CHEMICAL PROPERTIES OF MONOSACCHARIDES. STRUCTURE,
COMPOSITION AND PROPERTIES OF DISACCHARIDES. STRUCTURE, COMPOSITION AND
PROPERTIES OF POLYSACCHARIDES.
Introduction: Carbohydrates are the most abundant class of bioorganic molecules on planet
Earth. Although their abundance in the human body is relatively low,
carbohydrates constitute about 75% by mass of dry plant materials.
Green (chlorophyll-containing) plants produce carbohydrates via
photosynthesis. In this process, carbon dioxide from the air and water from the
soil are the reactants, and sunlight absorbed by chlorophyll is the energy
source.
Carbohydrates are the most abundant
class of organic compounds found in living organisms. They originate as
products of photosynthesis, an endothermic reductive condensation of
carbon dioxide requiring light energy and the pigment chlorophyll
|
n
CO2 + n H2O + energy |
As noted here, the formulas of many carbohydrates can
be written as carbon hydrates, Cn(H2O)n, hence
their name. The carbohydrates are a major source of metabolic energy, both for
plants and for animals that depend on plants for food. Aside from the sugars
and starches that meet this vital nutritional role, carbohydrates also serve as
a structural material (cellulose), a component of the energy transport compound
ATP, recognition sites on cell surfaces, and one of three
essential components of DNA and RNA.
Plants have two main uses for the carbohydrates they produce. In the form
of cellulose, carbohydrates serve as structural elements, and in the form of
starch, they provide energy reserves for the plants.
Dietary
intake of plant materials is the major carbohydrate source for humans and
animals. The average human diet should ideally be about two-thirds carbohydrate
by mass.
Carbohydrates
have the following functions in humans:
1.
Carbohydrate oxidation provides energy.
2.
Carbohydrate storage, in the form of glycogen, provides а short- term energy
reserve.
3.
Carbohydrates supply carbon atoms for the synthesis of other biochemical
substances (proteins, lipids, and nucleic acids).
4.
Carbohydrates form part of the structural framework of DNA and RNA molecules.
5.
Carbohydrate "markers" on cell surfaces play key roles in cell -cell
recognition processes.
CARBOHYDRATE CLASSIFICATIONS
Most simple
carbohydrates have empirical formulas that fit the general formula СnН2nОn.
An early observation by scientists that this general formula can also be
written as Сn(Н2О)n is the basis for the term
carbohydrate - that is, "hydrate of carbon." It is now known that
this hydrate viewpoint is not correct, but the term carbohydrate still persists.
Today the term is used to refer to an entire family of compounds, only some of
which have the formula СnН2nОn.
Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that yield
such substances upon hydrolysis. The carbohydrate glucose is а polyhydroxy
aldehyde, and the carbohydrate fructose is а polyhydroxy ketone.
А striking
structural feature of carbohydrates is the large number of functional groups
present. In glucose and fructose there is а functional group attached to each carbon
atom. Carbohydrates are classified on the basis of molecular size as
monosaccharides, oligosaccharides, and polysaccharides.
Monosaccharides are carbohydrates that contain a single polyhydroxy aldehyde or
polyhydroxy ketone unit. Monosaccharides cannot be broken down into simpler
units by hydrolysis reactions. Both glucose and fructose are monosaccharides.
Naturally occurring monosaccharides have4rom three to seven carbon atoms; five-
and six-carbon species are especially common. Pure monosaccharides are
water-soluble, white, crystalline solids.
Oligosaccharides are
carbohydrates that contain from two to ten monosaccharide units. Disaccharides
are the most common type of oligosaccharide. Disaccharides
are carbohydrates composed of two monosaccharide units covalently bonded to
each other. Like monosaccharides, disaccharides are crystalline, water-soluble
substances. Sucrose (table sugar) and lactose (milk sugar) are disaccharides.
Within the human body, oligosaccharides are often found associated with proteins
and lipids in complexes that have both structural and regulatory functions.
Free oligosaccharides, other than disaccharides, are seldom encountered in
biological systems.
Complete hydrolysis of an oligosaccharide produces monosaccharides. Upon
hydrolysis, а disaccharide produces two monosaccharides, а trisaccharide three
monosaccharides, а hexasaccharide six monosaccharides, and so on.
Polysaccharides are carbohydrates made up of
many monosaccharide units. Polysaccharides, which are polymers, often consist
of tens of thousands of monosaccharide units. Both cellulose and starch are
polysaccharides. We encounter these two substances everywhere. The paper on
which this book is printed is mainly cellulose, as are the cotton in our
clothes and the wood in our houses. Starch is а component of many types of
foods including bread, pasta, potatoes, rice, corn, beans, and peas.
Chirality:
handedness in molecules
Before
considering structures for and reactions of specific carbohydrates, we will
consider handedness, а biologically important structural property exhibited by
most carbohydrates. Most carbohydrate molecules can exist in two forms - а
left-handed form and а right-handed form. Significantly, these different forms
often elicit different responses within the human body.
Mirror Images. The concept
of mirror images is the key to understanding molecular handedness. All objects,
including all molecules, have mirror images. The mirror
image of an object is the object’ reflection in а mirror. For
example: human hands.
Chirality. Objects that cannot be superimposed upon their mirror image are said to be
chiral objects. А chiral object is an
object that is not identical to its mirror image. Your hands and feet are
chiral objects, as are gloves and shoes. Objects that can be superimposed upon
their mirror images are achiral. An achiral
object is identical to its mirror image. Achiral objects include
tube socks, solid-colored ties and Т-shirts.
Molecules,
like larger objects, can be chiral or achiral. А simple example of а chiral
molecule is the trisubstituted methane bromochloroiodomethane.
The simplest example of а chiral carbohydrate is the three-carbon molecule
glyceraldehyde.
Trying to superimpose the mirror image of а molecule on that molecule
visually, is one way to determine molecular chirality. Another method, which is
much easier to apply, makes use of the observation that generally, whenever а
carbon atom in а molecule is bonded to four different groups, the molecule as а
whole is chiral.
Any organic
molecule containing а single carbon atom with four different groups attached to
it exhibits chirality. Such а carbon atom is called а chiral center. А chiral
center is an atom in а molecule that has four different groups
tetrahedrally bonded to it.
Chiral centers
within molecules are often denoted by а small asterisk. Note the chiral centers
in the following molecules.
2-butanol 1-chloto-1-iodoethane 3-methylhexane
Organic molecules, especially carbohydrates, may contain more than one
chiral center. For example, the following carbohydrate has two chiral centers.
Stereoisomers are isomers whose atoms are connected in the same way but differ in their
arrangement in space. The two nonsuperimposable mirror-image forms of а chiral
molecule are stereoisomers.
There are
two major causes of stereoisomerism: (1) the presence of а chiral center in а
molecule, and (2) the presence of "structural rigidity" in а
molecule. Structural rigidity is caused by restricted rotation about chemical
bonds. It is the basis for cis - trans stereoisomerism, а phenomenon found in
some substituted cycloalkanes and some alkenes.
Stereoisomers can be subdivided into two types: enantiomers and
diastereomers. Enantiomers are
stereoisomers whose molecules are nonsuperimposable mirror images of each
other. Left- and right-handed forms of а molecule with а single chiral center
are enantiomers.
Diastereomers are stereoisomers whose
molecules are not mirror images of each other. Cis - trans isomers (of both the
alkene and the cycloalkane types) are diastereomers. We will see additional
examples of carbohydrate diastereomers in the next section. Stereoisomers that
are not enantiomers are diastereomers; they must be one or the other.
Enantiomers Diastereomers
Fischer Projections. Drawing three-dimensional representations of chiral molecules, can be both
time-consuming and awkward. Fischer projections represent а method for giving
molecular chirality specifications in two dimensions. А Fischer projection is а two-dimensional
notation showing the spatial arrangement of groups about chiral centers in
molecules.
In а Fischer
projection, а chiral center is represented as the intersection of vertical and
horizontal lines. The atom at the chiral center, which is almost always carbon,
is not explicitly shown.
The tetrahedral arrangement of the four groups attached to the atom at the
chiral center is governed by the following conventions: (1) Vertical lines from
the chiral center represent bonds to groups directed into the printed page. (2)
Horizontal lines from the chiral center represent bonds to groups directed out
of the printed page.
Fischer
projection
Our
immediate concern is Fischer projections for monosaccharides. Such projections
have the monosaccharide carbon chain positioned vertically with the carbonyl
group (aldehyde or ketone) at or near the top.
The smallest
monosaccharide that has а chiral center is the compound glyceraldehydes
(2,3-dihydroxypropanal). The structural formula and Fischer projections for the
two enantiomers of glyceraldehyde are
D-glyceraldehyde 
L-glyceraldehyde
The handedness (right and left) of these two enantiomers is specified by
using the designations D and L. The enantiomer with the chiral center - ОН
group on the right in the Fischer projection is by definition the right-handed
isomer (в-glyceraldehyde), and the enantiomer with the chiral center - ОН group
on the left in the Fischer projection is by definition the left-handed isomer
(L-glyceraldehyde).
We now consider Fischer projections for the compound
2,3,4-trihydroxybutanal, а monosaccharide with four carbons and two chiral
centers.
There are four stereoisomers for this compound - two pairs of enantiomers.
A B C D
First enantiomeric pair Second enantiomeric pair
In the first enantiomeric pair, both chiral center - ОН groups are on the
same side of the Fischer projection, and in the second enantiomeric pair, the
chiral center - ОН groups are on opposite sides of the Fischer projection.
These are the only - ОН group arrangements possible.
The D, L system used to designate the handedness of glyceraldehyde
enantiomers is extended to monosaccharides with more than one chiral center in
the following manner.
A B C D
D-isomer
L-isomer D-isomer L- isomer
The carbon
chain is numbered, starting at the carbonyl group end of the molecule, and the
highest-numbered chiral center is used to determine D or L configuration.
The D, L nomenclature gives the configuration (handedness) only at the
highest-numbered chiral center. The configuration at other chiral centers in а
molecule is accounted for by assigning а different common name to each pair of
D, L enantiomers. In our present example, compounds А and В (the first
enantiomeric pair) are D-erythrose and L-erythrose; compounds С and D (the
second enantiomeric pair) are D-threose and L-threose.
А and С are
diastereomers, stereoisomers that are not mirror images of each other. Other
diastereomeric pairs in our example are А and D, В and С, and В and D. These
four pairs are epimers. Epimers are
diastereomers that differ only in the configuration at one chiral center.
In general,
а compound that has n chiral centers may exist in а maximum of 2n
stereoisomeric forms. For example, when three chiral centers are present, at
most eight stereoisomers (23 = 8) are possible (four pairs of
enantiomers).
Stereoisomers. 1. Isomers in which the atoms
have the same connectivity but differ in spatial arrangement.
2. Stereoisomerism results either from the presence of а chrial center or
from structural rigidity caused by restricted rotation about chemical bonds.
Enantiomers. 1. Stereoisomers that are
nonsuperimposable mirror images of each other.
2.Handedness (D and L configuration) is determined by the configuration at
the highest-numbered chiral center.
3. Enantiomers rotate plane-polarized light in different directions. (+)
Enantiomers are dextrorotatory (clockwise), and (-) enantiomers are
levorotatory (counterclockwise).
Diastereomers. 1. Stereoisomers that are not
mirror images of each other.
2. Epimers are diastereomers whose configurations differ only at one chiral
center.
Properties of Enantiomers. Structural isomers differ in most chemical and physical properties. For
example, structural isomers have different boiling points and melting points.
Diastereomers also differ in most chemical and physical properties. They also
have different boiling points and freezing points. In contrast, nearly all the
properties of а pair of enantiomers are the same; for example, they have
identical boiling points and freezing points. Enantiomers exhibit property
differences in only two areas: their interaction with plane-polarized light and
their interaction with other chiral substances.
•
An
enantiomer that rotates plane-polarized light to the right is said to be
dextrorotatory (the Latin dexter means "right"). An enantiomer that
rotates plane-polarized light to the left is said to be levorotatory (the Latin
laevus means "left").
•
А plus or
minus sign inside parentheses is used to denote the direction of rotation of
plane-polarized light by а chiral compound. The notation (+) means rotation to
the right (clockwise), and (-) means rotation to the left (counterclockwise).
Thus the dextrorotstory enantiomer of glucose is (+)-glucose.
•
An equimolar
mixture of two enantiomers is called а racemic mixture, or а racemate.
Since а racemic mixture contains equal numbers of dextrorotating and
levorotating molecules, the net optical rotation is zero. А racemic mixture is
often specified by prefixing the name of the compound with the symbol (± )
Interactions between chiral
compounds. А left-handed baseball player (chiral) and а right-handed baseball player
(chiral) can use the same baseball bat (achiral) or wear the same baseball hat
(achiral). However, left- and right-handed baseball players (chiral) cannot use
the same baseball glove (chiral). This nonchemical example illustrates that the
chirality of an object becomes important when the object interacts with another
chiral object.
Applying this generalization to molecules, we find that the two members of
an enantiomeric pair, because of their differing chirality, interact
differently with other chiral molecules. We find that:
1. А pair of enantiomers has the same solubility in an achiral solvent,
such as ethanol, but differing solubilities in а chiral solvent, such as
в-2-butanol.
2. The rate and extent of reaction of enantiomers with another reactant are
the same if the reactant is achiral but differ if the reactant is chiral. The
different reactions that different enantiomers undergo are further considered
in the paragraph that follows.
3. Enantiomers have identical boiling points, freezing points, and
densities, because such properties depend on the strength of intermolecular
forces, and intermolecular force strength does not depend on chirality.
Intermolecular force strength is the same for both forms of а chiral molecule,
because both forms have identical sets of functional groups.
The two enantiomeric forms of а chiral molecule often generate different
responses from the human body. Sometimes both enantiomers are biologically
active, each form giving a different response; sometimes both give the same
response, but one isomer’s response is many times greater than that of the
other; and sometimes only one of the two forms is biologically active, the
other form giving no response. For example, the body’s response to the D isomer
of the hormone epinephrine is 20 times greater than its response to the L
isomer. Epinephrine exerts its effect by binding to specialized receptors. It
binds to the receptor site by means of а three-point contact, D-epinephrine
makes а perfect three-point contact with the receptor surface, but the
biologically weaker L-epinephrine can make only а two-point contact. Because of
the poorer fit, the binding of the isomer is weaker, and less physiological
response is observed.
Classification of Monosaccharides. Now that we have considered
molecular chirality and its consequences, we return to the subject of
carbohydrates by considering further details about monosaccharides, the
simplest carbohydrates.
Although there is no limit to the number of carbon atoms that can be
present in а monosaccharide, only monosaccharides with three to seven carbon
atoms are commonly found in nature. А three-carbon monosaccharide is called а
triose, and those that contain four, five, and six carbon atoms are called
tetroses, pentoses, and hexoses, respectively.
Monosaccharides
are classified as aldoses or ketoses on the basis of type of carbonyl group
present. Aldoses are monosaccharides that contain an aldehyde group. Ketoses
are monosaccharides that contain а ketone group.
Monosaccharides
are often classified by both their number of carbon atoms and their functional
group. А six-carbon monosaccharide with an aldehyde functional group is an
aldohexose; а five-carbon monosaccharide with а ketone functional group is а
ketopentose.
Monosaccharides
are also often called sugars. Hexoses are six-carbon sugars, pentoses
five-carbon sugars, and so on. The word sugar is associated with
"sweetness," and most (but not all) monosaccharides have а sweet
taste. The designation sugar is also applied to disaccharides, many of which
also have а sweet taste. Thus sugar is а general designation for either а
monosaccharide or а disaccharide.
The simplest aldose and ketose are the trioses
glyceraldehyde and dihydroxyacetone.
glyceraldehydes dihydroxycaetone
The D and L designations specify the configuration at the highest-numbered
chiral center in а monosaccharide. The configurations about other chiral
centers are accounted for by assigning а different common name to each pair of
D and L enantiomers. This naming system, for simple aldoses, is given
The L forms
are mirror images of the molecules shown.
А major difference between glyceraldehyde and dihydroxyacetone is that the
latter does not possess а chiral carbon atom. Thus, D and L forms are not
possible for dihydroxy acetone. This reduces by half (compared with aldoses)
the number of stereoisomers possible for ketotetroses, ketopentoses, and
ketohexoses. An aldohexose has four chiral carbon atoms, but а ketohexose has
only three. atoins.
Cyclic forms of monosaccharides. So far in this chapter, the structures of monosaccharides have been
depicted as open-chain polyhydroxy aldehydes or ketones. However, experimental
evidence indicates that for monosaccharides containing five or more carbon
atoms, such open-chain structures are actually in equilibrium with two cyclic
structures, and the cyclic structures are the dominant forms at equilibrium.
The cyclic forms of monosaccharides result from the ability of their
carbonyl group to react intramolecularly with а hydroxyl group. The result is а
cyclic hemiacetal or cyclic hemiketal. Such an intramolecular cyclization
reaction for D-glucose is shown:
Structure 2 is а rearrangement of the projection formula for D-glucose in
which the carbon atoms have locations similar to those found for carbon atoms
in а six-membered ring. All hydroxyl groups drawn to the right in the original
projection formula appear below the ring. Those to the left in the projection
formula appear above the ring.
Structure 3 is obtained by rotating the groups attached to carbon-5 in а
counterclockwise direction so that they are in the positions where it is
easiest to visualize intramolecular hemiacetal formation. The intramolecular
reaction occurs between the hydroxyl group on carbon-5 and the carbonyl group
(carbon-1). The - ОН group adds across the carbon - oxygen double bond,
producing а heterocyclic ring that contains five carbon atoms and one oxygen
atom.
Addition
across the carbon - oxygen double bond with its accompanying ring formation
produces а chiral center at carbon-l, so two stereoisomers are possible. These
two forms differ in the orientation of the - ОН group on the hemiacetal carbon
atom (carbon-1). In a-D-glucose, the - ОН group is on the opposite side of the ring from the CH2OH
group attached to carbon-5. In b-D-glucose, the СН2ОН
group on carbon-5 and the - ОН group on carbon-1 are on the same side of the
ring.
In an
aqueous solution of в-glucose, а dynamic equilibrium exists among the a, b, and open-chain forms, and there is continual interconversion among them.
For example, a freshly mixed solution of pure a-D-glucose slowly converts to а mixture of both a- and b-D-glucose by an opening and а closing of the cyclic structure. When
equilibrium is established, 63 % of the molecules are b-D-glucose, 37 % are a-D-glucose, and less than 0.01
% are in the open-chain form.
Intramolecular
cyclic hemiacetal formation and the equilibrium between forms associated with
it is not restricted to glucose. All aldoses with five or more carbon atoms
establish similar equilibria, but with different percentages of the alpha,
beta, and open-chain forms. Fructose and other ketoses with а sufficient number
of carbon atoms also cyclize; here, cyclic hemiketal formation occurs.
Galactose, like glucose, forms а six-membered ring, but both D-fructose and
D-ribose form а five-membered ring.
D-fructose
D-ribose
D-Fructose cyclization involves carbon-2 (the keto group) and carbon-5,
which results in two CH2OH groups being outside the ring (carbons 1
and 6). D-Ribose cyclization involves carbon-1 (the aldehyde group) and
carbon-4.
In а
The D or L
form of а monosaccharide is determined by the position of the terminal СН2ОН
group on the highest-numbered ring carbon atom. In the в form, this group is
positioned above the ring. In the ь form, which is not usually encountered in
biological systems, the terminal CH2OH group is positioned below the
ring.
a or b configuration is determined by the position of the -ОН group on carbon-1
relative to the CH2OH group that determines D or L series. In а b configuration, both of these groups point in the same direction; in an a configuration, the two groups point in opposite directions.
b-D-Monosaccharide a-D-Monosaccharide b-L-Monosaccharide
Where a or b configuration does not matter, the -ОН group on carbon-1 is placed in a
horizontal position, and а wavy line is used as the bond that connects it to
the ring.
The specific
identity of а monosaccharide is determined by the positioning of the other: -
ОН groups in the
b-form a-form
Reactions of monosaccharides. Five important reactions of
monosaccharides are oxidation, reduction, glycoside formation, phosphate ester
formation, and amino sugar formation. In considering these reactions, we will
use glucose as the monosaccharide reactant. Remember, however, that other
aldoses as well as ketoses undergo similar reactions.
Oxidation. Monosaccharide oxidation can
yield three different types of oxidation products. The oxidizing agent used
determines the product.
Weak oxidizing agents, such as Tollens, Fehling's, and Benedict's
solutions, oxidize the carbonyl group end of а monosaccharide to give an -onic
acid. Oxidation of the aldehyde end of glucose produces gluconic acid,
and oxidation of the aldehyde end of galactose produces galactonic acid. The
structures involved in the glucose reaction are
Because
monosaccharides act as reducing agents in such reactions, they are called
reducing sugars. With Tollens solution, glucose reduces Ag+ ion to
Ag, and with Benedict's and Fehling's solutions, glucose reduces Cu2+
ion to Cu+ ion. А reducing sugar is a carbohydrate that gives
a positive test with Tollens, Benedict's and Fehling's solutions. All
monosaccharides are reducing sugars.
Tollens, Fehling's, and Benedict's solutions can be used to test for
glucose in urine, а symptom of diabetes. For example, using Benedict's
solution, we observe that if no glucose is present in the urine (а normal
condition), the Benedict's solution remains blue.
The presence of glucose is indicated by the formation of а red precipitate.
Testing for the presence of glucose in urine is such а common laboratory
procedure that much effort has been put into the development of easy-to-use
test methods.
Strong oxidizing agents can oxidize both ends of а monosaccharide at the
same time (the carbonyl group and the terminal primary alcohol group) to
produce а dicarboxylic acid. Such polyhydroxy dicarboxylic acids are known as -aric
acids. For glucose, such an oxidation produces glucaric acid.
Although it is difficult to do in the laboratory, in biological systems
enzymes can oxidize the primary alcohol end of an aldose such as glucose,
without oxidation of the aldehyde group, to produce а -uronic acid.
For glucose, such an oxidation produces D-glucuronic acid.
D-Glucose 
D-Glucuronic
acid
Reduction. The carbonyl group present in
а monosaccharide (either an aldose or а ketose) can be reduced to а hydroxyl
group, using hydrogen as the reducing agent. For aldoses and ketoses, the
product of the reduction is the corresponding polyhydroxy alcohol, which is
sometimes called а sugar alcohol. For example, the reduction D-glucose gives
D-glucitol.
D-Glucitol
is also known by the common name D-sorbitol. Hexahydroxy alcohols such as
D-sorbitol have properties similar to those of the trihydroxy alcohol glycerol.
These alcohols are used as moisturizing agents in foods and cosmetics because
of their affinity for water. D-Sorbitol is also used as а sweetening agent in
chewing gum; bacteria that cause tooth decay cannot use polyalcohols as food
sources, as they can glucose and many other monosaccharides.
Mutarotation. a- and b-forms of monosaccharides are readily interconverted when dissolved in
water. This spontaneous process, called mutarotation,
results in an equilibrium mixture of a- and b-furanose forms and a- and b-pyranose forms. The open chain that is formed during muterotation can
participate in oxidation-reduction reactions.
O-Glycoside Formation. As you
know, that hemiacetals and hemiketals can react with alcohols in acid solution
to produce acetals and ketals. Because the cyclic forms of monosaccharides are
hemiacetals and hemiketals, they react with alcohols to form acetals and
ketals, as is illustrated for the reaction of b-D-glucose with methyl alcohol.
b-D-glucose
Methyl-b-D-glucoside and Methyl-a-D-glucoside
The general
name for monosaccharide acetals and ketals is glycoside. А glycoside is
an acetal or а ketal forpined pот а cyclic monosaccharide. More specifically, а
glycoside produced from glucose is а glucoside, from galactose а galactoside,
and so on. Glycosides, like the hemiacetals and hemiketals from which they are
formed, can exist in both a and b forms. Glycosides are named by listing the alkyl or aryl group attached to
the oxygen, followed by the name of the monosaccharide involved, with the
suffix –ide appended to it.
Methyl-a-D-glucoside
Methyl-b-D-glucoside
N-Glycoside.
Phosphate ester formation. The hydroxyl groups of а
monosaccharide can react with inorganic oxyacids to form inorganic esters.
Phosphate esters, formed from phosphoric acid and various monosaccharides, are
commonly encountered in biological systems. For example, specific enzymes in the
human body catalyze the esterification of the carbonyl group (carbon-1) and the
primary alcohol group (carbon-6) in glucose to produce the compounds glucose
1-phosphate and glucose б-phosphate, respectively.
a-D-Glucose-1-phosphate a-D-Glucose-6-phosphate
These phosphate esters of glucose are stable in aqueous solution and play
important roles in the metabolism of carbohydrates.
Acylation of
Monosaccharides:
Alkylation of
Monosaccharides:
Amino Sugar Formation. Amino
sugars of glucose, mannose, and galactose are common in nature. Such sugars are
produced by replacing the hydroxyl group on carbon-2 on the monosaccharide with
an amino group. Amino sugars and their N-acetyl derivatives are important
building blocks of polysaccharides found in cartilage.
a-D-Glucosamine a-D-Glalactosamine N-acety1a-D-glucosanune
The N-acetyl derivatives of D-glucosamine and D-galactosamine are present
in the biochemical markers on red blood cells, which distinguish the various
blood types.
Isomerization. Monosaccharides
undergo several types of isomerization, for example, after several hours an
alkaline solution of D-glucose will also contain D- mannose and D- fructose.
Both isomerizations involve an intramolecular shift of a hydrogen atom and a
charge in the location of a double bond.
The intermediate that is
formed during this process is called an enediol.
The reversible transformation of glucose to fructose is an example of an
aldose-ketose interconversion. Because there is a change in the conversion of
glucose to mannose is referred to as an epimerization. Several enzyme-catalyzed
reactions involving enediols occur in carbohydrate metabolism.
Biologically important monosaccharides. Of the many monosaccharides,
the most important in the human body are the D-forms of glucose, galactose,
fructose, and ribose. Glucose and galactose are aldohexoses, fructose is а
ketohexose, and ribose is an aldopentose. All four of these monosaccharides are
water-soluble, white, crystalline solids.
D-Glucose. Of all monosaccharides,
о-glucose is the most abundant in nature and the most important from а
nutritional standpoint. Its Fischer projection is
D-Glucose
Ripe fruits,
particularly ripe grapes (20% - 30% glucose by mass), are а good source of
glucose, which is often referred to as grape sugar. Two other names for
о-glucose are dextrose and blood sugar. The name dextrose draws attention to
the fact that the optically active D-glucose, in aqueous solution, rotates
plane-polarized light to the right. The term blood sugar draws attention to the
fact that blood contains dissolved glucose. The concentration of glucose in
human blood is fairly constant; it is in the range of 70 - 100 mg per 100 mL of
blood. Cells use this glucose as а primary energy source.
А 5% (m/v)
glucose solution is often used in hospitals as an intravenous source of
nourishment for patients who cannot take food by mouth. The body can use it as
an energy source without digesting it.
Foods high in
carbohydrate include fruits, sweets, soft drinks, breads, pastas, beans,
potatoes, bran, rice, and cereals. Carbohydrates are a common source of energy
in living organisms; however, no carbohydrate is an essential nutrient in humans.
Carbohydrates
are not necessary building blocks of other molecules, and the body can obtain
all its energy from protein and fats.[ The brain and neurons generally
cannot burn fat for energy, but use glucose or ketones.
Humans can synthesize some glucose (in a set of processes known as gluconeogenesis)
from specific amino acids, from the glycerol backbone
in triglycerides and
in some cases from fatty acids. Carbohydrate and protein contain 4 calories
per gram, while
fats contain 9 calories per gram. In the case of protein, this is somewhat
misleading as only some amino acids are usable for fuel.
Organisms
typically cannot metabolize all types of carbohydrate to yield energy. Glucose
is a nearly universal and accessible source of calories. Many
organisms also have the ability to metabolize other monosaccharides and Disaccharides,
though glucose is preferred. In Escherichia coli,
for example, the lac operon will express enzymes for the digestion of
lactose when it is present, but if both lactose and glucose are present
the lac operon is repressed, resulting in the glucose being used
first. Polysaccharides are also common sources of
energy. Many organisms can easily break down starches into glucose, however,
most organisms cannot metabolize cellulose or other polysaccharides like chitin and arabinoxylans.
These carbohydrates types can be metabolized by some bacteria and
protists. Ruminants and termites, for
example, use microorganisms to processcellulose
Even though these complex
carbohydrates are not very digestible, they represent an important dietary
element for humans, called dietary fiber. Fiber enhances digestion, among other
benefits.
The biological significance of carbohydrates in living
organisms
Two common Monosaccharides, (single sugars) Glucose and
Fructose
Sugars are most often found in the form of a
"RING". The glucose molecule in the image above and the one in the
image below (Glc) are really the same molecule, just arranged differently. The
corners of the "stop sign" represent Carbon atoms even thought they
are not labeled with a "C"
(its chemistry shorthand). To form these rings, the Carbonyl (C=0) Carbon of the
straight-chain form (above) forms a bond with the next to last Carbon in the
chain, making the ring.
![Описание: Pastas and whole-grain breads contain complex carbohydrates, which are long strands of glucose molecules. Nutritionists recommend that 55–60 percent of calories come from carbohydrates, and especially complex carbohydrates. [Photograph by James Noble. Corbis. Reproduced by permission.]](04.%20Corbohydrates.%20mono-,%20di-,%20polysaccharides.files/image114.jpg)
Functions of Carbohydrate
Carbohydrate
functions as Bio Fuel
· Polysaccharides such as starch
and glycogen are first hydrolyzed by enzymes to Glucose.
· Glucose is then oxidized to produce carbon dioxide and water.
· Energy is released in this
process which is used for functioning of the cells.
Carbohydrate
functions as Primary Source of Energy
Carbohydrate
functions as storage food
Different forms of
Carbohydrate are stored in living organism as storage food.
· Polysaccharide starch acts as storage food for plants.
· Glycogen stored in liver and muscles acts as storage food for animals.
· Inulin acts as storage food of
dahlias, onion and garlic.
Thus carbohydrate
performs the function of storing food.
Carbohydrate
functions as framework in body
Different
Carbohydrates especially Polysaccharides act as framework in living organism.
· Cellulose forms cell wall of plant cell along with hemicelluloses and
Pectin
· Chitin forms cell wall of fungal cell and exoskeleton of
arthropods
· Peptidoglycan forms cell wall of bacteria and cyanobacteria.
Thus carbohydrates
function as contributing material to the cellular structure.
Carbohydrate
functions as Anticoagulant
Carbohydrate
functions as Antigen
Carbohydrate
functions as Hormone
Carbohydrates
provide raw material for industry
D-Galactose. А comparison of the Fischer projections for D-galactose and D-glucose
shows that these two compounds differ only in the configuration of the - ОН
group and - Н group on carbon-4.]
D-Galactose D-Glucose
D-Galactose
and D-glucose are epimers.
D-Galactose is seldom encountered as а free monosaccharide. It is, however,
а component of numerous important biochemical substances. In the human body,
galactose is synthesized from glucose in the mammary glands for use in lactose
(milk sugar), а disaccharide consisting of а glucose unit and а galactose unit.
D-Galactose is sometimes called brain sugar because it is а component of
glycoproteins (protein-carbohydrate compounds) found in brain and nerve tissue.
D-Galactose is also present in the chemical markers that distinguish various
types of blood - А, В, АВ, and O.
D-Fructose is the most important ketohexose. It is also known as levulose and fruit
sugar. Aqueous solutions of naturally occurring D-fructose rotate
plane-polarized light to the left hence the name levulose. The sweetest-tasting
of all sugars, D-fructose is found in many fruits and is present in honey in
equal amounts with glucose. It is sometimes used as a dietary sugar, not
because it has fewer calories per gram than other sugars but because less is
needed for the same amount of sweetness.
From the
third to the sixth carbon, the structure of D-fructose is identical to that of
D-glucose. Differences at carbons 1 and 2 are related to the presence of а
ketone group in fructose and an aldehyde group in glucose.
D-Fructose D-Glucose
D-Ribose. The three monosaccharides previously discussed in this section have all
been hexoses. D-Ribose is а pentose. If carbon-3 and its accompanying - Н and -
ОН groups were eliminated from the structure of D-glucose, the remaining
structure would be that of D-ribose.
D-Glucose 
D-Ribose
D-Ribose is
а component of а variety of complex molecules, including ribonucleic acids
(RNAs) and energy-rich compounds such as adenosine triphosphate (ATP). The
compound 2-deoxy-D-ribose is also important in nucleic acid chemistry.
This monosaccharide is а component of DNA molecules. The prefix deoxy- means
"minus an oxygen"; the structures of ribose and 2-deoxyribose differ
in that the latter compound lacks an oxygen atom at carbon-2.
D-Ribose 
2-Deoxy-D-ribose
OLIGOSACCHARIDES
The term “oligosaccharide” is often used for carbohydrates that consist of
between two and ten monosaccharide units. Oligosaccharides
are carbohydrates that contain from two to ten monosaccharide units.
Disaccharides are the most common type of oligosaccharide. Disaccharides are carbohydrates composed of
two monosaccharide units covalently bonded to each other. Like monosaccharides,
disaccharides are crystalline, water-soluble substances. Sucrose (table sugar)
and lactose (milk sugar) are disaccharides.
Within the
human body, oligosaccharides are often found associated with proteins and
lipids in complexes that have both structural and regulatory functions. Free
oligosaccharides, other than disaccharides, are seldom encountered in biological
systems.
Complete
hydrolysis of an oligosaccharide produces monosaccharides. Upon hydrolysis, а
disaccharide produces two monosaccharides, а trisaccharide three
monosaccharides, а hexasaccharide six monosaccharides, and so on.
Carbohydrates
are the most abundant class of bioorganic molecules on planet Earth. Although
their abundance in the human body is relatively low, carbohydrates constitute
about 75% by mass of dry plant materials.
Green
(chlorophyll-containing) plants produce carbohydrates via photosynthesis. In
this process, carbon dioxide from the air and water from the soil are the
reactants, and sunlight absorbed by chlorophyll is the energy source.
Plants have
two main uses for the carbohydrates they produce. In the form of cellulose,
carbohydrates serve as structural elements, and in the form of starch, they
provide energy reserves for the plants.
Dietary intake of plant materials is the major carbohydrate source for
humans and animals. The average human diet should ideally be about two-thirds
carbohydrate by mass.
Carbohydrates have the following functions in humans:
1. Carbohydrate oxidation provides energy.
2. Carbohydrate storage, in the form of glycogen, provides а short- term
energy reserve.
3. Carbohydrates supply carbon atoms for the synthesis of other biochemical
substances (proteins, lipids, and nucleic acids).
4. Carbohydrates form part of the structural framework of DNA and RNA
molecules.
5. Carbohydrate "markers" on cell surfaces play key roles in cell
-cell recognition processes.
As mentioned earlier, disaccharides are those sugars which on hydrolysis
give two moles of monosaccharides general these are sweet-testing crystalline,
water-soluble substances, easily hydrolysed by enzymes and dilute mineral
acids. The common disaccharides have the general formula C12H22O11
which during hydrolysis take un one molecule of water to form two hexoses.
Disaccharides are formed by
intermolecular dehydration between two monosaccharide molecules, e.g.
In the formation of
disaccharides, at least one monosaccharide unit is linked to the other through
the glycosidic carbon. In other words we can say that in the formation of
disaccharide, reducing property of at least one hexose unit is lost. Hence
disaccharides may be considered .as glycosides in which both components of the
molecules are sugars. Disaccharides may be of two types, namely non-reducing
and reducing depending on the fact that С1 of one hexose is linked
to the carbonyl carbon at in or any other carbon atom of other hexose.
Weak oxidizing agents, such as
Tollens, Fehling's, and Benedict's solutions, oxidize the carbonyl group end of
а monosaccharide to give an -onic
acid.
(1) Nоn-reducing disaccharides. In these disaccharides the
two hexose units are linked together through their reducing (i е. aldehydic or
ketonic) groups which is С, in aldoses and С, in ketoses. Now in such cases
since the reducing groups of both the hexoses are lost, the resulting compound
(disaccharide) will be non-reducing. Hence such disaccharides do not form
osazone do not show mutarotation and do not react with reagents like Fehling
solution, Tollen’s reagent, etc. Important example of non-reducing disaccharides
is sucrose.
(2) Reducing disaccharides. In these
disaccharides, one hexose unit is linked through its reducing carbon to the
non-reducing carbon (C4 or С6) of the other Now since the
reducing group of one of the hexoses is not involved, the resulting
disaccharide will be а reducing sugar. Maltose and lactose are examples of
reducing disaccharides
Disaccharides. А monosaccharide that has
cyclic forms (hemiacetal or hemiketal) can react with an alcoho1 to form а
glycoside (acetal or ketal). This same type of reaction can be used to produce
а disaccharide, а carbohydrate in which two monosaccharides are bonded
together. In disaccharide formation, one of the monosaccharide reactants
functions as а hemiacetal or hemiketal, and the other functions as an alcohol.
Monosaccharide + monosaccharide = disaccharide + Н2O
Glycosidic linkage
The bond that links the two monosaccharides of а disaccharide together is called
а glycosidic linkage. А glycosidic linkage is the carbon-oxygen-carbon bond
that joins the two components of а glycoside together.
We now examine the structures and properties of four important
disaccharides: maltose, cellobiose, lactose, and sucrose. As we consider
details of the structures of these compounds, we will find that the
configuration (а or p) at carbon-1 of the reacting monosaccharides is often of
prime importance.
Maltose, often
called malt sugar, is produced whenever the polysaccharide starch breaks down,
as happens in plants when seeds germinate and in human beings during starch
digestion. It is а common ingredient in baby foods and is found in malted milk.
Malt (germinated barley that has been baked and ground) contains maltose; hence
the name malt sugar.
Structurally, maltose is made up of two D-glucose units, one of which must
be a-D-glucose. The formation of maltose from two glucose molecules is as
follows:
a-D-Glucose a-D-Glucose
a-(1-4)-linkage
The
glycosidic linkage between the two glucose units is called an a(1 - 4) linkage. The two -ОН groups that form the linkage are attached,
respectively, to carbon-1 of the first glucose unit (in a configuration) and to
carbon-4 of the second.
Maltose is а
reducing sugar, because the glucose unit on the right has а hemiacetal carbon
atom (С-1). Thus this glucose unit can open and close; it is in equilibrium
with its open-chain aldehyde form. This means there are actually three forms of
the maltose molecule: a-maltose, b-maltose, and the open-chain form. In the solid state, the b-form is dominant.
The most
important chemical reaction of maltose is that of hydrolysis. Hydrolysis of
D-maltose, whether in а laboratory flask or in а living organism, produces two
molecules of D-glucose.
Cellobiose is produced as an intermediate in the hydrolysis of the polysaccharide
cellulose. Like maltose, cellobiose contains two D-glucose monosaccharide
units. It differs from maltose in that one of D-glucose units - the one
functioning as а hemiacetal - must have а b configuration instead of the а configuration for maltose. This change in
configuration results in а b(1 - 4) glycosidic linkage.
b-D-Glucose
b(1 - 4)-linkage
Like
maltose, cellobiose is a reducing sugar, has three isomeric forms in aqueous
solution, and upon hydrolysis produces two D-glucose molecules.
Despite
these similarities, maltose and cellobiose have different biological behaviors.
These differences are related to the stereochemistry of their glycosidic linkages.
Maltase, the enzyme that breaks the glucose-glucose a(1 - 4) linkage present in maltose, is found both in the human body and in
yeast. Consequently, maltose is digested easily by humans and is readily
fermented by yeast. Both the human body and yeast lack the enzyme cellobiase
needed to break the glucose - glucose b(1 - 4) linkage of cellobiose.
Thus cellobiose cannot be digested by humans or fermented by yeast.
In maltose
and cellobiose, the two units of the disaccharide are identical - two glucose
units in each case. However, the two monosaccharide units in а disaccharide
need not be identical.
Lactose is made up
of а b-D-galactose unit and а D-glucose unit joined by b-(1 - 4) glycosidic linkage.
b-D-galactose a-D-Glucose
b(1 - 4)-linkage
The glucose hemiacetal center is unaffected when galactose bonds to glucose
in the formation of lactose, so lactose is а reducing sugar (the glucose ring
can open to give an aldehyde).
Lactose is
the major sugar found in milk. This accounts for its common name, milk sugar.
Enzymes in mammalian mammary glands take glucose from the bloodstream and
synthesize lactose in а four-step process. Epimerization of glucose yields
galactose, and then the b(1 - 4) linkage forms between
а galactose and а glucose unit. Lactose is an important ingredient in
commercially produced infant formulas that are designed to simulate mother' s
milk. Souring of milk is caused by the conversion of lactose to lactic acid by
bacteria in the milk. Pasteurization of milk is а quick-heating process that
kills most of the bacteria and retards the souring process.
Lactose can
be hydrolyzed by acid or by the enzyme lactase, forming an equimolar mixture of
galactose and glucose.
In the human
body, the galactose so produced is then converted to glucose by other enzymes.
The genetic condition lactose intolerance, an inability of the human digestive
system to hydrolyze lactose.
Sucrose, common
table sugar, is the most abundant of all disaccharides and occurs throughout
the plant kingdom. It is produced commercially from the juice of sugar cane and
sugar beets. Sugar cane contains up to 20 % by mass sucrose, and sugar beets
contain up to 17 % by mass sucrose.
The two monosaccharide units present in a-D-sucrose molecule are a-D-glucose and b-D-fructose. The glycosidic linkage is not а (1 - 4) linkage, as was the
case for maltose, cellobiose, and lactose. It is instead an a,b(1 - 2) glycosidic linkage. The - ОН group on carbon-2 of D-fructose (the
hemiketal carbon) reacts with the - ОН group on carbon-l of D-glucose (the
hemiacetal carbon).
Sucrose, unlike maltose, cellobiose, and lactose, is а nonreducing sugar.
No helmiacetal or hemiketal center is present in the molecule, because the
glycosidic linkage involves the reducing ends of both monosaccharides. Sucrose,
in the solid state and in solution, exists in only one form - there are no a and b isomers, and an open-chain form is not possible.
Sucrase, the enzyme needed to break the a,b(1 - 2) linkage in sucrose, is present in the human body. Hence sucrose is
an easily digested substance. Sucrose hydrolysis (digestion) produces an
equimolar mixture of glucose and fructose called invert sugar.
When sucrose is cooked with acid-containing foods such as fruits or
berries, partial hydrolysis takes place, forming some invert sugar. Jams and
jellies prepared in this manner are actually sweeter than the pure sucrose
added to the original mixture, because one-to-one mixtures of glucose and
fructose taste sweeter than sucrose.
Sucrose is dextrorotatcry. On hydrolysis it gives one
molecule of glucose and one molecule of fructose. Now since fructose is more
strongly laevorotatory than the dextrorotatory property of glucose, the mixture
(product) after hydrolysis will be laevorotatory.
dextrorotatcry laevorotatory
This reaction is also as inversion
of sugar because the dextrorotatory case sugar is converted into
laevorotatory product due to hydrolysis. The mixture of glucose and fructose is
called invert sugar.
Raffinose is a trisaccharide
composed of galactose,
fructose,
and glucose.
It can be found in beans,
cabbage,
brussels
sprouts, broccoli, asparagus,
other vegetables,
and whole grains.
Raffinose can be hydrolyzed to D-galactose
and sucrose
by the enzyme α-galactosidase, an
enzyme not found in the human digestive tract. α-galactosidase also
hydrolyzes other α-galactosides
such as stachyose,
verbascose,
and galactinol,
if present. The enzyme does not cleave β-linked galactose, as in lactose.
Using:
Legume seeds (peas, beans,
lentils) contain 5 to 15 % raffinose in their dry weight. During the production
of beet sugar, major amounts of raffinose accumulate in the molasses, which can
be used to produce some kinds of brown sugars. Technically, raffinose can be
used as a antifreezing agent (freezing medical preparates, cryopreservation).
POLYSACCHARIDE
А polysaccharide (glucans) contains many monosaccharide units bonded to each
other by glycosidic linkages. The number of monosaccharide units varies with
the polysaccharide from а few hundred to hundreds of thousands. Polysaccharides
are polymers. In some, the monosaccharides are bonded together in а linear
(unbranched) chain. In others, there is extensive branching of the chains.
Unlike monosaccharides and most disaccharides, polysaccharides are not
sweet and do not test positive in Tollens, Benedict’s, and Fehling’s solutions.
They have limited water solubility because of their size. However, the - ОН
groups present can individually become hydrated by water molecules. The result
is usually а thick colloidal suspension of the polysaccharide in water. Polysaccharides,
such as flour and cornstarch, are often used as thickening agents in sauces,
desserts, and gravy.
The
following table lists the biologically important polysaccharides and their
functions.
|
Name of the Polysaccharide |
Composition |
Occurrence |
Functions |
|
1. Starch |
Polymer of glucose containing a straight chain of
glucose molecules (amylose) and a branched chain of glucose molecules
(emylopectin) |
In several plant species as main storage
carbohydrate |
Storage of reserve food |
|
2. Glycogen |
Polymer of glucose |
Animals (equivalent of starch) |
Storage of reserve food |
|
3. Callose |
Polymer of glucose |
Different regions of a plant, In the sieve tubes of
phloem |
Formed often as a response to wounds |
|
4. Insulin |
Polymer of fructose |
In roots and tubers (like Dahlia) |
Storage of reserve food |
|
5. Cellulose |
Polymer of glucose |
Plant cell wall (most abundant organic molecule on
the_Earth) |
Cellwall matrix |
|
6. Pectin |
Polymer of galactose and its derivatives |
Plant cellwall |
Cellwall matrix |
|
7. Hemicellulose |
Polymer of pentoses and sugar acids |
Plant cellwall |
Cellwall matrix |
|
8. lignin |
Polymer of glucose |
Plant cellwall (dead cells like sclerenchyma) |
Cellwall matrix |
|
9. Chitin |
Polymer of glucose |
Bodywall of arthropods. In some fungi also |
Exoskeleton Impermeable to water |
|
10. Murein |
Polysaccharide cross linked with amino acids |
Cell wall of prokaryotic cells |
Structural, protection |
|
11. Hyaluronic acid |
Polymer of sugar acids |
Connective tissue matrix. Outer coat of mammalian
eggs |
Ground substance, protection |
|
12. Chondroitin sulphate |
Polymer of sugar acids |
Connective tissue matrix |
Ground substance |
|
13. Heparin |
Closely related to chondroitin |
Connective tissue cells |
Anticoagulant |
|
14. Gums and mucilages |
Polymers of sugars and sugar acids |
Gums - barks of trees. Mucilages-flower |
Retain water in dry seasons |
Biomolecular
chemistry is a major category within organic chemistry which is frequently
studied by biochemists. Many complex multi-functional group molecules are
important in living organisms. Some are long-chain biopolymers, and these
include peptides, DNA, RNA and the polysaccharides such as starches in animals
and celluloses in plants. The other main classes are amino acids (monomer
building blocks of peptides and proteins), carbohydrates (which includes the
polysaccharides), the nucleic acids (which include DNA and RNA as polymers),
and the lipids. In addition, animal biochemistry contains many small molecule
intermediates which assist in energy production through the Krebs cycle, and
produces isoprene, the most common hydrocarbon in animals. Isoprenes in animals
form the important steroid structural (cholesterol) and steroid hormone
compounds; and in plants form terpenes, terpenoids, some alkaloids, and a class
of hydrocarbons called biopolymer polyisoprenoids present in the latex of
various species of plants, which is the basis for making rubber.
Biopolymers
are polymers produced by living organisms. Since they are polymers, biopolymers
contain monomeric units that are covalently bonded to form larger structures.
There are three main classes of biopolymers, classified according to the
monomeric units used and the structure of the biopolymer formed:
polynucleotides (RNA and DNA), which are long polymers composed of 13 or more
nucleotide monomers; polypeptides, which are short polymers of amino acids; and
polysaccharides, which are often linear bonded polymeric carbohydrate
structures.
Cellulose is the most common
organic compound and biopolymer on Earth. About 33 percent of all plant matter
is cellulose. The cellulose content of cotton is 90 percent, while wood's is 50
percent.
Linear and branched
structure of polysaccharides
Unlike monosaccharides and most disaccharides,
polysaccharides are not sweet and do not test positive in Tollens, Benedict’s,
and Fehling’s solutions. They have limited water solubility because of their
size. However, the - ОН groups present can individually become hydrated by
water molecules. The result is usually а thick colloidal suspension of the
polysaccharide in water. Polysaccharides, such as flour and cornstarch, are
often used as thickening agents in sauces, desserts, and gravy.
Although there are many naturally occurring
polysaccharides, in this section we will focus on only four of them: cellulose,
starch, glycogen, and chitin. All play vital roles in living systems -
cellulose and starch in plants, glycogen in humans and other animals, and
chitin in arthropods.
Polysaccharides
may be divided into two classes: homopolysaccharides, which are composed of one
type of monosaccharide units, and heteropolysaccharides, which contain two or
more different types of monosaccharide units.
Starch, glycogen and cellulose are homoglycans as they
are made of only glucose and are called glucans or glucosans. On the other
hand, mucopolysaccharides like hyaluronic acid and chondroitin sulphates are
heteroglycans as they are made up of different monosaccharide units.
Cellulose is the most abundant
polysaccharide. It is the structural component of the cell walls of plants.
Approximately half of all the carbon atoms in the plant kingdom are contained
in cellulose molecules. Structurally, cellulose is а linear (unbranched)
D-glucose polymer in which the glucose units are linked by b(1-4) glycosidic bonds.
Typically, cellulose chains contain about 5000 glucose units, which gives
macromolecules with molecular masses of about 900,000 amu. Cotton is almost
pure cellulose (95 %) and wood is about 50 % cellulose.
Even though it is а glucose polymer, cellulose is not а source of nutrition
for human beings. Humans lack the enzymes capable of catalyzing the hydrolysis
of b (1- 4) linkages in cellulose. Even grazing animals lack the enzymes
necessary for cellulose digestion. However, the intestinal tracts of animals
such as horses, cows, and sheep contain bacteria that produce cellulose, an
enzyme that can hydrolyze b (1- 4) linkages and produce
free glucose from cellulose. Thus grasses and other plant materials are а
source of nutrition for grazing animals. The intestinal tracts of termites
contain the same microorganisms, which enable termites to use wood as their source
of food. Microorganisms in the soil can also metabolize cellulose, which makes
possible the biodegradation of dead plants.
Despite its
nondigestibility, cellulose is still an important component of а balanced diet.
It serves as dietary fiber. Dietary fiber provides the digestive tract with
"bulk" that helps move food through the intestinal tract and
facilitates the excretion of solid wastes. Cellulose readily absorbs water,
leading to softer stools and frequent bowel action. Links have been found between
the length of time stools spend in the colon and possible colon cancer.
High-fiber food may also play а role in weight control. Obesity is not seen
in parts of the world where people eat large amounts of fiber-rich foods. Many
of the weight-loss products on the market are composed of bulk-inducing fibers
such as methylcellulose.
Some fibers
bind lipids such as cholesterol and carry them out of the body with the feces.
This lowers blood lipid concentrations and possibly the risk of heart and
artery disease.
Starch, like cellulose, is а polysaccharide containing only glucose units. It is
the storage polysaccharide in plants. If excess glucose enters а plant cell, it
is converted to starch and stored for later use. When the cell cannot get
enough glucose from outside the cell, it hydrolyzes starch to release glucose.
Iodine is
often used to test for the presence of starch in solution. Starch-containing
solutions turn а dark blue-black when iodine is added. As starch is broken down
through acid or enzymatic hydrolysis to glucose monomers, the blue-black color
disappears.
Two different
polyglucose polysaccharides can be isolated from most starches: amylose and
amylopectin. Amylose, а straight-chain glucose polymer, usually accounts for
15% — 20% of the starch; arnylopectin, а highly branched glucose polymer,
accounts for the remaining 80% — 85% of the starch.
In amylose's
structure, the glucose units are connected by a(1- 4) glycosidic linkages.
Starch (amylose)
The number
of glucose units present in an amylose chain depends on the source of the
starch; 300 – 500 monomer units are usually present.
Amylopectin,
the other polysaccharide in starch, is similar to amylose in that all linkages
are а linkages. It is different in that there is а high degree of branching in
the polymer. А branch occurs about once every 25 - 30 glucose units. The branch
points involve a(1 – 6) linkages:
Starch
(amylopectin)
Because of
the branching, amylopectin has а larger average molecular mass than the linear
amylose. The average molecular mass of amylose is 50,000 amu or more; it is
300,000 or more for amylopectin.
Note that
all of the glycosidic linkages in starch (both amylose and amylopectin) are of
the a type. In amylose, they are all a(1 - 4); in amylopectin, both a(1 -4) and a(1 -6) linkages are present. Because а linkages can be broken through
hydrolysis within the human digestive tract (with the help of the enzyme
amylase), starch has nutritional value for humans. The starches present in
potatoes and cereal grains (wheat, rice, corn, etc.) account for approximately
two-thirds of the world' s food consumption.
Glycogen, like cellulose and starch, is
а polysaccharide containing only glucose units. It is the glucose storage
polysaccharide in humans and animals. Its function is thus similar to that of
starch in plants, and it is sometimes referred to as animal starch. Liver cells
and muscle cells are the storage sites for glycogen in humans.
Glycogen has
а structure similar to that of amylopectin; all glycosidic linkages are of the a type, and both (1 - 4) and (1 - 6) linkages are present. Glycogen and amylopectin
differ in the number of glucose units between branches and the total number of
glucose units present in а molecule. Glycogen is about three times more highly
branched than amylopectin, and it is much larger, with а molar mass of up to
3,000,000 amu.
When excess
glucose is present in the blood (normally from eating too much starch), the
liver and muscle tissue convert the excess glucose to glycogen, which is then
stored in these tissues. Whenever the glucose blood level drops (from exercise,
fasting, or normal activities), some stored glycogen is hydrolyzed back to
glucose. These two opposing processes are called glycogenesis and
glycogenolysis, the formation and decomposition of glycogen, respectively.
Glycogen is
an ideal storage form for glucose. The large size of these macromolecules
prevents them from diffusing out of cells. Also, conversion of glucose to
glycogen reduces osmotic pressure. Cells would burst because of increased
osmotic pressure if all of the glucose in glycogen were present in cells in
free form. High concentrations of glycogen in а cell sometimes precipitate or
crystallize into glycogen granules. These granules are discernible in
photographs of cells under electron microscope magnification.
The glucose polymers
amylose, amylopectin, and glycogen compare as follows in molecular size and
degree of branching:
Amylose: Up to 1000 glucose units; no branching
Amylopectin: Up to 100,000 glucose units; branch points every 24-30 glucose units
Glycogen: Up to 1,000,000 glucose units; branch points every 8-12 glucose units
Glycogen
is the storage form of glucose in animals and humans which is analogous to the
starch in plants. Glycogen is synthesized and stored mainly in the liver and
the muscles. Structurally, glycogen is very similar to amylopectin with alpha
acetal linkages, however, it has even more branching and more glucose units are
present than in amylopectin. Various samples of glycogen have been measured at
1,700-600,000 units of glucose.
The structure of
glycogen consists of long polymer chains of glucose units connected by an alpha acetal linkage. The graphic on
the left shows a very small portion of a glycogen chain. All of the monomer
units are alpha-D-glucose, and all the alpha acetal links connect C # 1 of one
glucose to C # 4 of the next glucose.
The branches are
formed by linking C # 1 to a C # 6 through an acetal linkages. In glycogen, the
branches occur at intervals of 8-10 glucose units, while in amylopectin the
branches are separated by 12-20 glucose units.
Acetal Functional Group:
Carbon 1 is called the anomeric carbon and is the center of an acetal functional group. A
carbon that has two ether oxygens attached is an acetal.
The
Alpha position is defined as the
ether oxygen being on the opposite side of the ring as the C
Starch vs. Glycogen:
Plants
make starch and cellulose through the photosynthesis processes. Animals and
human in turn eat plant materials and products. Digestion is a process of
hydrolysis where the starch is broken ultimately into the various
monosaccharides. A major product is of course glucose which can be used
immediately for metabolism to make energy. The glucose that is not used
immediately is converted in the liver and muscles into glycogen for storage by
the process of glycogenesis. Any glucose in excess of the needs for energy and
storage as glycogen is converted to fat.
http://www.youtube.com/watch?v=oBL0OC3IavI
Start
with G-6-P, again note that this molecule is at a metabolic crossroads. First
convert to G-1-P using Phosphoglucomutase:
This
reaction is very much like PGA Mutase, requiring the bis phosphorylated
intermediate to form and to regenerate the phosphorylated enzyme intermediate.
Again a separate "support" enzyme, Phosphoglucokinase, is required to
form the intermediate, this time using ATP as the energy source:
Note
that this reaction is easily reversible, though it favors G-6-P.
UDP-glucose pyrophosphorylase,
which catalyzes the next reaction, has a near zero DG° ':
It
is driven to completion by the hydrolysis of the PPi to 2 Pi
by Pyrophosphatase with a DG° '
of about -32 kJ (approx. one ATP's worth of energy).
Finally
glycogen is synthesized with Glycogen
Synthase:
UDPGlucose
+ (glucose)n Æ UDP + (glucose)n+1
This
reaction is favored by a DG° ' of about 12 kcal, thus the overall synthesis of
glycogen from G-1-P is favored by a standard free energy of about 40 kJ. Note
that the glucose is added to the non-reducing end of a glycogen strand, and
that there is a net investment of 2 ATP equivalents per glucose (ATP to ADP and
UTP to UDP, regenerated with ATP to ADP). Note also that glycogen synthase
requires a 'primer.' That is it needs to have a glycogen chain to add on to.
What happens then in new cells to make now glycogen granules? Can use a special
primer protein (glycogenin). Thus glycogen granules have a protein core.
These
reactions will give linear glycogen strands, additional reactions are required
to produce branching. Branching enzyme
[amylo-a-(1,4) to a-(1,6)-transglycosylase] transfers a block of residues from
the end of one chain to another chain making a 1,6-linkage (cannot be closer
than 4 residues to a previous branch). (For efficient release of glucose
residues it has been determined that the optimum branching pattern is a new
branch every 13 residues, with two branchs per strand.)
Glycogen is broken down using Phosphorylase
to phosphorylize off glucose residues:
(glucose)n
+ Pi Æ (glucose)n-1 + G-1-P
Note
that no ATP is required to recover Glucose phosphate from glycogen. This is a
major advantage in anaerobic tissues, get one more ATP/glucose (3 instead of
2!). [Phosphorylase was originally thought to be the synthetic as well as
breakdown enzyme since the reaction is readily reversible in vitro.
However it was found that folks lacking this enzyme - McArdle's disease - can
still make glycogen, though they can't break it down.]
Glycogen
synthesis and degradation occurs in the liver cells. It is here that the
hormone insulin (the primary hormone responsible for converting glucose to
glycogen) acts to lower blood glucose concentration.
Crystal structure of glycogen synthase: homologous
enzymes catalyze glycogen synthesis and degradation
Alejandro Buschiazzo,
Juan E Ugalde, Marcelo E Guerin, William Shepard,
Rodolfo A Ugalde and Pedro M Alzari
Molecular surface representation of the GS
core, showing the equivalent position of the arginine clusters in the mammalian/yeast
(GT3) allosteric site (in red) with respect to the active center. Assuming an
extended main-chain conformation, approximate distances are shown for two
relevant phosphorylation sites, one in the N-terminal (2a) and the other in the
C-terminal (3a) extensions of GT3 enzymes.
Insulin. Chemical structure: protein. Insulin is formed in b-cells of Langerhans
islets (specialized endocrine regions of
the pancreas).
Proinsulin
is the biosynthetic precursor of insulin.
Effect of
insulin on carbohydrate metabolism:
-
increases
the permeability of cell membranes for glucose;
-
activates
the first enzyme of glycolysis - glucokinase and prevent the inactivation of
hexokinase;
-
activates
some enzymes of Krebs cycle (citrate synthase);
-
activates
the pentose phosphate cycle;
-
activates
glycogen synthetase;
-
activates
pyruvate dehydrogenase and a-ketoglutarate dehydrogenase;
-
inhibits
the gluconeogenesis;
-
inhibits
the decomposition of glycogen.
Effect of
insulin on protein metabolism:
-
increases
the permeability of cell membranes for amino acids;
-
activates
synthesis of proteins and nucleic acids;
-
inhibits
the gluconeogenesis.
Effect of
insulin on lipid metabolism:
-
enhances
the synthesis of lipids;
-
promotes
the lipid storage activating the carbohydrate decomposition;
-
inhibits
the gluconeogenesis.
Effect of
insulin on mineral metabolism:
-
activates
Na+, K+-ATP-ase (transition of K into the cells and Na
from the cells).
Target
tissue for insulin - liver, muscles and lipid tissue.
The release
of insulin from pancreas depends on the glucose concentration in the blood.
Some other hormones, sympathetic and parasympathetic nervous system also can
influence on the rate of insulin secretion.
The
deficiency of insulin causes diabetes mellitus.
Insulin is destroyed in the
organism by the enzyme insulinase that is produced by liver.
Insulin
Insulin crystals
Other names: insulin
Taxa expressing: Homo
sapiens; homologs: in metazoan taxa from invertebrates to
Antagonists: glucagon,
steroids, most stress hormomes
Insulin
(from Latin insula, "island", as it is produced in the Islets of
Langerhans in the pancreas) is a polypeptide hormone that regulates
carbohydrate metabolism. Apart from being the primary agent in carbohydrate
homeostasis, it has effects on fat metabolism and it changes the liver's
activity in storing or releasing glucose and in processing blood lipids, and in
other tissues such as fat and muscle. The amount of insulin in circulation has
extremely widespread effects throughout the body.
Chitin is а polysaccharide that is similar
to cellulose in both function and structure. Its function is to give rigidity
to the exoskeletons of crabs, lobsters, shrimp, insects, and other arthropods.
It also occurs in the cell walls of fungi.
Structurally, chitin is а linear polymer (no branching) with all b(1- 4) glycosidic linkages, as is cellulose. Chitin differs from cellulose
in that the monosaccharide present is an N-acetyl amino derivative D-glucose.
MUCOPOLYSACCHARIDES are
compounds that occur in connective tissue associated with joints in animals and
humans. Their function is primarily that of lubrication, а necessary
requirement if movement is to occur. The name mucopolysaccharide comes from the
highly viscous, gelatinous (mucus-like) consistency of these substances in
aqueous solution.
Unlike all the polysaccharides we have discussed up to this point,
mucopolysaccharides are heteropolysaccharides rather than homopolysaccharides.
А heteropolysaccharide is а
polysaccharide in which more than one (usually two) type of monosaccharide unit
is present.
One of the most common mucopolysaccharides is hyaluronic acid, а
heteropolysaccharide in which the following two glucose derivatives alternate
in the structure.
It is а highly viscous substance and has а molecular weight in several
hundred millions. Hyaluronic acid is а principal component of the ground
substance of connective tissue. Among other places it is found in skin,
synovial fluid, vitreous hemour of the eye, and umbilical cord. It exercises а
cementing function in the tissues and capillary walls, and forms а coating gel
round the ovum. It accounts for about 80% of the viscosity of synovial fluid
which contains about 0. 02 – 0.05% of hyaluronate.
(1,4)-O-b-D-Glucopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy-b-D-glucopyranose.
Hyaluronic acid is split up by the enzyme
hyalurorsidase into а number of small molecule. If fluid containing this enzyme
is injected into а tissue it spreads rapidly, from the site of injection and
thus this enzyme is sometimes referred to as the “spreading factor”. It
is found in relatively high concentration in the testis and seminal fluid, in
the venoms of certain snakes and insects, and in some bacteria. The enzyme also
has а physiological role in fertilization. The sperm is rich in the enzyme and
the former can thus advance better in the cervical canal and finally penetrates
the ovum.
CHONDROITIN SULPHATE. It has similar structure as hyaluronic acid with the difference that the
N-acetyl glucosamine unit of the latter is replaced by N-acetyl galactosamine 6
sulphate unit. The two other chondriotin sulphates are А and В; the type А nas
sulphate group in position 4 while the type В has L-iduronate (а stereoisomer
оf D-glucuronic acid) in place of D-glucuronic acid. Chondroitin sulphates are
found in cartilage, bone, heart valves, tendons and cornea.
(1,4)-O-b-D-Glucopyranosyluronic
acid-(1,3)-2-acetamindo-2-deoxy-6-O-sulfo-b-D-galactopyranose.
Dermatan sulfate. (Varying amounts of D-glucuronic acid may be present. Concentration
increases during aging process.)
(1,4)-O-a-L-Idopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy-4-O-sulfo-b-D-galactopyranose.
Heparin. It is naturally occurring
anticoagulant found mainly in the liver, and also in lung, spleen, kidney and
iatestinal mucosa. It prevents blood clotting by inhibiting the
prothrombin-thrombin conversion and thus eliminating the thrombin effect on
fibrinogen. This polysaccharide is composed of glucosamiae-N-sulphate aad
sulphate ester of glucuronic acid linked via 1 ® 4 — 1® 4 linkages (difference from hyaluronic acid and chondroitin sulphates).
(1,4)-O-a-D-Glucopyranosyluronic
acid-2-sulfo-(1,4)-2-sulfamindo-2-deoxy-6-O-sulfo-a-D-glucopyranose.
PECTIN (from Ancient Greek:
πηκτικός pēktikós,
"congealed, curdled") is a structural heteropolysaccharide
contained in the primary cell walls
of terrestrial plants.
It was first isolated and described in 1825 by Henri Braconnot. It is produced commercially as a
white to light brown powder, mainly extracted from citrus fruits, and is used in food as a gelling agent particularly in jams
and jellies. It is also used in fillings, medicines, sweets, as a stabilizer in
fruit juices and milk drinks, and as a source of dietary fiber.
Biology. In plant biology, pectin consists of a complex set of polysaccharides (see
below) that are present in most primary cell walls and are particularly abundant
in the non-woody parts of terrestrial plants. Pectin is present not only
throughout primary cell walls but also in the middle lamella between plant
cells, where it helps to bind cells together.
The amount, structure and chemical composition of pectin differs among
plants, within a plant over time, and in various parts of a plant. Pectin is an
important cell wall polysaccharide that allows primary cell wall extension and
plant growth. During fruit ripening, pectin is broken down
by the enzymes pectinase and pectinesterase, in which process the fruit becomes
softer as the middle lamellae break down and cells become separated from each
other. A similar process of cell separation caused by the breakdown of pectin
occurs in the abscission zone
of the petioles
of deciduous plants at leaf fall.
Pectin is a natural part of the human diet, but does not contribute significantly to nutrition. The daily intake of pectin from fruits
and vegetables can be estimated to be around 5 g (assuming consumption of
approximately 500 g fruits and vegetables per day).
In human digestion, pectin binds to cholesterol in the gastrointestinal
tract and slows glucose absorption by trapping carbohydrates. Pectin is thus a
soluble dietary fiber.
Consumption of pectin has been shown to reduce blood cholesterol levels. The
mechanism appears to be an increase of viscosity in the intestinal tract,
leading to a reduced absorption of cholesterol from bile or food. In the large
intestine and colon, microorganisms degrade pectin and liberate short-chain
fatty acids that have positive influence on health (prebiotic
effect).[citation needed]
Chemistry. Pectins, also known as pectic polysaccharides, are rich in galacturonic
acid. Several distinct polysaccharides have been identified and characterised
within the pectic group. Homogalacturonans are linear chains of
α-(1–4)-linked D-galacturonic acid.
Substituted galacturonans are characterized by the presence of saccharide
appendant residues (such as D-xylose or D-apiose
in the respective cases of xylogalacturonan and apiogalacturonan) branching
from a backbone of D-galacturonic acid residues. Rhamnogalacturonan I pectins
(RG-I) contain a backbone of the repeating disaccharide:
4)-α-D-galacturonic acid-(1,2)-α-L-rhamnose-(1. From many of the rhamnose residues,
sidechains of various neutral sugars branch off. The neutral sugars are mainly
D-galactose, L-arabinose and D-xylose, with the types and
proportions of neutral sugars varying with the origin of pectin.
Another structural type of pectin is rhamnogalacturonan II (RG-II), which
is a less frequent complex, highly branched polysaccharide. Rhamnogalacturonan
II is classified by some authors within the group of substituted galacturonans
since the rhamnogalacturonan II backbone is made exclusively of D-galacturonic
acid units.
Isolated pectin has a molecular weight of typically 60–130,000 g/mol, varying
with origin and extraction conditions.
In nature, around 80 percent of carboxyl groups of galacturonic acid are esterified
with methanol. This proportion is decreased to a
varying degree during pectin extraction. The ratio of esterified to
non-esterified galacturonic acid determines the behavior of pectin in food
applications. This is why pectins are classified as high- vs. low-ester pectins
(short HM vs. LM-pectins), with more or less than half of all the galacturonic
acid esterified.
The non-esterified galacturonic acid units can be either free acids
(carboxyl groups) or salts with sodium, potassium, or calcium. The salts of
partially esterified pectins are called pectinates, if the degree of
esterification is below 5 percent the salts are called pectates, the insoluble
acid form, pectic acid.
Some plants such as sugar beet,
potatoes and pears
contain pectins with acetylated galacturonic acid in addition to methyl esters.
Acetylation prevents gel-formation but increases the stabilising and
emulsifying effects of pectin.
Amidated pectin is a modified form of pectin. Here, some of the galacturonic acid is converted
with ammonia to carboxylic acid amide.
These pectins are more tolerant of varying calcium concentrations that occur in
use.
To prepare a pectin-gel, the ingredients are heated, dissolving the pectin.
Upon cooling below gelling temperature, a gel starts to form. If gel formation
is too strong, syneresis
or a granular texture are the result, whilst weak gelling leads to excessively
soft gels. In high-ester pectins at soluble solids content above 60% and a
pH-value between 2.8 and 3.6, hydrogen bonds and hydrophobic
interactions bind the individual pectin chains together. These bonds
form as water is bound by sugar and forces pectin strands to stick together.
These form a 3-dimensional molecular net that creates the macromolecular gel.
The gelling-mechanism is called a low-water-activity gel or sugar-acid-pectin
gel.
In low-ester pectins, ionic bridges are formed between calcium ions and the
ionised carboxyl groups of the galacturonic acid. This is idealised in the
so-called “egg box-model”. Low-ester pectins need calcium to form a gel, but
can do so at lower soluble solids and higher pH-values than high-ester pectins.
Amidated pectins behave like low-ester pectins but need less calcium and
are more tolerant of excess calcium. Also, gels from amidated pectin are
thermo-reversible; they can be heated and after cooling solidify again, whereas
conventional pectin-gels will afterwards remain liquid.
High-ester pectins set at higher temperatures than low-ester pectins.
However, gelling reactions with calcium increase as the degree of esterification
falls. Similarly, lower pH-values or higher soluble solids (normally sugars)
increase gelling speed. Suitable pectins can therefore be selected for jams and
for jellies, or for higher sugar confectionery jellies.
Sources and production. Apples, guavas,
quince, plums, gooseberries, oranges and other citrus
fruits, contain large amounts of pectin, while soft fruits like cherries,
grapes and strawberries contain small amounts of pectin.
Typical levels of pectin in plants are (fresh weight):
· apples, 1–1.5%
· apricot, 1%
· cherries, 0.4%
· oranges, 0.5–3.5%
· carrots approx. 1.4%
· citrus peels, 30%
The main raw-materials for pectin production are dried citrus peel or apple
pomace, both by-products of juice production.
Pomace from sugar-beet is also used to a small extent.
From these materials, pectin is extracted by adding hot dilute acid at
pH-values from 1.5 – 3.5. During several hours of extraction, the protopectin
loses some of its branching and chain-length and goes into solution. After
filtering, the extract is concentrated in vacuum and the pectin then precipitated
by adding ethanol or isopropanol. An old technique of precipitating pectin with
aluminium salts is no longer used (apart from alcohols and polyvalent cations;
pectin also precipitates with proteins and detergents).
Alcohol-precipitated pectin is then separated, washed and dried. Treating
the initial pectin with dilute acid leads to low-esterified pectins. When this
process includes ammonium hydroxide, amidated pectins are obtained. After
drying and milling, pectin is usually standardised with sugar and sometimes
calcium-salts or organic acids to have optimum performance in a particular
application.
Worldwide, approximately 40,000 metric tons of pectin are produced every
year.[citation needed]
Uses. The main use for pectin (vegetable agglutinate) is as a gelling agent,
thickening agent and stabilizer in food. The classical application is giving
the jelly-like consistency to jams or marmalades, which would otherwise be sweet juices.
For household use, pectin is an ingredient in gelling sugar (also known as "jam
sugar") where it is diluted to the right concentration with sugar and some
citric acid to adjust pH. In some countries, pectin is also available as a
solution or an extract, or as a blended powder, for home jam making. For
conventional jams and marmalades that contain above 60% sugar and soluble fruit
solids, high-ester pectins are used. With low-ester pectins and amidated
pectins less sugar is needed, so that diet products can be made.
Pectin can also be used to stabilize acidic protein drinks, such as
drinking yogurt, and as a fat substitute in baked goods. Typical levels of
pectin used as a food additive are between 0.5 and 1.0% – this is about the
same amount of pectin as in fresh fruit.
In medicine, pectin increases viscosity and volume of stool
so that it is used against constipation
and diarrhea. Until 2002, it was one of the main
ingredients used in Kaopectate
a drug to combat diarrhea, along with kaolinite. Pectin is also used in throat lozenges
as a demulcent. In cosmetic products, pectin acts as
stabilizer. Pectin is also used in wound healing preparations and specialty
medical adhesives, such as colostomy
devices.
Yablokov et al., writing in Chernobyl: Consequences of the Catastrophe for People and the
Environment, quote research conducted by the Ukrainian Center of
Radiation Medicine and the Belarussian Institute of Radiation Medicine and
Endocrinology with the conclusion that "adding pectin preparations to the
food of inhabitants of the Chernobyl-contaminated regions promotes an effective
excretion of incorporated radionuclides".
The authors report on the positive results of using pectin food additive
preparations in a number of clinical studies conducted on children in severely
polluted areas, with up to 50% improvement over control groups.
In ruminant nutrition, depending on the extent of
lignification of the cell wall, pectin is up to 90% digestible by bacterial
enzymes. Ruminant nutritionists recommend that the digestibility and energy
concentration in forages can be improved by increasing pectin concentration in
the forage.
In the cigar industry, pectin is considered an excellent
substitute for vegetable glue and many cigar smokers and collectors will use
pectin for repairing damaged tobacco wrapper leaves on their cigars.
The compounds that result from the covalent
linkages of carbohydrate molecules to both proteins and lipids are collectively
known as the glycoconjugates. These
substances have profound effects on the function of individual cells, as well
as the cell-cell interactions of multicellular organisms. There are two classes
of carbohydrate-protein conjugate: proteoglycans and glycoproteins. Although
both molecular types contain саrbohydrate and protein, their structures and
functions appear, in general, to be substantially different. The glycolipids,
which are oligosaccharide-containing lipid molecules, are found predominantly
on the outer surface of plasma membranes.
Glycolipids and glycoproteins. Before 1960, the biochemistry of carbohydrates was thought to be rather
simple. These compounds served (1) as energy sources for plants, humans, and
animals and (2) as structural materials for plants and arthropods.
Research
since that time has shown that carbohydrates (oligosaccharides) attached
through glycosidic linkages to lipids and proteins, which are called
glycolipids and glycoproteins, respectively, have а wide variety of cellular
functions.
It is now
known that carbohydrate units on cell surfaces (glycolipids and glycoproteins)
govern how individual cells interact with other cells, with invading bacteria,
and with viruses. In the human reproductive process, fertilization begins with
the binding of а sperm to а specific oligosaccharide on the surface of an egg.
GLYCOCONJUGATES
The compounds that result from the covalent
linkages of carbohydrate molecules to both proteins and lipids are collectively
known as the glycoconjugates. These
substances have profound effects on the function of individual cells, as well
as the cell-cell interactions of multicellular organisms. There are two classes
of carbohydrate-protein conjugate: proteoglycans and glycoproteins. Although
both molecular types contain сагbohydrate and protein, their structures and functions appear, in general,
to be substantially different. The glycolipids, which are
oligosaccharide-containing lipid molecules, are found predominantly on the
outer surface of plasma membranes.
Proteoglycans are distinguished from the тоге common glycoproteins by their extremely high carbohydrate content, which
may constitute as much as 95% of the dry weight of such molecules. These
molecules are found predominantly in the extracellular matrix (intercellular
material) of tissues. All
proteoglycans
contain GAG chains. The GAG chains are linked to protein molecules (known as
core proteins) by N- and O-glycosidic linkages. The diversity of proteoglycans
is а result of both the number of
different core proteins and the large variety of different classes and lengths
of the carbohydrate chains.
Fig. Proteoglycan structure.
Because proteoglycans contain large numbers of
GAGs, which are polyanions, large volumes of water and cations are trapped
within their structure. As а result, proteoglycan molecules occupy space that is thousands of times
that of а densely packed molecule of
the same mass. Proteoglycans contribute support and elasticity to tissues in which
they occur. Consider, for example, the strength, flexibility, and resilience of
cartilage. The structural diversity of proteoglycans allows them to serve а variety of structural and
functional roles in living organisms. Proteoglycans are particularly abundant
in the extracellular matrix of connective tissue. Together with matrix proteins
such as collagen, fibronectin, and laminin, they form an organized meshwork
that provides strength and support to multicellular tissues. Proteoglycans are
also present at the surface of cells, where they are directly bound to the
plasma membrane. Although the function of these latter molecules is not yet
clear, the suggestion has been made that they play an important role in
membrane structure and cell-cell interactions.
А number of genetic diseases
associated with proteoglycan metabolism, known as mucopolysaccharidoses,
have been identified. Because proteoglycans are constantly being synthesized
and degraded, their excessive accumulation (due to missing or defective lysosomal
enzymes) has very serious consequences. For example, in Hurler's syndrome, an
autosomal recessive disorder (а disease type in which one copy of the defective gene is inherited from
each parent), deficiency of а specific enzyme results in accumulation of dermatan sulfate. Symptoms
include mental retardation, skeletal deformity, and early childhood death.
Glycoproteins Glycoproteins are commonly defined as proteins that are
covalently linked to carbohydrate through O- or N-linkages. The carbohydrate composition
of glycoprotein varies from 1% to over 85% of total weight. The types of
carbohydrate that are found include monosaccharides and disaccharides such as
those attached to the structural protein collagen and the branched
oligosaccharides on plasma glycoproteins. Although the glycoproteins are
sometimes considered to include the proteoglycans, there appear to be
sufficient structural reasons to examine them separately.
These include the relative absence in
glycoproteins of uronic acids, sulfate groups, and the disaccharide repeating
units that are typical of proteoglycans. The carbohydrate groups of
glycoproteins are linked to the polypeptide by either (1) an N-glycosidic
linkage between N-acetylglucosamine (GlcNAc) and the amino acid asparagine (Asn)
or (2) an O-glycosidic linkage between N-acetylgalac-tosamine (GalNAc) and the
hydroxyl group of the aminoacids serine (Ser) or threonine (Thr). The former
glycoprotein class is sometimes referred to as asparagine-linked; the latter is
of- ten called mucin-type.
Asparagine-linked Carbohydrate. As was mentioned previously, three structural forms of asparagine-linked
oligosaccharide occur in glycoproteins: high- mannose, complex, and hybrid.
High-mannose type is com- posed of GlcNAc and mannose. Complex-type may contain
fucose, galactose, and sialic acid in addition to GlcNAc and mannose.
Hybrid-type oligosaccharides contain features of both complex and
high-mannose-type species. Despite these differences, the core structure of all
N-linked
oligosaccharides
is the same. This core, which is constructed on а membrane-bound lipid molecule, is covalently linked toasparagine during
ongoing protein synthesis. Several
additional reactions, which occur within the lumen of the endoplasmic reticulum
and the Golgi complex, result in the final N-linked oligosaccharide structures.
Mucin-type carbohydrate While all N-linked
oligosaccharides are bound to protein via GlcNAc-Asn, the linking groups of
0-glycosidic oligosac. charides are of several types. The most common of these
is GalNAc-Ser (or GalNAc-Thr). Mucin-type carbohydrate unit vary considerably
in size and structure, from disaccharide such as Gal-1,3-GalNAc, found in the
antifreeze glycoprotein of antarctic fish (Figure 10), to the complex
oligosaccharides of blood groups such as those of the ABO system
Fig. Antifreeze
glycoprotein structure.
Glycoprotein functions. Glycoproteins are а diverse group of molecules
that are ubiquitous constituents of most living organisms. They occur in cells, in both soluble and membrane-bound form, as well as in
extracellular fluids. Vertebrate animals are particularly rich in
glycoproteins. Examples of such substances include the metal-transport proteins
transferrin and ceruloplasmin, the blood-clotting factors, and many of the components
of complement (proteins involved in cell destruction during immune reactions). А number of hormones (chemicals
produced by certain cells that are transported by blood to other cells, where
they exert regulatory effects) are glycoproteins. Consider, for example,
follicle-stimulating hormone (FSH), which is produced by the anterior pituitary
gland. FSH stimulates the development of both eggs and sperm. Additionally,
many enzymes are glycoproteins. Ribonuclease
(RNase), the enzyme whose function is the degradation of ribonucleic acid, is а well-researched example. Other glycoproteins occur as integral membrane proteins. Of these, Na+-К+-ATPase (an ion pump found in the plasma membrane of animal cells) and the
major histo-cpmpatibility antigens (cell surface markers used to cross-match
organ donors and recipients) are especially interesting examples.
Although а wide varieties of glycoproteins have been studied, the role of
carbohydrate is still not clearly understood. 0espite challenging technical
problems, some progress has been made in discerning how the carbohydrate
component contributes to biological activity. Recent research has focused on
the ontribution of carbohydrate to the stability of protein molecules and to
the various complex recognition phenomena that occur in multicellular
organisms.
The presence of carbohydrate on protein
molecules has been shown to protect them from denaturation. For example, bovine
RNase А is more susceptible to heat
denaturation than its glycosylated counterpart RNase В. Several other studies have
shown that sugar-rich glycoproteins are relatively resistant to proteolysis
(splitting of polypeptides by enzyme-catalyzed hydrolytic reactions). Because
the carbohydrate occurs on the molecule's surface, it may act as а shield by hindering the
approach of proteolytic enzymes toward the polypeptide chain.
It is becoming apparent that the carbohydrates
in glycoproteins may affect biological function in а variety of ways. In some
glycoproteins this contribution is more easily discerned than in others. For
example, а large
content of sialic acid residues is known to be responsible for the viscosity of
salivary mucins (the lubricating glycoproteins of saliva). Another interesting
example is provided by: the antifreeze glycoproteins of antarctic fish.
Apparently, their disaccharide residues form hydrogen bonds with water
molecules. This process retards the growth of ice crystals.
Glycoproteins are now known to be important in
complex recognition phenomena such as cell-molecule, cell-virus, and cell-cell
interactions. Prime examples of glycoprotein involvement in cell-molecule
interactions include the insulin receptor, whose binding to insulin facilitates
the transport of glucose into numerous cell types. It does so, in part, by
recruiting glucose transporters to the plasma membrane. In addition, the
glucose transporter which is directly responsible for the transport of the
sugar into cells is also а glycoprotein. The interaction between gpl20, the target cell binding
glycoprote in of HIV (the AIDS virus), and host cells is а fascinating example of
cell-virus interaction. The attachment of gp120 to the CD4 receptor found on
the surface of several human cell types is now considered to be an early step
in the infective process. Removal of carbohydrate from purified gpl20 results
in а significant reduction in the
binding of the viral protein to the CD4 receptor. Paradoxically, the
oligosaccharides attached to С protein (the glycoprotein on the surface of vesicular stomatitis virus)
are not required for viral infectivity.
Cell surface glycoproteins, components of the
glycocalyx (also known as the cell coat), are now recognized as being involved
in the process of cellular adhesion. This process is а critical event in the
cell-cell interactions of growth and differentiation. The best-characterized of
these substances are called cell adhesion molecules (CAMs). CAMs are now
believed to be involved in the embryonic development of the mouse nervous
system. Sialic residues in the N-linked oligosaccharides of several CAMs have
been shown to be important in this phenomenon.
Recent improvements in technology have led to an
increased appreciation of the importance of carbohydrate in glycoproteins.
Consequently, there is currently а heightened interest in studying cellular glycosylation patterns.
Carbohydrate structure is now used as а tool in the investigation of normal processes such as nerve development
(mentioned above) and certain disease processes. For example, changes in the
galactose content of the antibody class IgG have recently been shown to be
directly related to the severity (i.е.,the degree of inflammation) of juvenile arthritis. Additionally, recent
evidence indicates that changes in glycosylation patterns accompany changes in
the behavior of cancer cells. This knowledge is currently making various
aspects of malignancy, such as tumor detection and the metastatic process (the
spread of cancerous cells from а tumor to other body parts), more accessible to investigation.