Nomenclature and Structure of
Carbohydrates
AILEEN ZAYDEL
Carbohydrate Nomenclature
 “carbon-hydrates” (CH2O)n
 Monosaccharides, disaccharides, oligosaccharides,
polysaccharides
 Complex carbohydrate: more than one
monosaccharide building unit, extended
branching polymer, a sugar attached to another
biomolecule that forms a glycoconjugate

glycoconjugate: glycoproteins, glycolipids,
proteoglycans
Basic Structure
Emil Fischer
• Polyhydroxy aldehydes and ketones
• (CH2O)n where n can be 3-9
• 2 classes: aldoses and ketoses
•Subclasses based on number of C
atoms
Aldose carbons are
numbered with the
carbonyl group dubbed
as C-1
Ketose carbons are
numbered with the
carbonyl group dubbed
as C-2
Aldose
Ketose
Sterioisomers
 All monosaccharides (except
dihydroxy acetone) have one or
more chiral C atoms
 Depending on the orientation of
the CHOH group on the chiral C,
a monosaccharide can have a “D”
or “L” configuration.
 If there is more than one chiral
C, choose the one furthest from
the aldehyde or ketone group
 Any sugar that differs only in the
orientation of 1 chiral C is called
an epimer
Total # sterioisomers = 2k
Where k = # of chiral
Carbons
Common Monosaccharides
Cyclization
 In solution,
carbonyl
group
condenses
with –OH
group to form
rings of
hemiacetals/
hemiketals
Cyclic Sterioisomers
 Generates new chiral center at
the original carbonyl carbon
atom called anomeric
carbon
 2 possible isomers: α or β
depends on position above or
below the plane of the ring
Haworth projection
Ring Conformations
 Because of the flexibilty of the tetrahedral bonds,
the planar ring model is actually inaccurate.
 Chair, boat, twist, envelope, crown, tub
The 2 conformations
are labeled 1C4 or
4C1 1st numeral =
number of the C
atom above the seat
of the chair
2nd numeral =
number of the C
atom below the seat.
The pyranose ring, for example, can have
2 chair conformations.
Saccharide Reactions- Mutarotation
 The conversion
between the α and β
forms of a ring
structure
 Acidic/basic
environment
 In solution at
equilibrium
Equilibrium reaction between alpha and
beta pyranose and furanose rings of glucose
Reaction- Oxidation
 Oxidation of the terminal groups to –COOH makes
acid
 Free aldehyde group functions as the reducing
terminus
Reaction- Reduction
 Reduction of terminal groups
with sodium borohydride
(NaBH4)
 Makes polyhydroxy alcohols
and sugar alcohols like alditol
Conversion of a monosaccharide to a
tritium-labeled alditol by reduction with
NaB3H4
Reaction- Schiff Base Formation
 Carbonyl groups can react with amines and
hydrazides to form imines and hydrazones
 Attaches monosaccharides to protein via amino
group of lysine residue through glycation
 High levels of products of glycation found in
diabetics.
Reaction- Glycosidic Bond Formation
 When an anomeric center of one monosaccharide
reacts with an –OH group in a second sugar
 Fundamental linkage of sugars
hemiacetal + alcohol = full acetal
Glycosidic linkage
 When 2 monosaccharides combine through a
glycosidic linkage, a disaccharide results.
Maltose formation
 When a monosaccharide and a hydroxyl
group of an amino acid react, a
glycoprotein results
Gycoprotein Linkage
 Protein- bound glycans can be
divided into two classes
1.
2.
N-linked: bound to the nitrogen
atom of asparagine side chains
O-linked: bound to the oxygen
atom of serine or threonine side
chains
Thy-1 glycoprotein
Chemistry of Hydroxyl Groups
 Methylation: used to cap hydroxyl groups of
saccharides to generate methyl ethers used in
structural analysis
 Esterification: hydroxyl groups esterified by
carboxylic acid. Required to allow interaction with
certain biomolecules
 Deoxygenation: hydroxyl groups replaced by
hydrogen atoms to form deoxysugars. E.g. DNA
biosynthesis
Reducing/non-reducing Ends
 Reducing end bears a
free anomeric center
available to form
linkages
 Structures written from
the non-reducing
end to the reducing
end
 Some structures are
non-reducing
Why are glycans so complex?
One monosaccharide already has two steriosomers
because of the anomeric C atom
2. The second monosacchaide can use any of its free
hydroxyl groups to bind to the anomeric carbon
3. A single monosaccharide can form a glycosidic linkage
with more than one other monosaccharide, effectively
forming a branching point
4. A glycosidic linkage is strong but flexible, allowing for
several conformations
5. A glycosidic linkage can form between any other
biomolecule with a hydroxyl group , effectively forming a
glycoconjugate
1.
References
 http://rpi.edu/dept/bcbp/molbiochem/MBWeb/mb
1/part2/sugar.htm