hydrogen
• deoxyribofuranose - part of DNA
• Aminosaccharides
• hydroxyl on usualy second C substituted by -NH2
• glucosamine, galactosamine - part of
  proteoglycans

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Reactions- oxidation
Cyclic form - inner hemiacetals, can form
glykosidic bond - glycosiduronates
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Reactions of monosaccharides
Oxidation of hemiacetal hydroxyl

• hemiacetal hydroxyl reduces Fehling (Cu 2) and
  Tollens (Ag) reagent
• proof of monosaccharides

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Reactions- Isomerization

• In alkaline solutions
• In organism catalyzed by enzymes

CH2-OH
ketose
O
CH-OH
endiol
CH-OH
CHO
aldose
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Reactions- Esterification

• With acids
• Most reactive is hemiacetal hydroxyl
• In organism catalyzed by enzymes, mainly with
• trihydrogenphosphoric a.
• sulphuric a.
• GLC-6-P, FRU-1,6-bisphosphate
  galaktosamine-6-sulphate

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Important monosacchrides

• Ribose
• Constituent of RNA, ATP, Coenzyme A, NAD, and
  other coenzymes
• Deoxyribose constituent of DNA
• Glucose
• Key sugar in metabolism, primary energy source of
  the body
• Constituent of the long chain carbohydrates
  starch and cellulose
• Also known as dextrose or blood sugar
• At equilibrium about 36 exists in the alpha form
  and 64 exists in the beta form.

• Galactose
• One monosaccharide in the disaccharide lactose
  (milk sugar)
• Found in plant gums and pectin polysaccharides
• Fructose
• One monosaccharide in the disaccharide sucrose
  (table sugar).
• Called levulose or fruit sugar
• Found in honey and fruits
• Sweeter than sucrose and glucose

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GLYCOSIDIC BOND

• By reactions of hemiacetal hydroxyl acetal bond
  is formed
• with hemiacetal hydroxyl of other saccharide
• nonreducing disaccharides
• with other then hemiacetal hydroxyl of other
  saccharide
• reducing disaccharides
• with nonsugar hydroxyl (alcohol, sterol,
  heterocycle)
• O-glycosides
• with amino(imino) group
• N-glycosides

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Oligosaccharides

• 2 to 10 monosaccharide units linked by
  O-glycosidic bond
• di-, tri-, tetrasaccharides etc.
• most important Disaccharides
• Classification
• Reducing
• One hemiacetal hydroxyl is free, not involved in
  bond
• Nonreducing
• Both hemiacetal hydroxyls form glycosidic bond
  together

?-lactose
sucrose
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Disaccharides

• Reducing
• Maltose ?-D-Glc-(1-4)-D-Glc
• Isomaltose ?-D-Glc-(1-6)-D-Glc
• Hydrolysis of starch
• Malt
• Malt is prepared from germinating grain (barley).
  Amylases are produced during germination and
  hydrolyse starch to dextrines and maltose
  isomaltose. Hydrolysis is stopped by hot water
  (50 to 55 C). Malt extract also containes
  vitamines B, E and C, small amount of glucose,
  peptides and minerál salts.
• Lactose ??-D-Gal-(1-4)-D-Glc ?
• Milk, 4-8
• Lactose intolerance - lactase deficiency
• Cellobiose ?-D-Glc-(1-4)-D-Glc ?

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Disaccharides

• Nonreducing
• Sucrose ?-D-Fru-(2-1)-?-D-Glc
• Sugar-beet (Beta vulgaris) - head contains cca 14
  to 18 of sucrose
• Sugar-cane (Saccharum officinarum) - contains cca
  8 to 17 of sucrose
• Nonreducing

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Relative sweetness of various carbohydrates

• fructose 173
• invert sugar 120
• sucrose 100
• honey 94
• glucose 74
• sorbitol 55
• mannitol 50
• maltose 33
• galactose 32
• lactose 15

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Polysaccharides
more than 10 units
classification

• homopolysaccharides
• One type of units
• Glc - starch, glycogen, cellulose
• Gal - agar
• Fru - inulin
• heteropolysaccharides
• More types of units

1.

• linear
• branched
• interlinked

2.
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Glucans
homopolysaccharides of glucose

• starch
• amylose - nonbranched ? (1-4 )
• amylopectin - branched ? (1-4 ) ? (1-6 )
• glycogen - branched ? (1-4 ) ? (1-6 )
• cellulose - nonbranched ? (1-4 )

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Glucans
Starch Major form of stored carbohydrate in
plants Natural starches contain 10-20 amylose
and 80-90 amylopectin

• amylose - nonbranched ? (1-4 )
• 200 to 20 000 glucose units
• helix
• stained blue by iodine

• amylopectin - branched ? (1-4 ) ? (1-6 )
• up to million glucose units
• cca 5 of Glc have side chains of about 10-30
  glucose units
• stained red-brown by iodine

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Glucans

• glycogen - branched ? (1-4 ) ? (1-6 )
• similar to amylopectin
• more highly branched, shorter branches (about 13
  glucose units)
• not stained by iodine
• storage polysaccharide of animals, muscles,
  liver
• cleaved by alpha amylase

• cellulose - nonbranched ? (1-4 )
• firm, insoluble, chemically very resistant
• major structural material of plants
• wood, cotton

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Other homopolysaccharides

• inulin fru - fru ? ( 1- 2 )
• storage polysaccharide in tubers and roots of
  plants - garlic, onion, dahlias, artichokes,
  dandelions

• agar gal -gal ? ( 1- 4 ) a ? ( 1- 3 )
• more precisely D-gal and L-3,6dehydrogal
• sea algaegelling agent - food, culture media,
  imprinting materials

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Ohter homopolysaccharides

• chitin N-acetyl-D-glukosamine ? ( 1- 4 )
• structural polysaccharide of exoskeleton of
  invertebrates (in complex with proteins and/or
  CaCO3)
• pectin D-galakturonic acid a (1-4)
• methylesters
• often side chains containing other
  monosaccharides
• small fruits, plants
• forms jelly

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Heteropolysaccharides - ALGINATES

• Salts of alginic acid - polyuronide made up of a
  sequence of two uronic acid residues
• b-D-mannuronic a.
• a-L-guluronic a.
• Residues distributed irregularly
• form blocks of up to twenty units
• depends on the species of seaweed
• determine the physical properties
• Imprinting materials
• less accurate than agar
• cheap, tasty

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Heteropolysaccharides
Most important GLYCOSAMINOGLYCANS Common
structure N-acetylhexosamine (glucosamine,
galactosamine) uronic acid (gluccouronic,
iduronic)
Always in complex with proteins -
proteoglycans Part of inercellular matrix (bone,
cartilage, ligament)
hyaluronic a. N-acetylglucosamine, glucuronic
a. chondoitinsulphate N-acetylgalactosaminesulphat
e, glucuronic a. dermatansulphate
N-acetylgalactosaminesulphate, iduronic
a. heparin glucosaminesulphate, both acids
(sulphated)
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Proteoglycans

• Ground of packing substance
• Molecule resembles forrest - proteoglycan
  subunits are noncovalently bond to molecule of
  hyaluronic acid
• Proteoglycan subunits
• protein core
• covalently bond glycosaminoglycans
  (mucopolysaccharides - keratansulphate,
  chondroitinsulphate)
• Hold large quantities of water, cushioning and
  lubrication of other structures
• Glycosylation in ER