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CARBOHYDRATES: CARBOHYDRATES: STRUCTURE AND STRUCTURE AND FUNCTIONFUNCTION
ObjectivesObjectives
To understand the structure of carbohydrates To understand the structure of carbohydrates of physiological significanceof physiological significance
To understand the main role of carbohydrates To understand the main role of carbohydrates in providing and storing of energy in providing and storing of energy
To understand the structure and function of To understand the structure and function of glycosaminoglycans glycosaminoglycans
OVERVIEWOVERVIEW
The most abundant organic molecules The most abundant organic molecules in naturein nature
– provide important part of energy in dietprovide important part of energy in diet– Act as the storage form of energy in the bodyAct as the storage form of energy in the body– are structural component of cell membraneare structural component of cell membrane
The empiric formula is (CHThe empiric formula is (CH22O)n – O)n –
“hydrates of carbon”“hydrates of carbon”
OVERVIEWOVERVIEW
Diseases associated with disorders Diseases associated with disorders of carbohydrate metabolism: of carbohydrate metabolism:
– Diabetes mellitus Diabetes mellitus
– GalactosemiaGalactosemia
– Glycogen storage diseasesGlycogen storage diseases
– Lactose intoleranceLactose intolerance
CONT’D
CLASSIFICATION:CLASSIFICATION: Monosaccharides:Monosaccharides: Simple sugar Simple sugar
Disaccharides: Disaccharides: 2 monosaccharide 2 monosaccharide unitsunits
Oligosaccharides: Oligosaccharides: 3-10 3-10 monosaccharide unitsmonosaccharide units
Polysaccharides:Polysaccharides: more than 10 more than 10 sugar unitssugar unitsHomopolysaccharides and heteropolysaccharidesHomopolysaccharides and heteropolysaccharides
MonosaccharidesMonosaccharides
Further classifiedFurther classified based on: based on:
1. No. of carbon atoms1. No. of carbon atoms 2. Functional group:2. Functional group:
Aldehyde group – aldosesAldehyde group – aldosesKeto group – ketosesKeto group – ketoses
So
me
So
me
Mo
no
sacc
har
ides
Mo
no
sacc
har
ides
All carbons in a monosaccharide are bonded to a hydroxyl group (-OH) except
for one which is bonded to a carbonyl group (=O) (note that this statement is true
only for the linear form of monosaccharides)
So
me
So
me
Mo
no
sacc
har
ides
Mo
no
sacc
har
ides
IsomerismIsomerism
IsomersIsomers
Compounds having same Compounds having same chemical formula but chemical formula but different structural different structural formulaformula
The No. of isomers depends The No. of isomers depends on the No. of asymmetric on the No. of asymmetric C C
So
me
So
me
Mo
no
sacc
har
ides
Mo
no
sacc
har
ides
The
tw
o si
mpl
est
suga
rs
Note Numerous Chiral Carbons
Sugar IsomersSugar Isomers
1.1. Aldo-ketoAldo-keto
2.2. EpimersEpimers
3.3. D- and L-FormsD- and L-Forms
4.4. αα- and - and ββ-anomers-anomers
Aldo-Keto IsomersAldo-Keto Isomers
Example:Example:
Glucose and Glucose and fructosefructose
EpimersEpimers EpimersEpimersCHO dimers that differ in CHO dimers that differ in
configuration around configuration around only only one one specific carbon atomspecific carbon atom
-Glucose and galactose, C4-Glucose and galactose, C4
-Glucose and Mannose, C2-Glucose and Mannose, C2
Galactose and mannose Galactose and mannose are are not not epimers, whyepimers, why??
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Figure: Epimers of glucose.Figure: Epimers of glucose.
Structures that are Structures that are mirror imagesmirror images of each of each other and are other and are designated as D- and designated as D- and L- sugars based on the L- sugars based on the position of –OH grp on position of –OH grp on the the asymmetric asymmetric carbon farthest from carbon farthest from the carbonyl carbonthe carbonyl carbon
Majority of sugars in Majority of sugars in humans are humans are D-sugarsD-sugars
Enantiomers Enantiomers (D- and L-Forms)(D- and L-Forms)
αα- and - and ββ-Forms-Forms
CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
HOH2C
OH
CH2OH
HOH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
H O
OH
H
OHH
OH
CH2OH
H
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose -D-glucose
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4
5
6
1 1
6
5
4
3 2
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
1
5
2
3
4
6
D-glucose (linear form)
Cyclization of Monosaccharides Monosaccharides with 5 or more carbon are predominantly found in the ring form
-The aldehyde or ketone grp reacts with the –OH grp on the same
sugar
-Cyclization creates an anomeric carbon (former carbonyl carbon) generating the α and β configurations
MutarotationMutarotationIn solution, the cyclic In solution, the cyclic αα and and ββ anomers of a anomers of a
sugar are in equilibrium with each other, sugar are in equilibrium with each other, and can be interconverted spontaneouslyand can be interconverted spontaneously
Fischer Projection Haworth Projection
DisaccharidesDisaccharides:: Joining of 2 monosaccharides Joining of 2 monosaccharides by by O-O-
glycosidic bond:glycosidic bond:
Maltose (Maltose (αα-1, 4) = glucose + glucose-1, 4) = glucose + glucose
Sucrose (Sucrose (αα-1,2) = glucose + fructose -1,2) = glucose + fructose
Lactose (Lactose (ββ-1,4) = galactose + glucose-1,4) = galactose + glucose
Lactulose (Lactulose (ββ-1,4) = galactose + fructose-1,4) = galactose + fructose
DisaccharideDisaccharide
DisaccharidesDisaccharides
LactoseLactose
CONT’D
PolysaccharidesPolysaccharides HomopolysaccharidesHomopolysaccharides::
BranchedBranched:: glycogen ( storage in humans) and starch glycogen ( storage in humans) and starch ( plants), both (( plants), both (αα-glycosidic polymer of glucose)-glycosidic polymer of glucose)
UnbranchedUnbranched: cellulose (: cellulose (ββ-glycosidic polymer)-glycosidic polymer)
HeteropolysaccharidesHeteropolysaccharides::
e.g., e.g., glycosaminoglycans (GAGs)glycosaminoglycans (GAGs)
Reducing SugarsReducing Sugars If the If the O on the anomeric CO on the anomeric C of a sugar is not of a sugar is not
attached to any other structure, that sugar can attached to any other structure, that sugar can act act as a reducing agentas a reducing agent
RReducing sugars reduce chromogenic agents educing sugars reduce chromogenic agents like like Benedict’s reagentBenedict’s reagent or or Fehling’s solutionFehling’s solution to give a colored periceptateto give a colored periceptate
UUrine is tested for the presence of reducing rine is tested for the presence of reducing sugars using these colorimetric testssugars using these colorimetric tests
Reducing SugarsReducing Sugars
Examples:Examples:
MonosaccharidesMonosaccharides
Maltose and LactoseMaltose and Lactose
Sucrose is non-reducing, Why?Sucrose is non-reducing, Why?
CONT’D
Complex CarbohydratesComplex Carbohydrates Carbohydrates attached to non-carbohydrate Carbohydrates attached to non-carbohydrate
structures by glycosidic bonds (O- or N-type) structures by glycosidic bonds (O- or N-type) e.g.e.g.
11. . Purine and pyrimidine bases in nucleic acidsPurine and pyrimidine bases in nucleic acids
2. Aromatic rings in steroids2. Aromatic rings in steroids
3. Proteins in glycoproteins and glycosaminoglycans3. Proteins in glycoproteins and glycosaminoglycans
4. Lipids found in glycolipids4. Lipids found in glycolipids
5. Bilirubin5. Bilirubin
Glycosidic BondsGlycosidic Bonds
N-GlycosidicN-Glycosidic
O-Glycosidic
Glycosaminoglycans (GAGs)Glycosaminoglycans (GAGs) GlycosaminoglycansGlycosaminoglycans (GAGs) are large complexes of (GAGs) are large complexes of
negatively charged heteropolysaccharidenegatively charged heteropolysaccharide chains chains are associated with a small amount of protein, forming are associated with a small amount of protein, forming
proteoglycansproteoglycans, which consist of over 95 percent , which consist of over 95 percent carbohydratecarbohydrate
bind with large amounts of water, producing the gel-like bind with large amounts of water, producing the gel-like matrix that forms body's ground substance matrix that forms body's ground substance
The viscous, lubricating properties of mucous secretions The viscous, lubricating properties of mucous secretions also result from GAGs, which led to the original naming also result from GAGs, which led to the original naming of these compounds as of these compounds as mucopolysaccharidesmucopolysaccharides
Glycosaminoglycans (GAGs)Glycosaminoglycans (GAGs) GAGs GAGs are are linear polymerslinear polymers of of repeating repeating disaccharidedisaccharide units units
[acidic sugar-amino sugar] n [acidic sugar-amino sugar] n The amino sugar (The amino sugar (usually sulfatedusually sulfated) is ) is either either
D-glucosamine or D-galactosamineD-glucosamine or D-galactosamine The acidic sugar is either The acidic sugar is either
D-glucuronic acid or L-iduronic D-glucuronic acid or L-iduronic acidacid GAGs are strongly negatively-charged: GAGs are strongly negatively-charged: carboxyl groups of acidic sugarscarboxyl groups of acidic sugars & & Sulfate groupsSulfate groups
Resilience of GAGsResilience of GAGs Being negatively charged GAG chains are Being negatively charged GAG chains are
extended in solution and repel each other and extended in solution and repel each other and when brought together, they "slip" past each when brought together, they "slip" past each otherother
This produces the This produces the ""slippery" consistency of slippery" consistency of mucous secretions and synovial fluidmucous secretions and synovial fluid
When a solution of GAGs is compressed, the When a solution of GAGs is compressed, the water is "squeezed out" and the GAGs are water is "squeezed out" and the GAGs are forced to occupy a smaller volume. When the forced to occupy a smaller volume. When the compression is released, the GAGs spring back compression is released, the GAGs spring back to their original, hydrated volume because of to their original, hydrated volume because of the repulsion of their negative chargesthe repulsion of their negative charges
This property contributes to the This property contributes to the resilience of resilience of synovial fluid and the vitreous humor of synovial fluid and the vitreous humor of the eyethe eye
Examples of GAGs are:Examples of GAGs are:
1.1. Chondroitin sulfatesChondroitin sulfates
2.2. Keratan sulfatesKeratan sulfates
3.3. Hyaluronic acidHyaluronic acid
4.4. HeparinHeparin
Members of GAGsMembers of GAGs
CHONDROITIN SULFATESCHONDROITIN SULFATES
Disaccharide unit:Disaccharide unit: Sulfated Sulfated N-acetyl-galactosamine N-acetyl-galactosamine
+Glucuronic acid+Glucuronic acid
MostMost abundantabundant GAG in the GAG in the bodybody
Form proteoglycan Form proteoglycan aggregatesaggregates
Found in cartilage, tendons, Found in cartilage, tendons, ligaments, and aortaIn ligaments, and aortaIn cartilage, they bind collagen cartilage, they bind collagen and hold fibers in a tight, and hold fibers in a tight, strong networkstrong network
KERATAN SULFATESKERATAN SULFATES Disaccharide unit:Disaccharide unit:
N-acetylglucosamine N-acetylglucosamine
Galactose Galactose ((no uronic acidno uronic acid)) Sulfate content is variable Sulfate content is variable
and may be present on C-6 and may be present on C-6 of either sugarof either sugar
Most heterogeneous GAGsMost heterogeneous GAGs Present in loose Present in loose
connective tissue and connective tissue and corneacornea
HYALURONIC ACIDHYALURONIC ACID Disaccharide unit:Disaccharide unit:
N-acetylglucosamine N-acetylglucosamine
Glucuronic acidGlucuronic acid Different from other GAGs: Different from other GAGs:
UnsulfatedUnsulfated
Not covalently attached to proteinNot covalently attached to protein
The only GAG found in The only GAG found in bacteriabacteria
Serves as a lubricant and shock Serves as a lubricant and shock absorberabsorber
Found in synovial fluid of joints, Found in synovial fluid of joints, vitreous humor of the eye, the vitreous humor of the eye, the umbilical cord, and cartilageumbilical cord, and cartilage
HEPARINHEPARIN Disaccharide unit:Disaccharide unit:
Glucosamine and Glucosamine and
Glucuronic or iduronic acidsGlucuronic or iduronic acids Sulfate is found on Sulfate is found on
glucosamine and uronic acid glucosamine and uronic acid Unlike other GAGs that are Unlike other GAGs that are
extracellular, heparin is an extracellular, heparin is an intracellularintracellular component of component of mast cells that line arteries, mast cells that line arteries, especially liver, lungs and especially liver, lungs and skinskin
Serves as Serves as anticoagulantanticoagulant