Transcript L refers to the left hand orientation of the OH on the chiral carbon

Carbohydrates
• are polyhydroxyderivatives of
aldehydes or ketones
• have the general molecular formula
CH2O
Roles of Carbohydrates
• Storage of metabolic fuel
• Structural framework of nucleic acids
• Structural elements in the cell walls of
bacteria and plants
• Linked to proteins and lipids which are
involved in cell recognition
Monosaccharides
• The basic units in
carbohydrates
• Could either be an
aldose or ketose
• Typically contain 3 to
nine carbons
Monosaccharides are classified
by their number of carbon atoms
Name
Formula
Triose
Tetrose
Pentos e
C3 H6 O3
C4 H8 O4
Hexose
Heptose
Octose
C5 H1 0 O 5
C6 H1 2 O 6
C7 H1 4 O 7
C8 H1 6 O 8
EXERCISE
• Draw the possible structure(s) of
1. aldotetrose
2. ketotetrose
Monosaccharides
Stereoisomers
Carbohydrates contain
many chiral carbons
OPTICAL ISOMERS
• Stereosiomers can be distinguished using plane
of polarized light
Naming stereoisomers
Based on the structure of L- and D-glyceraldehyde
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
D- sugars are the most common
carbohydrates. D-refers to the right
hand orientation of the OH on the
chiral carbon farthest from the
carbonyl carbon
L-sugars: L refers to the left hand
orientation of the OH on the chiral
carbon farthest from the carbonyl
carbon
Glucose
• The most common
sugar
• Known as dextrose
• Also known as
blood sugar
H
O
C
H
C
OH
HO
C
H
H
C
OH
H
C
OH
CH2OH
D-glucose
Galactose
• One of the monosaccharides in the
disaccharide lactose
• Found in plant gums and pectins
polysaccharides
• An epimer of glucose at C4
• Converted to glucose during
metabolism
Fructose
• One monosaccharide in the disaccharide
sucrose (table sugar).
CH2OH
• Called levulose or fruit sugar
C O
• Found in honey and fruits
HO C H
• Sweeter than sucrose and glucose
H C OH
• Commercially prepared as highH C OH
fructose sugars from corn starch for sweetness
CH2OH
• An epimer of glucose at C4
D-fructose
• Readily converted to glucose in metabolism by isomerization
Reactions of Monosaccharides
• Oxidation
Oxidation using a mild
oxidizing agent
Oxidation using a strong
oxidizing agent
Oxidation to Uronic Acids
CHO
enzymeH
OH
catalyzed
HO
H
oxidation
H
OH
H
OH
CH2 OH
D -Glu cose
H
HO
H
H
CHO
OH
H
OH
OH
COOH
COOH
HO
HO
D -Glucu ronic acid
(a u ronic acid )
O
OH
OH
Reduction to alditol
Reaction with dilute base
Reaction with a concentrated
base-fragmentation
Alcohols react readily with
aldehydes to form hemiacetals
ketones react readily with alcohols to
form hemiketals
Fructose forms either
CH2OH
1
HO
H
H
2C
O
C
H
C
OH
C
OH
3
4
5
6
HOH2C 6
CH2OH
D-fructose (linear)
H
5
H
1 CH2OH
O
4
OH
HO
2
3
OH
H
-D-fructofuranose
 a 6-member pyranose ring, by reaction of the C2 keto group
with the OH on C6, or
 a 5-member furanose ring, by reaction of the C2 keto group
with the OH on C5.
The formation of hemiketals and hemiacetals results
in an asymmetric carbon atom.
Isomers that differ only in their configuration about
the new asymmetric carbon are called anomers, the
new assymetric carbon is called anomeric carbon.
a-anomer has the hydroxyl group at the right side of
the anomeric carbon
-anomer has the hydroxyl group at the left side of
the anomeric carbon
Mutarotation: the change in specific rotation
that accompanies the equilibration of - and anomers in aqueous solution
• example: when either -D-glucose or -D-glucose is
dissolved in water, the specific rotation of the solution
gradually changes to an equilibrium value of +52.7°,
which corresponds to 64% beta and 36% alpha forms
HO
HO
CH2 OH
O
OH
OH
-D -Glucopyranose
[] D 2 5 = + 18.7°
HO
HO
CH2 OH
OH
O
HO
C
H
Open-chain form
HO
HO
CH2 OH
O
HO
OH
-D -Glucopyranose
[] D 2 5 = +112°
EXERCISE
D-Mannose exists in aqueous solution as
a mixture of  and  forms. Draw
Haworth projections.
Because of the tetrahedral nature of carbon
bonds, pyranose sugars actually assume a
"chair" or "boat" configuration,
depending on the sugar.
The representation above reflects the chair
configuration of the glucopyranose ring
more accurately than the Haworth
projection.
For pyranoses, the six-membered ring is
more accurately represented as a chair
conformation
HO
HO
CH2 OH
O
anomeric
carbon
OH()
OH
 -D -Glu copyran os e
( - D -Glucos e)
HO
HO
CH2 OH
OH
O
C
OH H
D -Glucos e
HO
HO
CH2 OH
O
HO
OH( )
- D -Glu copyran os e
(  - D -Glucose)
EXERCISE
Draw the chair conformations for -D
mannopyranose and -D-mannopyranose
Formation of Glycosides
• Treatment of a monosaccharide, all of which exist
almost exclusively in a cyclic hemiacetal form,
with an alcohol gives an acetal
anomeric
carbon
CH2 OH
O OH
H
+
H
H
+ CH3 OH
OH H
-H2 O
HO
H
glycos idic
H OH
CH2 OH
bond
-D -Glu copyran os e
O OCH3
H
(-D -Glu cose)
H
+
OH H
H
HO
CH2 OH
OH
H
H
OH H
HO
OCH3
H OH
H OH
Methyl -D -glu copyran os ide Methyl -D -glu copyran os ide
(Methyl -D -glu coside)
(Methyl -D -glucos ide)
Disaccharides
Maltose
– present in malt, the juice from sprouted
barley and other cereal grains
– maltose consists of two units of Dglucopyranose joined by an -1,4glycosidic bond
– maltose is a reducing sugar
Lactose
– lactose is the principal sugar present in milk;
it makes up about 5 to 8 percent of human
milk and 4 to 6 percent of cow's milk
– it consists of D-galactopyranose bonded by a
 -1,4-glycosidic bond to carbon 4 of Dglucopyranose
– lactose is a reducing sugar
Sucrose
– is the most abundant disaccharide in
the biological world; it is obtained
principally from the juice of sugar cane
and sugar beets
– sucrose is a nonreducing sugar
EXERCISE
• Draw the chair conformation of
1. methyl - and -D-mannopyranoside
2. -D-mannopyranosyl-(1-4)- -Dglucopyranoside
Polysaccharides
Glycogen
• is the energy-reserve carbohydrate for
animals
• glycogen is a branched polysaccharide of
approximately 106 glucose units joined by
-1,4- and -1,6-glycosidic bonds
• the total amount of glycogen in the body
of a well-nourished adult human is about
350 g, divided almost equally between
liver and muscle
Starch
• starch can be separated into amylose and
amylopectin
• amylose is composed of unbranched chains of up
to 4000 D-glucose units joined by -1,4-glycosidic
bonds
• amylopectin contains chains up to 10,000 Dglucose units also joined by -1,4-glycosidic
bonds; at branch points, new chains of 24 to 30
units are started by -1,6-glycosidic bonds
Cellulose
• is a linear polysaccharide of D-glucose units joined by
-1,4-glycosidic bonds
• it has an average molecular weight of 400,000 g/mol,
corresponding to approximately 2200 glucose units per
molecule
• cellulose molecules act like stiff rods and align
themselves side by side into well-organized waterinsoluble fibers in which the OH groups form
numerous intermolecular hydrogen bonds
• this arrangement of parallel chains in bundles gives
cellulose fibers their high mechanical strength
• it is also the reason why cellulose is insoluble in water
Bacterial Cell Walls
• The bacterial cell wall is a unique
structure which surrounds the cell
membrane.
• Maintaining the cell's characteristic shape
• Countering the effects of osmotic pressure
• Consist of many layers of peptidoglycan
connected by amino acid bridges
Peptidoglycan
• is composed of an alternating sequence of Nacetyl-muraminic acid (NAM) and Nacetylglucosamine (NAG) joined by -1,4glycosidic bonds.
CH2 OH O
O
O
H3 C CH
C= O
NH
NH
O= C
CH2 OH O
O
HO
O
N
O= C
CH3
CH3
L A la
D Gln
O
L Ly s ( CH2 ) 4 N H- C-( Gly ) 5 -N H-- -- -
D A la
C= O
O
N H-( Gly) 5 C- - --
To tetrapeptide
side chains
The NAM-NAG polysaccharide is in
turn cross-linked by small peptides
• in Staphylococcus aureus, the cross link is a
tetrapeptide
• this tetrapeptide is unusual in that it
contains two amino acids of the D-series,
namely D-Ala and D-Gln
• each tetrapeptide is cross linked to an
adjacent tetrapeptide by a pentapeptide of
five glycine units
PROTEOGLYCANS
Glycosaminoglycans (GAGs) are anionic
polyanionic polysaccharides chains made of
repeating disaccharide units
• play important roles in the structure and function of
connective tissues
• 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
• Glycosaminoglycans tend to be negatively charged,
because of the prevalence of acidic groups.
• GAGs are usually attached to proteins to form
proteoglycans
Common GAGs
• heparin: natural anticoagulant
• hyaluronic acid: a component of the
vitreous humor of the eye and the
lubricating fluid of joints
• chondroitin sulfate and keratan sulfate:
components of connective tissue
Chondroitin 6-sulfate
• The repeating disaccharide units of Nacetylgalactosamine ang glucoronic acid
• can be sulfated in either 4 or 6 position of
GalNAc
• They are prominent components of
cartilage tendons, ligaments and aorta
• Palate is made from chondroitin sulfate
Keratan Sulfate
• Repeating disaccharide unit of Nacetylglucosamine and galactose
• Ester sulfate on C6 of GalNAc
HEPARIN
• Glucosamine and d-glucoronic acid or Liduronic acid form the characteristic
disaccharide unit
• In addition to N-sulfate or O-sulfate on C6 of
glucosamine, heparin can also contain sulfate
on C3 of the hexosamine and C2 of the uronic
acid
• Functions predominantly as anticoagulant and
lipid clearing agent
Dermatan Sulfate
• Predominant uronic acid is L-iduronic
acid
• Antothrombotic like heparin but in
contrast, shows only minimal wholeblood anticoagulant and blood lipidclearing activities
• Found in skin
Hyaluronic acid
• Found in synovial fluid of the joints
• Forms a viscous solution that is an
excellent lubricant
Specific enzymes are
responsible for oligosaccharide
assembly
• Glycosyltransferase-catalyzes the
synthesis of oligosaccharides through the
formation of glycosidc bond
Glycoproteins
Components of the cell membrane, where
they play variety of roles in cell adhesion
Protein glycosylation takes part in
the lumen of the endoplasmic
reticulum and in the golgi complex
LECTINS
• Proteins that bind specific carbohydrate
structures
• Are ubiquitous, being found in plants,
animals and microorganisms
C-type (for Calcium requiring)-found in animals