Carbohydrates
Robert J Sturgeon,Heriot-Watt University, Edinburgh, UK
The carbohydrates comprise one of the major groups of naturally occurring organic
molecules and are amongst the most abundant constituents of plants, animals and
microorganisms. In general, carbohydrates are polyhydroxy-aldehydes or-ketones. They
may contain, in addition, amino, acetamido and carboxyl functional groups.
Introduction
The term carbohydrate includes monosaccharides, oligosaccharides
and polysaccharides. Also included are substances
derived from monosaccharides such as alditols,
which are derived by reduction of the carbonyl group and
carboxylic acids, which are derived by oxidation of one or
more terminal groups. Replacement of a hydroxyl group
with a hydrogen atom produces a deoxy-sugar and
replacement of a hydroxyl group with an amino group
produces an amino sugar. The term ‘sugar’ is frequently
applied to monosaccharides and lower molecular weight
oligosaccharides.
Classification
Carbohydrates are usually classified in three groups:
monosaccharides, oligosaccharides and polysaccharides.
Monosaccharides are simple sugars that cannot be
hydrolysed to smaller molecules. They exist in nature in
the free form or linked by glycosidic bonds to other
monosaccharides in the formation of oligosaccharides or
polysaccharides. Oligosaccharides are defined as simple
polymers of monosaccharides containing between two and
approximately 10 monosaccharide residues. They are
termed disaccharides, trisaccharides, tetrasaccharides,
pentasaccharides, and so on, according to the number of
monosaccharide units they contain. Polysaccharides (glycans)
are higher molecular weight polymers of monosaccharides.
Glycoconjugates are defined as either
glycoproteins, glycolipids and proteoglycans. Glycoproteins
are conjugated proteins containing either oligosaccharide
groups or polysaccharide groups, having a fairly
low molecular mass. Glycolipids are conjugated lipids
containing oligosaccharide groups, and proteoglycans are
proteins linked to polysaccharides of high molecular mass.
Monosaccharides
Monosaccharides arepolyhydroxyaldehydesH-[CHOH]xCHO
or polyhydroxyketones H-[CHOH]x-CO-[CHOH]yH
with three or more carbon atoms. Monosaccharides
bearing an aldehydic carbonyl group are called aldoses,
whereas those with a ketonic carbonyl group are called
ketoses. As monosaccharides are normally found as cyclic
structures, containing either a hemiacetal or hemiketal
group arising from ring closure of the linear polyhydroxycarbonyl
compounds, monosaccharides are considered to
contain a potential aldehydic carbonyl group (hemiacetal)
or potential ketonic carbohydrate group (hemiketal).
Cyclic hemiacetals or hemiketals of sugars with a fivemembered
(tetrahydrofuran) ring are described as furanoses
and those with a six-membered (tetrahydropyran)
ring are called pyranoses.
Monosaccharides are classified, according to the number
of carbon atoms they contain, as trioses, tetraoses,
pentoses, hexoses, etc.
Stereoisomerism and configuration
In the early 1890s Emil Fischer published detailed studies
on the configuration of aldoses. Molecules containing one
centre of chirality (asymmetrical carbon atom – which is a
carbon atom to which four different atoms or groups are
attached) exist in two isomeric forms. They are stereoisomers,
differing only in the arrangement of groups in
space. Thus the simplest sugar, glyceraldehyde, exists in
two nonsuperimposable stereoisomeric molecules, which
are mirror images. The two stereoisomers are called
enantiomers, or an enantiomeric pair. It is known that
these projections correspond to absolute configurations,
which are referred to as d-glyceraldehyde (I) and lglyceraldehyde
(II).
CH2OH
C
CHO
OHH C
CHO
HHO
(I) (II)
CH2OH
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Article Contents
Introductory article
. Introduction
. Monosaccharides
. Higher-order Structures
. Naturally Occurring Oligosaccharides and Glycosides
. Plant and Algal Polysaccharides
. Polysaccharides in Fungi and Invertebrates
. Polysaccharides and Glycoconjugates of Bacteria
. Polysaccharides and Glycoconjugates in Higher
Animals
. Carbohydrates in Recognition Processes
. Summary
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1ENCYCLOPEDIA OF LIFE SCIENCES © 2002, John Wiley & Sons, Ltd. www.els.net
The carbon atoms of a monosaccharide are numbered
consecutively, with the (potential) aldehyde group as
number 1. The monosaccharides are then assigned to the
d or l series according to the highest-numbered centre of
chirality. The structure below is the Fischer projection for
the acyclic form of d-glucose (III) and l-glucose (IV).
C
C
C
HO
C OH
OH OH
H
H
OH
H
(III) (IV)
CH2OH CH2OH
CHO
C
C
C
HO
HO
HO C OH
OH
H
H
CHO
H
1
2
3
4
5
Comparison of these structures with those of d-and lglyceraldehyde
illustrate the relationship of the configurations.
The structures of monosaccharides (triose, tetrose,
pentose and hexose) in the aldehydic acyclic form together
with their trivial names and three-letter abbreviations for
hexoses and pentoses are given in Figure1. Only the d forms
are shown.
Ring structure
Most monosaccharides exist as cyclic hemiacetals or
hemiketals, and are customarily presented in the Haworth
representation. Using d-glucose as an example, the
relationship between the acyclic Fischer representation
and the Haworth representation is shown in Figure 2.
Groups appearing to the right of the Fischer projection
appear below the plane of the ring in the Haworth
representation, and those on the left of the Fischer
projection appear above the plane of the ring in the
Haworth representation. The formation of the ring form
produces a new asymmetric carbon atom at C-1 in aldoses
(or C-2 in ketoses). This is called the anomeric carbon atom
and the isomers are called anomers, distinguished by the
terms a and b.
In the Haworth structures, one is visualizing the groups
attached to carbon atoms as being either above or below
the plane of the ring, implying that it is a flat rigid ring.
However, monosaccharides assume conformations that
are not planar. For consideration of the conformation of
monosaccharides (the arrangement in space of the atoms),
b-d-glucopyranose assumes a chair conformation. For
clarity, in most representations the hydrogen atoms
bonded to carbon atoms are omitted.
Physical and chemical properties
Monosaccharides are mainly crystalline solids, soluble in
water and capable of forming viscous solutions. The
specific rotation (a) of freshly prepared aqueous solutions
of monosaccharides such as glucose change rapidly, before
reaching a constant value in a phenomenon known as
mutarotation. Mutarotation generally follows first-order
kinetics, in whichring opening ofa hemiacetalor hemiketal
sugar takes place followed by ring closure with the
production of quantities of the other anomer.
A few sugars (l-arabinose, d-galactose and d-fructose)
exhibit a more complex form of mutarotation, where at
equilibrium a mixture of both pyranose and furanose
forms is obtained.
As most monosaccharides contain a primary hydroxyl
group, several secondary hydroxyl groups and a potential
carbonyl group, they can undergo a variety of chemical
reactions to produce ethers, acetals, esters and glycosides
(Figure 3). Methyl ethers (Figure 3(i)) are among the most
common derivatives and have been widely used in
analytical work, mainly in the identification of the linkage
positions between individual monosaccharides in oligosaccharides
and polysaccharides. Benzyl ethers are
amongst the most commonly used protecting groups in
carbohydratechemistry, wherethey areused mainly forthe
temporary substitution of hydroxy groups on sugar
derivatives required for synthetic purposes. Provided that
the monosaccharides do not contain base-sensitive groups,
traditional methods, involving reagents as benzyl halides
with sodium hydroxide or silver oxide may be used. Benzyl
ethers have similar stability to methyl ethers. Debenzylation
is easily achieved by catalytic hydrogenation.
Glycosides (Figure 3(ii)) are sugar derivatives bearing a
substituent (the aglycone) at the anomeric oxygen atom of
an aldose or ketose to produce a hemiketal. These products
are stable to alkali, but labile to acid, do not undergo
reversible ring opening, and therefore do not undergo
mutarotation, as do the parent aldoses or ketoses.
The conversion of aldoses to glycosides is a common
procedure for the protection of the anomeric centre of the
sugar, prior to the synthesis of other derivatives. Frequently,
sugars are converted to glycosides by alcoholysis
in the presence of an acid catalyst, in a reaction known as
Fischer glycosidation. The method is nonspecific, and
mixtures of a-and b-pyranosides are frequently formed.
Acetals (Figure 3(iii)) are produced by condensation of
two alcoholic groups and a carbonyl group. A wide variety
of derivatives may be produced due to the possibility of
intermolecular and intramolecular reactions. The products
are often crystalline and serve as convenient intermediates
in synthesis. Reaction, for example, of glucose with
acetone and in the presence of a dehydrating agent
produces 1,2:3,4-di-O-isopropylidene glucose.
The hydroxyl groups of sugars and their derivatives may
be readily esterified (Figure 3(iv)), with acetates and
benzoates being used frequently in synthetic work. Glucose
treated with acetic anhydride in the presence of sodium
acetate will furnish mainly penta-O-acetyl-b-d-glucopyranose.
When sodium acetate is replaced by zinc chloride,
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Carbohydrates
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the main product is penta-O-acetyl-a-d-glucopyranose.
Sugar esters are readily cleaved under acidic or basic
conditions.
Monosaccharides are readily reduced to alditols
(Figure3(v)), using water-soluble reductants such as sodium
borohydride. Reduction of the aldehyde or potential
aldehyde group in aldoses gives one product. Glucose is
converted to glucitol, but ketoses such as fructose gives two
products, glucitol and mannitol, due to the creation of a
new asymmetrical carbon atom at C-2.
Mild oxidizing reagents such as bromine water or
sodium hypochlorite oxidize aldoses at C-1 to produce
the corresponding aldonic acid (Figure 3(vi)). Glucose is
converted to gluconic acid, which readily gives a stable
lactone. Ketoses are not oxidized by means of these
procedures.
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CHO
CH OH
CH2OH
Glyceraldehyde
H C
CHO
OH
CH OH
CH2OH
Erythrose
HO C
CHO
H
CH OH
CH2OH
Threose
H C
CHO
OH
CH OH
CH2OH
Ribose
(Rib)
CH OH
HC
CHO
HO
CH OH
CH2OH
Arabinose
(Ara)
CH OH
OHC
CHO
H
CHO H
CH2OH
Xylose
(Xyl)
CH OH
HC
CHO
HO
CHO H
CH2OH
Lyxose
(Lyx)
CH OH
H C
CHO
OH
CH OH
CH2OH
Allose
(All)
CH OH
CH OH
H C
CHO
OH
CHO H
CH2OH
Glucose
(Glc)
CH OH
CH OH
HO C
CHO
H
CHO H
CH2OH
Mannose
(Man)
CH OH
CH OH
H C
CHO
OH
CH OH
CH2OH
Gulose
(Gul)
CH OH
CHO H
HO C
CHO
H
CH OH
CH2OH
Idose
(Ido)
CH OH
CHO H
H C
CHO
OH
CHO H
CH2OH
Galactose
(Gal)
CH OH
CHO H
C H
C H
Talose
(Tal)
CH
C H
HO C
CHO
H
CH OH
CH2OH
Altrose
(Alt)
CH OH
CH OH
HO
CHO
HO
CH2OH
OH
HO
Figure 1 Aldose sugars of the D series.
Carbohydrates
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3
Higher-order Structures
Oligosaccharides
Oligosaccharides are molecules in which two or more
monosaccharide units are joined by a glycoside linkage(s).
They are classified according to the number of monosaccharide
units, into di-, tri-tetra-saccharides, etc. The
glycosidic bond is formed between the hydroxyl group on
the anomeric carbon atom of one monosaccharide and any
hydroxyl group on another monosaccharide. If the
hydrogen atom of an alcoholic hydroxy group is replaced
by a glycosyl unit, the resulting oligosaccharide has a free
hemiacetal group and is thus referred to as a reducing
oligosaccharide. If the glycosidic bond is formed by
reaction of the two glycosidic (anomeric) hydroxyl groups,
the resulting oligosaccharide is referred to as a nonreducing
oligosaccharide.
Examples of reducing oligosaccharides are maltose (V)
and lactose (VI). A nonreducing oligosaccharide is sucrose
(VII).
Polysaccharides
Polysaccharides are macromolecules, widely distributed in
nature, that exhibit a variety of functions, such as
structural material in plants, animals and microorganisms.
Examples of structural polysaccharides include cellulose in
plants and chitin in the exoskeletons of insects and
crustaceans. Other polysaccharides such as starch, glycogen
and seed galactomannans act as major energy sources
for specific living organisms.
Polysaccharides (glycans) may be described as homopolysaccharides
if they contain only a single type of
monosaccharide residue. Examples include cellulose and
starch, which are both glucans because they are composed
of glucose residues. Excluding those polysaccharides that
have trivial names (e.g. cellulose, starch, chitin), polysaccharide
names are obtained by replacing the ‘ose’
ending of the sugar by ‘an’ (e.g. xylan for polymers of
xylose, and mannan for polymers of mannose). When the
configuration, d or l, and the glycosidic linkage designation
are known, these are also included in the name. Thus
glycogen is an a-d-glucan or, more specifically, an a(1!4)(1!6)-d-glucan.
Polysaccharides that contain two
or more kinds of sugar residue are called heteropolysaccharides
(heteroglycans).
Glycoconjugates
When monosaccharides and oligosaccharides are covalently
linked to lipid or proteins, they are called glycolipids
and glycoproteins, and known collectively as glycoconjugates.
The lipids are hydrophobic molecules and can be
long-chain fatty acids, alkanols, amines, steroids or
terpenoids. Polymers containing covalently bound monosaccharide
and amino acid residues are termed glycoproteins,
glycopeptides or peptidoglycans. Glycoproteins are
proteins bearing oligosaccharide or low molecular mass
polysaccharides covalently attached to them. Peptidoglycans
have polysaccharide chains covalently linked to
peptide chains, and proteoglycans are polysaccharides of
high molecular mass covalently linked to proteins.
Naturally Occurring Oligosaccharides
and Glycosides
Oligosaccharides
Only a relatively few disaccharides, such as sucrose and
lactose, are found free and abundantly in nature. Most
other disaccharides and oligosaccharides are produced in
degradation of their constituent polymers.
Sucrose
Sucrose is one of the most abundant natural products. It is
widely distributed in the plant kingdom, with sugar cane
(20% by weight) and sugar beet (15% by weight) being the
two main commercial sources for its production. Sucrose is
the major soluble carbohydrate reserve and source of
energy in plants, and is an important dietary material in
humans.
Sucrose (VII) is characterized as an a-d-glucopyranosyl(1!2)-b-d-fructofuranoside
(or a b-d-fructofuranosyl(2!1)-a-d-glucopyranoside).
Since the glycosidic bond is
formed between two anomeric carbon atoms, this disaccharide
has no reducing properties. Hydrolysis of
sucrose to give equimolar amounts of d-glucose and dfructose
is readily achieved by heating solutions of the
sugar under mild acidic conditions. This process is called
‘inversion’ and the resulting product is referred to as ‘invert
sugar’, because the optical rotation of the sucrose changes
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O
OH
OH
O
OH
OH
OHOH
O
O
OH
OH
OH O
OH
OH
OH
O
O
OH
OHOH
O
OH
OH
O
CH2OH
(V) (VI)
(VII)
CH2OH CH2OH
CH2OH CH2OH CH2OH CH2OH
Carbohydrates
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4
from dextro to laevo owing to the high laevorotation of the
d-fructose.
In human metabolism, sucrose, as part of the diet, is
readily converted by a-glucosidases present on the
intestinal wall. The resulting d-glucose and d-fructose
are readily transported through the intestinal wall into the
bloodstream, and transported to and utilized by specific
cells in the body.
A number of other oligosaccharides based on the
structure of sucrose are important in the metabolism of
plants. For example, raffinose, a naturally occurring plant
trisaccharide, has a distribution in nature almost as
ubiquitous as that of sucrose, but in much smaller amounts
(approximately 0.05% in sugar beet and 1.9% in soy
beans). Raffinose is an O-a-d-galactopyranosyl-(1!6)-ad-glucopyranosyl-(1!2)-b-d-fructofuranoside.
It is readily
converted by the action of b-fructofuranosidase
(invertase) to the corresponding disaccharide melibiose
(O-a-d-galactopyranosyl-(1!6)-a-d-glucopyranose).
Lactose
Lactose (VI) occurs in mammalian milk, from between 2.0
and 8.5 per cent, depending on the mammal. It was
originally named ‘milk sugar’ and is 4-O-b-d-galactopyranosyl-d-glucose,
and thus a reducing sugar. Lactose is
prepared industrially from whey, which is obtained as a
byproduct in the manufacture of cheese.
The disaccharide maltose (V) (O-a-d-glucopyranosyl(1!4)-d-glucose)
is an intermediate formed during the
metabolic mobilization of starch, a common reserve
polysaccharide in plant tissues. It is produced, together
with other malto-oligosaccharides, in the saccharification
of starch by amylases during the development, maturation
and germination of cereal grains. The major pathway for
the metabolic utilization of maltose in plants is its cleavage
to free glucose, by the action of a-glucosidase (maltase), an
enzyme found in starch-producing tissues.
Various food preparations contain large amounts of
maltose, which when ingested by humans is hydrolysed to
glucose by an a-glucosidase in the intestinal tract before
transport across the intestinal membrane into the bloodstream
can take place. The glucose is then available for
distribution to specific cells for further metabolism. Starch
itself, which forms part of the human diet, is first broken
down by salivary and pancreatic amylases to maltose and
malto-oligosaccharides prior to conversion to glucose by
the intestinal a-glucosidases.
Most other oligosaccharides, which may occur freely,
but in very small concentrations, may be prepared either by
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H C
CHO
OH
CHO H
CH2OH
CH OH
CH OH
C
H
CH2OH
OH
OH
H
H
C
H
HO
CHO
OH
C
CH2OH
OH
OH
H
H
H
C
H
HO
CHO
OH
C
CH2OH
O
OH
H
H
C
H
C
H
HO
C
H
OHC
OH
C
CH2OH
O
OH
H
H
C
H
C
H
HO
C
OH
HC
OH
+
O
OH
HOH2C
OH
O
OH
HOH2C
OH
HO
HO
HO
HO
α-D-Glucose β-D-Glucose
Figure 2 Acyclic (Fischer representation) and cyclic (Haworth
representation) forms of D-glucose.
CH2OH
OH
OH
OH
CH2OH
OH
CH2OAc
O
OAc
OAc
OH
OAc
CH2OCH3
O
OCH3
OCH3
OH
OCH3
CH2OH
OH
OH
OH
COOH
OH
CH2OH
O
OH
OH
OH
OH
CH2OH
O
OH
OH
OCH3
OH
(vi)
(iv)
(i) (ii)
(v) (iii)
CH2OH
O
O
O
O
O C
H3C
CH3
C
CH3
H3C
Figure 3 Derivatization of D-glucose. (i) Ether formation to produce
2,3,4,6-tetra-O-methyl-D-glucose. (ii) Glycoside formation to produce
methyl-a-D-glucoside. (iii) Acetalation to form 1,2,3,4-di-O-isopropylidene
D-glucose. (iv) Ester formation to form 2,3,4,6-tetra-O-acetyl-D-glucose. (v)
Reduction to form D-glucitol. (vi) Oxidation to form D-gluconic acid.
Carbohydrates
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controlled partial acid hydrolysis or by enzymic hydrolysis
of the native polysaccharide. These methods usually
produce mixtures (mono-, di-, tri-saccharides, etc.).
Individual saccharides may then be obtained following
some chromatographic procedure.
Glycosides
A host of natural compounds in which sugars are
glycosidically linked to nonsugar constituents (aglycone)
have been identified. These include simple alcohols,
alkaloids, carotenes, natural pigments and steroids.
Many of these compounds have been shown to be
pharmacologically active, including cardiac glycosides
and antibiotics such as streptomycin and the anthracyclins.
d-Glucose, b-linked to the aglycone, is commonly but not
exclusively found, and many plant glycosides have been
shown to contain disaccharide and occasionally trisaccharide
units. Microbial products have revealed a much
larger range of glycosidically linked sugars containing
structural variations such as amino, carboxyl and deoxy
groups.
Plant and Algal Polysaccharides
Plants can be considered in two separate groups: land
plants and aquatic plants (algae).
Plant polysaccharides
Cellulose
Land plants generally contain high levels of polysaccharides,
with cellulose (VIII), a (1!4)-b-d-glucan and the
primary component of the cell wall, being the most
abundant. Associated with cellulose are other heteropolysaccharides
such as arabinoxylans, arabinogalactans and
pectic substances.
O
OH
O
OH
O
OH
O
O
OH
(VIII)
CH2OH CH2OH
Reserve, or storage, polysaccharides are synthesized by
plant cells to be later enzymically hydrolysed to produce
the monosaccharides, which re-enter the cell’s metabolism.
These polysaccharides are generally synthesized during
periods of high photosynthetic activity. Starch, which is
economically the most important seed storage polysaccharide,
is formed inside the plastids. Resting seeds of
many plant species may contain little or no starch. In these
cases, polysaccharide reserves are stored outside the
plasmalemma and are described as cell wall storage
polysaccharides, represented by mannans, xyloglucans
and galactans. Cellulose is the most abundant organic
substance found on Earth. It occurs as 94% of the cotton
fibre, approximately 80% in flax, 60–70% in jute, 40–50%
in wood and as little as 4% in endosperm cells.
In cellulose, the molecules are entirely linear, consisting
of up to 14 000 d-glucose units linked uniformly by (1!4)b-glycosidic
bonds. The b-configuration between glucose
units, together with hydrogen bonding which arises from
the ring oxygen of one glucose residue and the C-3
hydroxyl group on the neighbouring glucose residue,
results in the formation of an extended ribbon-like chain.
In the plant cell, these polymer chains stack on top of each
other, joined by hydrogen bonds, in a way that staggers the
chains – as bricks are staggered to give stability to a wall.
This results in the cellulose existing in the form of
microfibrils, where both crystalline and amorphous areas
can be observed. Native cellulose is not very reactive
chemically, except in the amorphous regions. Strong acids,
such as hydrochloric or sulfuric acid, are necessary to
cleave glycosidic bonds to produce glucose and oligosaccharides
(cellodextrins).
The traditional uses of wood cellulose in the paper
industry and cotton cellulose in the textile industry have
been extended to the industrial production of a number of
derivatives of cellulose. When cellulose is preswollen by the
use of alkali, the polymer chains are much more susceptible
to chemical modifications. Products such as cellulose
acetate can be used to produce fibres (rayon), lacquers
and films. Cellulose nitrate is used in the plastics, film and
explosive industries. A water-soluble, viscous derivative of
cellulose, carboxymethyl cellulose, is used in the food
industry as a thickening agent in adhesives, laxatives and
low-calorie foods.
Starch
Starch is the principal food reserve polysaccharide in the
plant kingdom, and occurs within the protoplasts of many
plant cells in the form of granules. These granules contain
two separately different polysaccharides, amylose (IX) and
amylopectin (X), in the approximate ratio of 1 : 4. The
former consists of long, mostly linear, chains of (1!4)-alinked
d-glucose residues. Depending on the source of the
starch, amylose may contain several hundred to several
thousand glucose residues. The branching is due to the
presence of (1!6)-a-d-glucosidic linkages.
O- 5 (1!4)-a-d-glucosylO!
5 (1!6)-a-d-glucosylAmylopectin
has a branched structure, in which the
linear chains of approximately 20 to 25 (1!4)-a-d-glucose
residues are linked via (1!6)-a-d-glucosidic linkages to
form a macromolecule of molecular weight in the range 107
to 108
, the variation being dependent on the botanical
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origin of the starch. Amylose and amylopectin differ with
regard to enzymic breakdown and to a phenomenon
known as retrogradation. High yields of maltose are
produced by the action of b-amylase on amylose. Action
starts at the nonreducing end of the polysaccharide with
hydrolysis of alternate(1!4)-linkages. Theenzymecannot
cleaveorbypass the(1!6)-glucosidic linkagesofany ofthe
few branchpoints in the amylose. Action of b-amylase on
the highly branched amylopectin releases maltose from the
(1!4)-linked chains, degrading about 55% of the polysaccharide
and leaving an enzyme-resistant fraction, blimit
dextrin.
Retrogradation is a term applied to the physical
reassociation that takes place between starch molecules
in solution. With the progress of retrogradation, starch
solutions become less viscous, increasingly cloudy, and
increasingly resistant to enzyme action. Starch contributes
a major fraction of the human diet. The breakdown of the
starch is complex and requires the presence of a number of
enzymes. First, attack is by a-amylase in the saliva. A
limited amount of depolymerization takes place as food
passes to the stomach and then to the intestine. A second aamylase
is secreted from the pancreas. Its action is to cleave
a-(1!4)-a-glucosidic bonds only, on either sides of the
(1!6)-branchpoints to produce glucose, maltose, maltotriose
and low molecular weight dextrins that will contain
at least one (1!6)-a-glucosidic linkage. The oligosaccharides
are then hydrolysed to glucose by a number of aglucosidases
which are membrane-bound to the intestinal
wall. The final product, glucose, is then diffused through
the intestinal wall and transported in the blood for further
metabolism.
In the commercial preparation of a number of starchcontaining
foodstuffs, a considerable amount of retrogradation
may be introduced to starch molecules. This
results in incomplete enzymic hydrolysis of the starch in the
digestive system, and thus contributes to ‘dietary fibre’.
The food and beverage industry have a very large
requirement for the hydrolysis products of starch to
syrups. The production of these products depends on the
same basic system of hydrolysis of starchwith thermophilic
a-amylases and subsequent conversion, using b-amylase or
glucoamylase. This results in the conversion of starch to
either glucose syrups (mainly glucose) or maltose syrups
(mainly glucose maltose and maltotriose). Since d-fructose
is much sweeter than either sucrose or glucose, the glucose
arising from starch hydrolysis may be converted to an
equilibrium mixture of glucose and fructose in the presence
of glucose isomerase. This product has applications in the
soft drink, conserves and canned fruit industry.
Pectins
Primary cell walls and intercellular layers of land plants
contain pectins, which contain d-galacturonic acid as the
main component. Because of their strong gelling properties,
they are used commercially for the gelation of fruit and
fruit juices to conserves and jellies, the major sources of
these polysaccharides being apple pulp and the rinds of
citrus fruits. Pectins (XI) are esterified polymers of (1!4)a-d-galacturonic
acid residues. The linear chains are
frequently interrupted at intervals by the insertion of lrhamnose
residues. Pectic substances also include deesterified
pectic acid and other neutral polysaccharides
containing d-galactose and l-arabinose residues.
OH
COOH COOH
O
OH
O
OH
OH
O
O
O
4)-α-D-GalA-(1 4)-α-D-GalA-(1
(XI)
Algal polysaccharides
Algae grow abundantly in both fresh and sea water, and
include unicellular organisms. The multicellular algae
(seaweeds) have been studied extensively and have been
shown to contain a variety of polysaccharides, a number of
which are of great commercial importance. Some of the
polysaccharides function as food reserves, antidesiccants
or structural materials.
Starch hasbeen characterized in a numberof algalphyla,
such as blue-green, red and green algae, as a food-reserve
polysaccharide. Occasionally the amount of amylose
detected is very low and the amylopectin-type polysaccharide
resembles animal glycogen in the nature of its
branching. However, in the brown algae starch is replaced
by the water-soluble glucan laminarin as the principal
reserve polysaccharide.
Alginic acid is a linear polyuronic acid isolated from the
mucilage of brown seaweeds. The capacity of alginates,
which are viscous, to form gels by reaction with calcium
ions is an important property. The ability of alginates to
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hold water, form gels and stabilize emulsions has led to
numerous industrial and food applications.
The major structural polysaccharides of red seaweed are
the carrageenans, a family of partially sulfated linear
galactans. The polysaccharide (XII) has a regular disaccharide
repeat structure of 3-linked b-d-galactose and 4linked
3,6-anhydro-a-d-galactose residues. Sulfate ester
groups are located at different sites on the polysaccharide
to produce three different molecules: iota (i), kappa (k) and
lambda (l) carrageenans. Carrageenan produces highviscosity
solutions and gels in water, and is widely used in
the food industry. It reacts with the milk protein, casein,
and is used to prepare high-strength milk gels.
OH
O
O
OOH
OH
O
CH2OH
O O
3)-β-D-Gal-(1 4)-3,6-anhydro-α-D-Gal-(1
(XII)
CH2
Red seaweeds are the source of agar, which is a mixture of
at least three polysaccharides: agarose (XIII), a neutral
polysaccharide with alternating (1!3)-b-d-galactosyl and
(1!4)-linked 3,6-anhydro-a-l-galactosyl residues, and
two other acidic polysaccharides.
O
O
OOH
OH
O
CH2OH
O
O
H2C
O
3)-β-D-Gal-(1 4)-3,6-anhydro-α-L-Gal-(1
(XIII)
Agarose is the gelling portion of agar, and has a doublehelical
structure. The double helices aggregate to give a
three-dimensional framework, capable of holding water
molecules within the interstices of the framework. As these
gels are thermoreversible, agar is an ideal material for the
production of microbiological media for culturing micro-
organisms.
Polysaccharides in Fungi and
Invertebrates
Fungal polysaccharides
The cell walls of fungi are rich in polysaccharides, generally
containing an inner layer composed of either chitin or
cellulose, embedded in other polymers, and an outer layer.
The outer layer is generally soluble in dilute alkali, with the
inner layer being an insoluble residue. The inner layer of
most taxonomic groups normally contains two different
polysaccharides: a highly branched b-glucan and chitin,
which is a polymerof (1!4)-linked b-N-acetylglucosamine
residues.
Saccharomyces cerevisiae is probably the yeast studied in
greatest detail, and principal components of its cell wall are
polysaccharides, glucan and mannan, which together
account for approximately 80–90% of the dry weight of
the cell. The glucan component is responsible for the tensile
strength, rigidity and shape of the cell. In addition to the
cell wall glucan, S. cerevisiae contains an intracellular
storage polysaccharide, glycogen. This is a highly
branched a-glucan of similar structure to plant amylopectins.
When grown in the presence of a suitable carbohydrate
source, Aureobasidium pullulans (formerly Pullularia
pullulans) excretes large quantities of a viscous watersoluble
a-glucan. This polysaccharide, referred to as
pullulan, is a linear polysaccharide containing mainly
maltotriose units connected by (1!6)-a-linkages. Pullulan
is produced on an industrial scale, with applications in
forming films for food packaging and in the formulation of
certain foods.
Invertebrate polysaccharides
Chitin (XIV) is the major invertebrate polysaccharide and
forms 15–30% of the shells of crustacea, the rest being
mainly calcium carbonate. It is a fibrillar (1!4)-linked 2acetamido-2-deoxy-b-d-glucan.
It can be considered to
have a similar structure to cellulose, with the d-glucose
units in that polysaccharide being replaced by 2-acetamido-2-deoxy-d-glucose
(N-acetylglucosamine) units. Thus
the two polysaccharides have similar physical properties.
The chains of chitin are of the extended ribbon type,
possessing intramolecular hydrogen bonds.
O
O
OH
O
OH
O
O
(XIV)
NHCOCH3NHCOCH3
CH2OHCH2OH
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Although native chitin is extremely resistant to chemical
attack, it can be dissolved in a number of hydrogen bondbreaking
solvents. Prolonged heating in 10 mol L2 1
hot
alkali removes the majority of the acetyl groups to give
chitosan, whereas treatment with strong (5–10 mol L2 1
)
acid at 1008C is necessary to hydrolyse chitin to
glucosamine and acetic acid.
Chitosan is an important water-soluble viscous polysaccharide.
As the majority of N-acetyl groups of chitin are
replaced by amino groups, at low pH values these amino
groups exist in a charged (N1
H3) form. At high pH,
chitosan becomes insoluble.
Chitosan has proved to be useful in the food
and biomedical fields – from the removal of anionic
particulates from fruit juices to wound-healing tech-
nology.
Polysaccharides and Glycoconjugates of
Bacteria
Many polysaccharides and glyconjugates are synthesized
by bacteria and originate either in the cell wall or as
intracellular or extracellular polymers. More than 100
different monosaccharides have been identified as components
of bacterial polysaccharides isolated from different
species or groups of bacteria. Included among these
monosaccharides are mono-and di-deoxy sugars, aminodi-deoxy
and amino-uronic acids.
Bacteria are normally classified according to the
Gram stain as either Gram-positive or Gram-negative.
Most Gram-negative bacteria contain a thin inner
sheet of peptidoglycan together with an outer membrane
composed of lipoprotein, phospholipid and lipopolysaccharide.
In Gram-positive bacteria peptidoglycan is
the major cell wall component, together with
smaller quantities of teichoic acids (polyol phosphates)
or other polysaccharides. The peptidoglycan is the major
supporting structure of the bacterial cell wall and is a
glycan composed of alternating units of N-acetylglucosamine
and N-acetylmuramic acid. Individual glycan
chains are cross-linked with either tetra-or penta-peptides.
Lipopolysaccharides, which are located on the
outer membrane of Gram-negative bacterial cell walls,
are responsible for a number of biological properties
including toxicity and immunogenicity. The complete
lipopolysaccharide consists of three parts: lipid A, the
core region and the O-specific side-chains. Lipid A is a
phosphorylated polysaccharide containing esterified fatty
acid groups, and carries the main endotoxic properties of
the lipopolysaccharide. The core region is attached to lipid
A, and in turn is attached to the O-antigenic side-chains,
which contain the structural information recognized by
antibodies.
Many bacteria produce extracellular polysaccharides,
which together with the O-antigenic parts of the lipopolysaccharides
form the principal immunogens and antigens
of the bacteria.
A number of microbial polysaccharides are produced
on an industrial scale. Dextran is a high molecular
weight a-d-glucan produced from strains of Leuconostoc
mesenteroides. This polysaccharide and its derivatives
are used in the pharmaceutical and fine chemical
industries. Xanthan gum is produced as an extracellular
polysaccharide, by Xanthomonas campestris.
Solutions of the polysaccharide have high viscosity at
low concentration. Xanthan is used widely as an additive in
the food industry, as well as having many industrial
applications.
Polysaccharides and Glycoconjugates in
Higher Animals
Polysaccharides
The extracellular space of many mammalian tissues
contains polysaccharides, composed of amino sugars
(glycosaminoglycans) that are covalently attached to a
protein backbone. In the older literature they were
described as mucopolysaccharide–protein complexes;
now they are referred to as proteoglycans. A major
function ofproteoglycans is in theformationof a structural
matrix for protein components of connective tissue
and skin. A single proteoglycan may contain more
than one type of glycosaminoglycan chain. For example,
the cartilage proteoglycan (molecular weight 2.5 Â 106
Da,
90% carbohydrate) has a core protein with 80 to 100
chondroitin sulfate chains, 30 to 50 keratan sulfate
chains, together with oligosaccharide chains that are
directly linked to amino acids in the polypeptide
backbone.
The glycosaminoglycans are generally composed of
repeating disaccharide units, composed of an acidic and a
basic monosaccharide. The amino groups of the basic
monosaccharides are N-acetylated or N-sulfated amino
groups. O-Sulfate ester groups are also present, resulting in
the production of highly acidic polysaccharides. Glycoside
linkages are alternatively (1!3) and (1!4) linked.
Glycosaminoglycans that have been characterized
include hyaluronic acid (XV) (composed of N-acetylglucosamine
and glucuronic acid residues), chondroitin
sulfates (XVI) (N-acetylgalactosamine 4-or 6-sulfates and
glucuronic acid), dermatan sulfate (XVII) (N-acetylgalactosamine
4-sulfate and l-iduronic acid) and keratan sulfate
(XVIII) (galactose 6-sulfate and N-acetylglucosamine 6-
sulfate).
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3)-β-D-GalNAc-(1 4)-β-D-GlcA-(1
O
OH
O
OH
O
O
O
NHCOCH3
O
OOH
OH
OO
OH
O
OH
O
OH
COOH
O
O
OH
CH2OH
O
NHCOCH3
3)-β-D-GlcNAc-(1 4)-β-D-GlcA-(1
O
OH
O
OH
COOH
O
O
CH2OR′
O
NHCOCH3
OR
(XVI)
(R=SO3H and R′=H or R=H and R′=SO3H)
COOH
3)-β-D-GalNAc-4-SO3H-(1
(XVII)
O
CH2OH
O
OH
NHSO3H
OH
O O
OSO3H
O
O
3)-β-D-Gal-6-SO3H-(1 4)-β-D-GlcNAc-6-SO3H-(1
(XVIII)
COOH
4)-α-D-GlcNSO3H-(1 α-L-ldA-2-SO3H-(14)-
(XIX)
(XV)
4)-α-L-IdA-(1
HO3SO
CH2SO3H CH2SO3H
NHCOCH3
CH2OH
Heparin (XIX) is a linear polysaccharide containing
mainly d-glucosamine and l-iduronic acid residues, with
some d-glucuronic acid residues. The proportion of these
residues is variable, as is the number of sulfate ester groups
present. Heparin is a degradation product of a native
proteoglycan found exclusively in the granules of mast
cells, which are commonly associated alongside blood
vessels, and is best known as a powerful anticoagulant. The
anticoagulant activity arises from the activation of
antithrombin III, a protease inhibitor that is responsible
for the inhibition of several proteolytic enzymes of the
blood coagulation cascade. Hyaluronic acid is a nonsulfated
glycosaminoglycan located in joints, umbilical cord
and vitreous humour of the eyes. This high molecular
weight viscous polysaccharide is not covalently attached to
protein as in proteoglycans, but physically associates with
proteoglycan.
Glycogen, a highly branched a-d-glucan with a structure
generally similar to that of amylopectin, is the major
storage polysaccharide of animals. It is located in the liver
and skeletal muscle, and acts as a primary energy store in
cells. The breakdown of glycogen requires the presence of
two enzymes, a phosphorylase and a debranching enzyme,
to produce mainly glucose 1-phosphate and glucose. The
glucose 1-phosphate can then be converted to glucose 6phosphate,
then glucose, before use in various metabolic
pathways.
Mammalian glycoconjugates
Glycoconjugates include glycoproteins and glycolipids –
proteins or lipids to which carbohydrates are glycosidically
attached. Glycolipids are found in animal cells, the major
group being the glycosphingolipids. The lipid moiety is
ceramide, a molecule in which a long-chain fatty acid is
attached to sphingosine, a 2-amino-1,3-dihydroxy-4-transd-octadecene,
via an amide bond. The primary hydroxy
group of the ceramide is linked glycosidically to oligosaccharides,
usually by a b-d-glucosyl residue. Families of the
glycosphingolipids exist, depending on whether a tetra-,
tri-, di-or mono-glycosyl saccharide is involved. Gangliosides
are a special group of animal glycolipids, in which the
glycosylceramides contain at least one sialic acid residue.
Sialic acid is a nine-carbon carboxylated sugar, which thus
makes the gangliosides anionic.
Glycolipids are generally associated with cell membranes.
As a result of their amphipathic nature, the
hydrophobic ceramide portion of the molecule forms
hydrophobic interactions with the phospholipid of the cell
membranes. The covalently attached hydrophilic carbohydrate
portion is then located at the outer surface of the
cell membrane. A number of human disease states,
including Krabbe disease, Gaucher disease and several
forms of gangliosidosis, are recognized by the accumulation
in the body of specific glycolipids.
Mammalian glycoproteins are found in both soluble and
membrane-bound forms. More than 300 proteins have
been characterized from human serum or plasma, the vast
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majority of these being glycoproteins. A wide variety of
properties has been assigned to these glycoproteins,
including acting as protease inhibitors, blood clotting
factors, hormones, antibodies and the transport of various
ions. The presence of carbohydrate in a glycoprotein is
known to modify the physicochemical properties of
proteins. The presence of oligosaccharide chains will
influence the final conformation of proteins. Sialic acid
residues, which are normally present at the terminal
nonreducing ends of these oligosaccharide chains, are
important in determining the correct arrangement of
glycoproteins in cell membranes. In a number of
cases, the presence of the carbohydrate moiety of
glycoproteins confers resistance of these molecules to
attack by proteolytic enzymes. Cell-surface carbohydrate
moieties may be involved in adhesion, communication
recognition, antigenic specificity and regulation of cell
growth.
Oligosaccharides are conjugated to mammalian proteins,
either by O-glycosyl linkages to the hydroxyl groups
of serine or threonine (O-glycans) or by N-glycosyl
linkages to the amide nitrogen of asparagine (N-glycans).
N-glycan chains may have two to four branches, and
generally contain between 11 and 18 sugar residues,
whereas O-glycan chains are simpler and may be limited
to trisaccharides.
In membrane glycoproteins, the hydrophilic glycan
chains on the polypeptide backbone are located outside
the cell membrane. A sequence of mainly hydrophobic
amino acids is involved in hydrophobic–hydrophobic
binding in the membrane itself, with the remainder of the
polypeptide chain being in the cytoplasm.
Carbohydrates in Recognition
Processes
Many carbohydrate-specific proteins have been isolated
and characterized. Carbohydrates serve as recognition
determinants on glycoconjugates. This may involve
molecule–molecule, cell–molecule and cell–cell interactions.
The most obvious molecule–molecule interactions
include enzymic synthesis and degradation of carbohydrates,
and the immunological response to carbohydrate
antigens.
One of the earliest examples of molecule–cell interactions
involved the observation that certain seed proteins
have the ability to agglutinate animal blood cells. These
proteins are termed lectins. The initial event is the
noncovalent binding of a monosaccharide or oligosaccharide
segment of a glycoconjugate or polysaccharide to a
binding site on the protein. Inhibition of the binding
(agglutination) by free monosaccharides or oligosaccharides
gives information on the nature of the carbohydrate
involved in the binding. Although many lectins have been
isolated from plants, these proteins have also been
discovered in animals and microorganisms. These interactions
are important in the attachment of toxins, viruses and
bacteria to cells.
The adherence of bacteria to mammalian cells is a
prerequisite for colonization and infection, while the initial
adherence of bacterial toxins to cells is necessary before the
toxic effects take place. A number of strains of Escherichia
coli contain a mannose-binding lectin, which binds to
mannose-rich portions of glycoproteins on the epithelial
cells of the intestine. Cholera toxin is a protein with two
subunits. One subunit carries the toxic components, which
have no activity until they enter the cells, while the other
subunit binds to the gangliosides on intestinal epithelial
cells, after which the toxic subunit penetrates the cell.
The binding of influenza virus to host cells is a
prerequisite for the initiation of infection. This is mediated
by the influenza haemagglutinin, a lectin that is specific for
the recognition of sialic acid and is located on the viral
surface. Binding of this viral lectin is to the sialylated
glycoconjugates on cell surfaces.
One of the first animal lectins to be identified was the
hepatic lectin. Serum glycoproteins bear sialic acid residues
at the nonreducing termini of the glycan chains and
circulate normally in the bloodstream. Removal of this
sugar from the chains exposes d-galactose at the new
reducing termini. The resulting asialoglycoproteins are
rapidly removed from circulation owing to their uptake in
liver cells by the hepatic lectin.
Cell–cell interactions are exemplified by the homing of
leucocytes to sites of inflammation. Inflammation is the
body’s reaction to physical damage or invasion by an
infectious reagent or antigen. Leucocytes circulate normally
and do not bind to cells until the onset of
inflammation. Adhesion molecules (selectins), which are
membrane bound, allow the leucocytes to interact with
epithelial cells. This is achieved by recognition of the
carbohydrate sequences on the surface glycoconjugates of
the cells.
Summary
Carbohydrates are the most abundant organic substances
found in nature. They occur as simple sugars (monosaccharides)
and in bound form as oligosaccharides,
polysaccharides or glycoconjugates. In these higher forms,
the monosaccharide units are joined together by glycosidic
linkages.
Carbohydrates have numerous important functions in
living material. In animals, glucose and glycogen are the
main sources of energy. In starch, the polysaccharide is
produced as a result of photosynthesis. Polysaccharides,
such as cellulose, form part of the cell wall of plants.
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Peptidoglycans and lipopolysaccharides form the protective
outer cell wall of microorganisms.
Many mammalian tissues contain proteoglycans in
which polysaccharides are attached to a protein
backbone, and are components of connective tissue and
skin.
Glycoconjugates are frequently located on the surface of
membranes in animal cells. The carbohydrate portion of
these molecules may be antigenic and, in certain cases,
sequences of oligosaccharides are recognized by specific
proteins (lectins) in complementary cells. This recognition
mechanism is responsible for numerous cell–cell and cell–
molecule interactions.
Further Reading
Binkley RW (2000) Modern Carbohydrate Chemistry. New York: Marcel
Dekker.
Bush CA, Martin-Pastor M and Imberty A (1999) Structure and
conformation of complex carbohydrates of glycoproteins, glycolipids
and bacterial polysaccharides. Annual Review of Biophysics and
Biomolecular Structure 28: 1–269.
Kuberan B and Lindhardt RJ (2000) Carbohydrate-based vaccines.
Current Organic Chemistry 4: 653–677.
Lindhorst TK (2000) Essentials of Carbohydrate Chemistry and
Biochemistry. London: Wiley-VCH.
Lis H and Sharon N (1998) Lectins: carbohydrate-specific proteins that
mediate cellular recognition. Chemistry Review 98: 637–674.
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