Carbohydrates
Contents
 Functions of carbohydrates
 Classification of carbohydrates
 Isomers and epimers
 Stereochemistry
 Hemiacetal and hemiketals
 Fischer’s projection formula
 Haworth projection formula
 Chain and ring forms
 Chair conformations
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 Carbohydrates are one of the four major classes of
biomolecules along with proteins, nucleic acids, and lipids.
 Carbohydrates are aldehyde or ketone compounds with
multiple hydroxyl groups.
 They make up most of the organic matter on earth because
of their extensive roles in all forms of life.
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Functions of Carbohydrates
 Carbohydrates serve as energy stores, fuels, and metabolic
intermediates.
 Second, ribose and deoxyribose sugars form part of the
structural framework of RNA and DNA.
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Functions of Carbohydrates
 Third, polysaccharides are structural elements in the cell walls of
bacteria and plants.
 Eg., Cellulose in plants
 Chitin in insects
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Functions of Carbohydrates
 Fourth, carbohydrates are linked to many proteins and lipids,
where they play key roles in mediating interactions among
cells and interactions between cells and other elements in the
cellular environment.
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Classification
 Monosaccharides
 Disaccharides
 Oligosaccharides
 Polysaccharides
 According to the number of sugar units.
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Monosaccharides
 Monosaccharides, the simplest carbohydrates, are aldehydes or
ketones that have two or more hydroxyl groups;
 The empirical formula of many is (C-H2O)n, literally a
“carbon hydrate.”
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Monosaccharides
 Monosaccharides are important fuel molecules as well as
building blocks for nucleic acids.
 The smallest monosaccharides, for which n = 3, are
dihydroxyacetone and d- and l-glyceraldehyde.
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Monosaccharides
 They are referred to as
trioses (tri- for 3).
 Dihydroxyacetone is called
a ketose because it contains
a keto group, whereas
glyceraldehyde is called an
aldose because it contains an
aldehyde group.
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 All carbohydrates contain
at least one asymmetrical
(chiral) carbon and are,
therefore, optically active.
 In addition, carbohydrates
can exist in either of two
conformations, as
determined by the
orientation of the hydroxyl
group about the
asymmetric carbon farthest
from the carbonyl.
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 With a few exceptions,
those carbohydrates that
are of physiological
significance exist in the Dconformation.
 The mirror-image
conformations, called
enantiomers, are in the
L-conformation.
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Isomers and epimers
 Compounds that have the same chemical formula but have
different structures are called isomers.
 For example, fructose, glucose, mannose, and galactose are
all isomers of each other, having the same chemical formula,
C6H12O6.
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Isomers and epimers
 Carbohydrate isomers that differ in configuration around
only one specific carbon atom are defined as epimers of each
other.
 For example, glucose and galactose are C-4 epimers—their
structures differ only in the position of the -OH group at
carbon 4.
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C-2 and C-4 epimers and an isomer of
glucose.
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 Glucose and mannose are C-2 epimers.
 However, galactose and mannose are NOT epimers—they
differ in the position of -OH groups at two carbons (2 and 4)
and are, therefore, defined only as isomers .
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 The names of monosaccharides are frequently abbreviated;
most common are three-letter abbreviations for simple
monosaccharides (e.g., Gal, Glc, Man, Xyl, Fuc).
 Glycan-A generic term for any sugar or assembly of sugars,
in free form or attached to another molecule, used
interchangeably with saccharide or carbohydrate.
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Stereochemistry
 Saccharides with identical functional groups but with
different spatial configurations have different chemical and
biological properties.
 Stereochemisty is the study of the arrangement of atoms in
three-dimensional space.
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Stereochemistry
 Stereoisomers are compounds in which the atoms are linked
in the same order but differ in their spatial arrangement.
 Compounds that are mirror images of each other but are not
identical, comparable to left and right shoes, are called
enantiomers.
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Enantiomers
 A special type of isomerism is found in the pairs of structures
that are mirror images of each other.
 The vast majority of the sugars in humans are D-sugars.
 Enzymes known as racemases are able to interconvert D- and
L-isomers.
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Enantiomers
 These mirror images are
called enantiomers, and the
two members of the pair are
designated as a D- and an Lsugar.
 In the D isomeric form, the
OH group on the asymmetric
carbon (a carbon linked to
four different atoms or
groups) farthest from the
carbonyl carbon is on the
right, while in the L-isomer it
is on the left.
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Monosaccharides
 The monosaccharides commonly found in humans are
classified according to the number of carbons they contain in
their backbone structures.
 The major monosaccharides contain four to six carbon
atoms.
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Classification
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# Carbons
Name
Example
3
Triose
Glyceraldehyde,
Dihydroxyacetone
4
Tetrose
Erythrose
5
Pentose
Ribose, Ribulose, Xylulose
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Hexose
Glucose, Galactose,
Mannose, Fructose
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Heptose
Sedoheptulose
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Nonose
Neuraminic acid, also called
sialic acid
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d-Aldoses containing three, four, five,
and six carbon atoms
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d-Aldoses containing three, four, five,
and six carbon atoms
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 d-Aldoses contain an aldehyde group (shown in blue) and
have the absolute configuration of d-glyceraldehyde at the
asymmetric center (shown in red) farthest from the aldehyde
group.
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 d-Ribose, the carbohydrate component of RNA, is a five-
carbon aldose.
 d-Glucose, d-mannose, and d-galactose are abundant sixcarbon aldoses.
 d-glucose and d-mannose differ in configuration only at C-2.
 Sugars differing in configuration at a single asymmetric
center are called epimers.Thus, d-glucose and d-mannose are
epimeric at C-2; d-glucose and d-galactose are epimeric at C4.
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Ketose
 Dihydroxyacetone is the simplest ketose.
 d-Fructose is the most abundant ketohexose.
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d -Ketoses containing three- four, five,
and six carbon
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Pentoses and Hexoses Cyclize to Form
Furanose and Pyranose Rings
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 The predominant forms of
ribose, glucose, fructose,
and many other sugars in
solution are not open
chains.
 Rather, the open-chain
forms of these sugars
cyclize into rings.
 In general, an aldehyde can
react with an alcohol to
form a hemiacetal.
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 For an aldohexose such as
glucose, the C-1 aldehyde
in the open-chain form of
glucose reacts with the C-5
hydroxyl group to form an
intramolecular hemiacetal.
 The resulting cyclic
hemiacetal, a sixmembered ring, is called
pyranose because of its
similarity to pyran.
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 Similarly, a ketone can
react with an alcohol to
form a hemiketal.
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 The C-2 keto group in the
open-chain form of a
ketohexose, such as fructose,
can form an intramolecular
hemiketal by reacting with
either the C-6 hydroxyl
group to form a sixmembered cyclic hemiketal
or the C-5 hydroxyl group to
form a five-membered cyclic
hemiketal .
 The five-membered ring is
called a furanose because of its
similarity to furan
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Furan & Pyran
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 The depictions of glucopyranose and fructofuranose shown
are Haworth projections.
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Chain and Ring forms
 Many simple sugars can exist in a chain form or a ring form,
as illustrated by the hexoses above.
 The ring form is favored in aqueous solutions, and the
mechanism of ring formation is similar for most sugars.
 The glucose ring form is created when the oxygen on
carbon number 5 links with the carbon comprising the
carbonyl group (carbon number 1) and transfers its hydrogen
to the carbonyl oxygen to create a hydroxyl group.
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Chain and Ring forms
 The rearrangement produces alpha glucose when the
hydroxyl group is on the opposite side of the -CH2OH
group, or beta glucose when the hydroxyl group is on the
same side as the -CH2OH group.
 Isomers, such as these, which differ only in their
configuration about their carbonyl carbon atom are called
anomers.
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 a-D-glucose and b-D-glucose differ by the positioning of
hydroxyl group on C1 carbon
 Both a-D-glucose serves as basic units of glycogen and starch
 b-D-glucose is the basic unit of cellulose

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Chain and Ring forms
 The little D in the name derives from the fact that natural
glucose is dextrorotary, i.e., it rotates polarized light to the
right, but it now denotes a specific configuration.
Monosaccharides forming a five-sided ring, like ribose, are
called furanoses. Those forming six-sided rings, like
glucose, are called pyranoses.
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 The Haworth representations are preferably drawn with the
ring oxygen atom at the top (for furanose) or the top righthand corner (for pyranose) of the structure; the numbering
of the ring carbons increases in a clockwise direction.
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Chair Conformation
 The planar Haworth structures are distorted representations
of the actual molecules. The preferred conformation of a
pyranose ring is the chair conformation, similar to the
structure of cyclohexane.
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β-D-Glucose in Haworth projection and in its 4C1
and 1C4 chair conformations
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 An additional asymmetric
center is created when a
cyclic hemiacetal is
formed.
 In glucose, C-1, the
carbonyl carbon atom in
the open-chain form,
becomes an asymmetric
center. Thus, two ring
structures can be formed:
α-d-glucopyranose and βd-glucopyranose.
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Anomers
 For d sugars drawn as
Haworth projections, the
designation α means that the
hydroxyl group attached to C1 is below the plane of the
ring;
 β means that it is above the
plane of the ring. The C-1
carbon atom is called the
anomeric carbon atom, and
the α and β forms are
called anomers.
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 The same nomenclature
applies to the furanose ring
form of fructose, except
that α and β refer to the
hydroxyl groups attached
to C-2, the anomeric
carbon atom.
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Reducing sugars
 If the oxygen on the anomeric carbon of a sugar is not
attached to any other structure, that sugar can act as a
reducing agent and is termed a reducing sugar.
 Such sugars can react with chromogenic agents (for example,
Benedict's reagent or Fehling's solution) causing the reagent
to be reduced and colored, with the anomeric carbon of the
sugar becoming oxidized.
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The End
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