CARBOHYDRATE STRUCTURES
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Carbohydrates
Carbohydrates, the most abundant biomolecules in nature, are
a direct link between solar energy and the chemical energy of
living systems
The name carbohydrate arises from the basic molecular formula
(CH2O)n, which can be rewritten (C.H2O)n to show that these
substances are hydrates of carbon, where n=3 or more
Carbohydrates constitute a versatile class of molecules. Energy
from the sun captured by green plants, algae and some bacteria
during photosynthesis is stored in the form of carbohydrates
In turn, carbohydrates are the metabolic precursors of virtually
all other biomolecules
Glucose is the most important carbohydrate in humans ; most
dietary carbohydrate is absorbed into the bloodstream as
glucose formed by hydrolysis of dietary starch and
disaccharides, and other sugars are converted to glucose in the
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liver
• In addition to being the major metabolic fuel, glucose is the
precursor for synthesis of all the other carbohydrates in the
body, including glycogen for storage; ribose and deoxyribose in
nucleic acids; galactose in lactose of milk
• Glucose and its derivatives are also major components of
glycoconjugates
• Three types of glycoconjugates –glycolipids, glycoproteins and
proteoglycans – are important components of cell membranes,
cell walls and extracellular structures in plants, animals, and
bacteria
• In addition to the structural roles that glycoconjugates play,
investigations of biological processes such as signaltransduction, cell-cell interactions and endocytosis have
revealed that they typically involve the binding of glycoproteins
glycolipids, or free carbohydrate molecules with
complementary receptors
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 carbohydrates have informational capacities
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Carbohydrate Nomenclature
Carbohydrates are generally classified into three groups:
monosaccharides (and their derivatives), oligosaccharides,
and polysaccharides
The monosaccharides are also called simple sugars
Oligosaccharides derive their name from the Greek word
oligo, meaning “few,” and consist roughly of from two to
twelve simple sugar molecules
Disaccharides are common in nature, and trisaccharides also
occur frequently. Four- to six-sugar-unit oligosaccharides are
usually bound covalently to other molecules, including
glycoproteins
As their name suggests, polysaccharides are polymers of the
simple sugars and their derivatives. They may be either linear
or branched polymers and may contain hundreds or even
thousands of monosaccharide units
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Monosaccharides
• Monosaccharides are aldehyde or ketone derivatives of
polyhydroxy alcohols
• Monosaccharides with an aldehyde functional group are called
aldoses and those with a ketone group are called ketoses
• The simplest aldose and ketose are glyceraldehyde and
dihydroxyacetone, respectively
• Simple sugars are also classified according to the number of
carbon atoms they contain. The smallest sugars, called trioses,
contain three carbon atoms; tetroses, pentoses, hexoses,…
• The most abundant monosaccharides found in living cells are
the pentoses and the hexoses
• Simple sugars are usually represented by Fischer projections:
the carbohydrate backbone is drawn vertically with the most
highly oxidized carbon usually shown at the top; the horizontal
lines are understood to project toward the viewer, and the
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vertical lines recede from the viewer
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Stereochemistry of Monosaccharides
Aldoses with at least three carbons and ketoses with at least
four carbons contain chiral centers
When the number of chiral carbons increases in optically active
compounds, the number of possible optical isomers also
increases
The total number of isomers is given as 2n, where n is the
number of chiral carbon atoms. Half of these isomers are going
to be D isomers and the rest are L isomers
The reference carbon is the asymmetric carbon that is farthest
from the carbonyl carbon; its configuration is similar to that of
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the asymmetric carbon in either D- or L- glyceraldehyde
• Almost all naturally occurring sugars have the D-configuration
just like L-amino acids
• These preferences, established in apparently random choices
early in evolution, persist uniformly in nature because of the
stereospecificity of the enzymes that synthesize and
metabolize these small molecules
• In the D-aldose family of sugars, which contains most
biologically important monosaccharides, the hydroxyl group is
to the right of the chiral carbon atom farthest from the most
oxidized carbon
• The designation D or L merely relates the configuration of a
given molecule to that of glyceraldehyde and does not specify
the sign of rotation of plane-polarized light
• Stereoisomers that are not enantiomers are called
diastereomers
• Diasteromers that differ in the configuration at a single
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asymmetric carbon atom are called epimers
Aldose Sugars
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Ketose Sugars
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Cyclic Structures and Anomeric Forms
Although Fischer projections are useful for presenting the
structures of particular monosaccharides and their
stereoisomers, they don’t take into account the ability of sugars
to form cyclic structures with formation of an additional
asymmetric center
Alcohols react readily with aldehydes to form hemiacetals
The linear forms of glucose (and other aldohexoses) could
undergo a similar intramolecular reaction to form a cyclic
hemiacetal. The resulting six-membered, oxygen-containing
ring is similar to pyran and is designated a pyranose
In a similar manner, ketones can react with alcohols to form
hemiketals
The analogous intramolecular reaction of a ketose sugar such
as fructose yields a cyclic hemiketal
The five-membered ring thus formed is reminiscent of furan
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and is referred to as a furanose
Cyclic Aldoses
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Cyclic Ketoses
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Fischer Projection Formulas of Cyclic Glucose
Fischer Projection Formulas of Cyclic Fructose
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• The cyclic pyranose and furanose forms are the preferred
structures for monosaccharides in aqueous solution
• At equilibrium, the linear aldehyde or ketone structure is only a
minor component of the mixture (generally much less than 1%)
• When hemiacetals and hemiketals are formed, the carbon atom
that carried the carbonyl function becomes an asymmetric
carbon atom
• Isomers of monosaccharides that differ only in their
configuration about that carbon atom are called anomers,
designated as α or β, and the carbonyl carbon is thus called the
anomeric carbon
• When the hydroxyl group at the anomeric carbon is on the same
side of a Fischer projection as the oxygen atom at the highest
numbered asymmetric carbon, the configuration at the
anomeric carbon is α as in α-D-glucopyranose
• When the anomeric hydroxyl is on the opposite side of the
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Fischer projection, the configuration is β, as in β -D-glucopyranose
• The anomers of monosaccharides are readily interconverted
when dissolved in water
• This spontaneous process, called mutatrotation, produces a
mixture of α and β forms in both furanose and pyranose ring
structures
• This process occurs more rapidly in the presence of cellular
enzymes called mutarotases
• The proportion of each form differs with each sugar type
• The open chain formed during mutarotation can particpate in
oxidation-reduction reactions
• Although Haworth projections are convenient for display of
monosaccharide structures, they do not accurately portray the
conformations of pyranose and furanose rings
• Neither pyranose nor furanose rings can adopt true planar
structures. Instead, they take on puckered conformations, and
in the case of pyranose rings, the two favored structures are the
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chair conformation and the boat conformation
Chair and Boat Conformations of
Pyranose Sugars
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Important Monosaccharides
• Among the most important monosaccharides that occur in
living things are glucose, fructose and galactose
Glucose
• D-glucose, originally known as dextrose is the primary energy
source for living cells
• It is the preferred energy source for brain cells and cells with
little or no mitochondria such as erythrocytes; and for cells with
limited supply of oxygen such as the eyeball and the renal medulla
Fructose
• D-fructose, originally called levulose, is often referred to as fruit
sugar because of its high content in fruit
• It is synthesized in the seminal vesicles and then incorporated
into semen. Sperm use the sugar as an energy source
Galactose
• Galactose is involved in the synthesis of biomolecules such as
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lactose, glycolipids, glycoproteins and proteoglycans
• Synthesis of these substances is not diminished by diets that
lack galactose or its main dietary source, lactose, because
galactose can be synthesized from glucose by an epimerase
Derivatives of Monosaccharides
• A variety of chemical and enzymatic reactions produce
derivatives of the simple sugars
Sugar Acids
• Sugars with free anomeric carbon atoms are reasonably good
reducing agents and will reduce hydrogen peroxide, certain
metals (e.g. Cu2+ ) and other oxidizing agents.Such reactions
convert the sugar to a sugar acid
• For example, addition of alkaline CuSO4 (called Fehling’s
solution) to an aldose sugar produces a red cuprous oxide
(Cu2O) precipitate:
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• Carbohydrates that can reduce oxidizing agents in this way are
referred to as reducing sugars
• Sugars whose aldehyde group has been oxidized are known as
aldonic acids such as gluconic acid
• If the carbon containing the terminal hydroxyl group is oxidized,
the sugar is called a uronic acid (e.g., glucuronic acid)
Sugar Alcohols
• Sugar alcohols, or alditols, are designated by the addition of
-itol to the name of the parent sugar. The alditols are mostly
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linear molecules that cannot cyclize in the manner of aldoses
• Sugar alcohols are not metabolized as rapidly as their
precursors, and reconversion to the precursor is also slow
• Alditols are characteristically sweet -tasting, and sorbitol
(D-glucitol), mannitol and xylitol are widely used to sweeten
sugar-free gums
• Sorbitol buildup in the eyes is implicated in cataract formation
caused by diabetes and galactosemia
• Clinically, mannitol is administered intravenously as an osmotic
diuretic in patients with acute renal failure. It is not metabolized
appreciably, is filtered by the glomerulus, and is not reabsorbed
by the tubules; hence it is excreted in urine
• The non-reabsorbable solute holds water and thus maintains
urine volume in the presence of decreased glomerular function
• Intravenous mannitol is also used to relieve an increase in
pressure and in volume of cerebrospinal fluid
• Glycerol and myo-inositol, a cyclic alcohol, are components of
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lipids
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Deoxy Sugars
• The deoxy sugars are monosaccharides with one or more
hydroxyl groups replaced by hydrogens
• 2-Deoxy-D-ribose whose systematic name is 2-deoxy-Derythropentose, is a constituent of DNA in all living things
• Deoxy sugars also occur frequently in glycoproteins and
polysaccharides
• L-Fucose and L-rhamnose, both 6-deoxy sugars, are
components of some cell walls, and rhamnose is a
component of ouabain, a highly toxic cardiac glycoside
found in the bark and root of the ouabaio tree
• L-Fucose is often found among the carbohydrate
components of glycoproteins, such as those of the ABO
blood group determinants on the surface of red blood cells
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Amino sugars
• Amino sugars are obtained by replacing a hydroxyl group
of a monosaccharide by an amino group
• The most common amino sugars are the 2aminoaldohexoses, namely, D-glucosamine and Dglucosamine and D-galactosamine
• The amino groups usually occur as N-acetyl derivatives
• Amino sugars are components of structural
polysaccharides and of glycolipids of membranes
• N-Acetyl muramic acid , a constituent of a bacterial cell
wall polysaccharide (hence named as an amine isolated
from bacterial cell wall polysaccharides; murus is Latin for
“wall”), has a lactyl side chain linked to C3 of glucosamine
• The polysaccharide of bacterial cell walls is a polymer of
alternating N-acetylmuramic acid and N-acetyl26
glucosamine
• N-Acetylneuraminic acid, NANA (an amine isolated from
neural tissue) is made from the linkage of Nacetylmannosamine with pyruvic acid
• The N-acetyl and N-glycolyl derivatives of neuraminic acid
are collectively known as sialic acids and are distributed
widely in membrane systems in bacteria and animals
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Glycosides
• The hydroxyl group on the anomeric carbon of a
monosaccharide can react with an –OH or an –NH group of
another compound to form a glycosidic bond
• When the second compound is not another monosaccharide,
it is known as an aglycone
• be another monosaccharide
• These bonds are known as, respectively, O-glycosidic and Nglycosidic bonds
• The linkage may be either α or β , depending on the position
of the atom attached to the anomeric carbon of the sugar
• N-glycosidic bonds are found in nucleosides, nucleotides and
glycoproteins
• For example, in the adenosine moiety of ATP, the nitrogenous
base adenine is linked to the sugar ribose through a β -N28
glycosidic bond
• In contrast, O-glycosidic bonds join sugars to each other or
attach sugars to the hydroxyl group of an amino acid on a
protein
• The glycosides that are important in medicine because of their
action on the heart (cardiac glycosides) all contain steroids as the
aglycone
• These include digitalis and ouabain, which are inhibitors of the
Na+-K+ ATPase of cell membranes. Other glycosides include
antibiotics such as streptomycin.
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Oligosaccharides
• The simplest oligosaccharides , disaccharides, consist of two
monosaccharide units linked by an O-glycosidic bond
• As in proteins ,each individual unit in an oligosaccharide is
termed a residue
• The disaccharides that are most commonly found in nature and
are of physiologically importance are sucrose, maltose and
lactose
• The oxidation of a sugar’s anomeric carbon by cupric or ferric ion
occurs only with the linear form, which exists in equilibrium with
the cyclic form(s)
• When the anomeric carbon is involved in a glycosidic bond, that
sugar residue cannot take the linear form and therefore
becomes a non-reducing sugar
• In describing disaccharides or polysaccharides, the end of a chain
with a free anomeric carbon (one not involved in a glycosidic
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bond) is commonly called the reducing end
• Maltose, isomaltose,cellobiose and trehalose are all
homodisaccharides because they each contain only one kind
of monosaccharide, namely, glucose
• Maltose is a is a component of malt, a substance obtained
substance obtained by allowing grain to soften in water and
germinate
• Maltose contains two D-glucose residues joined by a
glycosidic linkage between C-1 (the anomeric carbon) of one
glucose residue and C-4 of the other
• Because it retains a free anomeric carbon, maltose is a
reducing sugar. The linkage in maltose is represented as
(α1 → 4) and maltose could be written as Glc (α1 → 4) Glc
• Cellobiose is Glc (β1 → 4)Glc, isomaltose Glc (α 1 → 6)Glc
and trehalose Glc (α1 → 1 α) Glc
• Lactose, which yields D-galactose and D-glucose on
hydrolysis, occurs naturally only in milk
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Common Disaccharides
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• The anomeric carbon of the glucose residue is available for
oxidation, and thus lactose is a reducing disaccharide
• Lactose’s abbreviated name is Gal (β1 → 4)Glc
• Sucrose (table sugar) is a disaccharide of glucose and fructose.
It is formed by plants but not by animals
• In contrast to maltose and lactose, sucrose contains no free
anomeric carbon atom; the anomeric carbons of both
monosaccharide units are involved in the glycosidic bond.
Sucrose is therefore a nonreducing sugar
• Sucrose can be represented either as Glc (α 1 → 2 β) Fru or
Fru (β2 → 1α) Glc
 Stachyose is typical of the oligosaccharide components found in
substantial quantities in beans, peas, bran and whole grains
These oligosaccharides are not digested by stomach enzymes, but
are metabolized readily by bacteria in the intestines. This is the
source of the flatulence that often accompanies the consumption
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of such foods
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Polysaccharides
The majority of carbohydrate material in nature occurs in the
form of polysaccharides
Polysaccharides, also called glycans, consist of
monosaccharides and their derivatives
If a polysaccharide contains only one kind of monosaccharide
molecule, it is a homopolysaccharide, or homoglycan, whereas
those containing more than one kind of monosaccharide are
heteropolysaccharides
The most common constituent of polysaccharides is D-glucose,
but D-fructose, D-galactose, L-galactose, D-mannose, Larabinose, and D-xylose are also common
Common monosaccharide derivatives in polysaccharides
include the amino sugars (D-glucosamine and D-galactosamine), their derivatives (N-acetylneuraminic acid and
N-acetylmuramic acid), and simple sugar acids (glucuronic and
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iduronic acids)
• Based on their functions, polysaccharides have been
traditionally classified into storage materials, structural
components or protective substances
• An improvement on this classification is the discovery that
polysaccharides and oligosaccharides function as information
carriers: they serve as destination labels for some proteins
and as mediators of specific cell-cell interactions and
interactions between cells and the extracellular matrix
Storage Polysaccharides
• The most important storage polysaccharides are starch in
plant cells and glycogen in animal cells
• Both polysaccharides occur intracellularly as large clusters or
granules
• Starch and glycogen molecules are heavily hydrated, because
they have many exposed hydroxyl groups available to
hydrogen-bond with water
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• Starch contains two types of glucose polymer, amylose and
amylopectin
• Amylose consists of long, unbranched chains of D-glucose
residues connected by (α1 → 4) linkages. Such chains vary in
molecular weight from a few thousand to more than a million
• Unlike proteins, polysaccharides generally do not have definite
molecular weights. This is because proteins are synthesized on a
template (messenger RNA) of defined sequence and length
• Amylopectin also has a high molecular weight (up to 100
million) but unlike amylose is highly branched
• The glycosidic linkages joining successive glucose residues in
amylopectin chains are (α1 → 4); the branch points (occurring
every 24 to 30 residues) are (α1 → 6) linkages
• Glycogen is found mainly in the liver (where it may amount to
as much as 10% of liver mass) and skeletal muscle (where it
accounts for 1 to 2% of muscle mass)
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• Like amylopectin, glycogen is a polymer of (α1 → 4) -linked
subunits of glucose, with (α1 → 6) -linked branches, but
glycogen is more extensively branched (on average, every 8 to
12 residues) and more compact than starch
• In hepatocytes glycogen is found in large granules, which are
themselves clusters of smaller granules composed of single,
highly branched glycogen molecules with an average molecular
weight of several million
• Because each branch in glycogen (and also amylopectin) ends
with a non-reducing sugar unit, a glycogen molecule has as
many non-reducing ends as it has branches, but only one
reducing end
• Storing glucose in the form of glycogen significantly reduces
the osmolality that the cell would have had if glucose was
stored in free form
 the threat of osmotic lysis is averted
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Storage Polysaccharides
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• Dextrans are bacterial and yeast polysaccharides made up of
(α1→6)-linked poly-D-glucose; all have (α1→3)branches, and
some also have (α1→2) or (α1→4) branches
• Dental plaque, formed by bacteria growing on the surface of
teeth, is rich in dextrans
• Inulin is a polysaccharide of fructose. It is readily soluble in
water and is used to determine the glomerular filtration rate,
but it is not hydrolyzed by intestinal enzymes
Structural Polysaccharides
• The structural polysaccharides have properties that are
dramatically different from those of the storage
polysaccharides, even though the compositions of these two
classes are similar
• The structural polysaccharide cellulose is the most abundant
natural polymer found in the world
• Found in the cell walls of nearly all plants, cellulose is one of the
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principal components providing physical structure and strength
• Like amylose, the cellulose molecule is a linear, unbranched
homopolysaccharide, consisting of 10,000 to 15,000 D-glucose
units
• But there is a very important difference: in cellulose the glucose
residues have the β-configuration
• The glucose residues in cellulose are linked by (β 1→4)
glycosidic bonds
• This linkage leads to cellulose assuming a fully-extended
conformation that permits efficient interchain hydrogen
bonding, the basis of much of the strength of cellulose
• The water content of cellulose structure is low because
extensive interchain hydrogen bonding between cellulose
molecules satisfies their capacity for hydrogen-bond formation
• Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in β-linkage
• The only chemical difference from cellulose is the replacement
of the hydroxyl group at C-2 with an acetylated amino group40
• Chitin forms extended fibers similar to those of cellulose
• Chitin is the principal component of the hard exoskeletons of
nearly a million species of arthropods—insects, lobsters, and
crabs, for example— and is probably the second most abundant
polysaccharide in nature
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• Chitin forms extended fibers similar to those of cellulose
• Chitin is the principal component of the hard exoskeletons of
nearly a million species of arthropods—insects, lobsters, and
crabs, for example— and is probably the second most abundant
polysaccharide in nature
• The rigid component of bacterial cell walls is a heteropolymer
of alternating (β 1→4) -linked N-acetylglucosamine and Nacetyl-muramic acid residues
• The polysaccharide chains are cross-linked through short
peptide chains, and the overall structure is known as a
peptidoglycan
• The peptides are composed of both D and L amino acids and
they weld the polysaccharide chains into a strong sheath that
envelops the entire cell and prevents cellular swelling and lysis
due to the osmotic entry of water
• The enzyme lysozyme (also called muramidase) kills bacteria by
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hydrolyzing the (β 1→4) bond
The Structure of Peptidoglycan
• Penicillin and related antibiotics kill bacteria by preventing
synthesis of the cross-links, leaving the cell wall too weak to
resist osmotic lysis
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Protective Polysaccharides
The extracellular space in the tissues of multicellular animals is
filled with a gel-like material, the extracellular matrix (ECM),
also called ground substance, which holds the cells together
and provides a porous pathway for the diffusion of nutrients
and oxygen to individual cells
The ECM is composed of an interlocking meshwork of
heteropolysaccharides and fibrous proteins such as collagen,
elastin, fibronectin and laminin
These heteropolysaccharides, the glycosaminoglycans, are a
family of linear polymers composed of repeating disaccharide
units
One of the two monosaccharides is always either N-acetylglucosamine or N-acetylgalactosamine; the other is in most
cases a uronic acid, usually D-glucuronic or L-iduronic acid
In some glycosaminoglycans, one or more of the hydroxyls of
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the amino sugar are esterified with sulfate
• The combination of of sulfate groups and the carboxylate
groups of the uronic acid residues gives glycosaminoglycans a
very high density of negative charge
• To minimize repulsion between neighboring charged groups,
these molecules assume an extended conformation in solution
• Heparin, with the highest net negative charge of the
disaccharides shown, is a natural anticoagulant substance. It
binds strongly to antithrombin III and inhibits blood clotting
• Hyaluronates are important components of the vitreous humor in
the eye and of synovial fluid, the lubricant fluid of joints in the
body
• The chondroitins and keratan sulfate are found in tendons,
cartilage, and other connective tissue, whereas dermatan
sulfate, as its name implies, is a component of the extracellular
matrix of skin
• Glycosaminoglycans (except hyaluronate),in combination with
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proteins, give rise to proteoglycans
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Carbohydrates as Informational Molecules: The Sugar Code
• Cells use specific oligosaccharides located on glycoproteins and
glycolipids to encode important information about intracellular
targeting of proteins, cell-cell interaction, tissue development
and extracellular signals
• Branched structures, not found in nucleic acids or proteins, are
common in oligosaccharides. These means that more
information can be encoded by an oligosaccharide that contains
the same number of residues as a protein or a nucleic acid 47
• The sugar code is “read” by lectins: lectins, found in all
organisms, are proteins that bind carbohydrates with high
affinity and specificity
• Biological processes that involve lectin binding include a vast
array of cell-cell interactions
• Prominent examples include infections by microorganisms, the
mechanisms of many toxins and physiological processes such as
leukocyte rolling
Infections
• Infection by many bacteria is initiated when they become
firmly attached to host cells
• Often, attachment is mediated by the binding of bacterial
lectins to oligossacharides on the cell’s surface
• Helicobacter pylori ,a major causative agent of gastritis and
stomach ulcers, possesses several lectins that allow it to
establish a chronic infection in the mucous membrane of the
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stomach
• One of these lectins binds with high affinity to a portion of the
type O blood group determinant, an oligosaccharide, a
circumstance that explains the observation that humans with
type O blood are at considerably greater risk of developing ulcers
• Several animal viruses, including the influenza virus, attach to
their host cells through interactions with oligosaccharides
displayed on the host cell surfaces
• The lectin of the influenza virus, the HA protein, is essential for
viral entry and infection. After initial binding of the virus to a sialic
acid–containing oligosaccharide on the host surface, a viral
sialidase removes the terminal sialic acid residue, triggering the
entry of the virus into the cell. Inhibitors of this enzyme are used
clinically in the treatment of influenza
Entry of toxins into the cell
• The damaging effects of many bacterial toxins occur only after
endocytosis into the host cell, a process that is initiated by lectinligand binding
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• The binding of the B subunit of cholera toxin to a glycolipid on
the surface of intestinal cells results in the uptake of toxic A
subunit
• Once the A subunit is internalized, it proceeds to disrupt the
mechanism that regulates chloride transport, a process that
results in a life-threatening diarrhea
Leukocyte rolling
• When a tissue becomes damaged in an animal either by
infection or physical trauma, it emits signal molecules that
create an inflammation
• In response to certain of these molecules, the endothelial cells
that line nearby blood vessels produce and insert a protein
known as selectin into their plasma membranes
• The selectins are a family of lectins that act as cell-adhesion
molecules. Once selectin is displayed on the surface of the
endothelial cells, it binds transiently to its ligand
oligosaccharides on white blood cells such as neutrophils 50
• This relatively weak binding serves to slow the rapid motion of
neutrophils as they flow in blood so that they appear to roll
along the lumenal surface of the blood vessel
• Once rolling has been initiated, and white blood cells approach
the inflammation site, they encounter other signal molecules
that cause them to express another lectin called integrin on
their surfaces
• The binding of integrin with its oligosaccharide ligand on the
endothelial surface of the blood vessel causes the WBC to roll
• Subsequently, the WBC undergo changes that allow them to
squeeze between the cells of the endothelium and migrate to
the infected where they proceed to consumed and degrade
bacteria and cellular debris
 Normal physiological processes mediated by oligosaccharidelectin interactions include protein targeting to cellular
compartments and hepatic degradation of certain peptide
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hormones
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