The word 'carbohydrate' was coined
more than 100 years ago to describe a large group of compounds include polymers
and other compounds synthesized from polyhydroxylated aldehydes and ketones. In
general carbohydrates have the empirical formula (CH2O)n.
They are polyhydroxylated aldehydes or ketones and their derivatives.
Chemically, carbohydrates are molecules that are composed of carbon, along with
hydrogen and oxygen. The compounds carbohydrates have common same functional
groups, glyceraldehydes and gulose are classifed as aldoses and ribulose and
dihydroxyacetone as ketoses. All of these compounds are alcohols with many
hydroxyl groups. They are polyhydroxylated and either aldehydes or ketones.
Classification of carbohydrates
Monosaccharides
The simplest and smallest unit of
the carbohydrates is the monosaccharide, (mono = one, saccharide = sugar). Monosaccharides
are either aldehydes or ketones, with one or more hydroxyl groups; the
six-carbon monosaccharides glucose (an aldohexose) and fructose (a keto hexose)
have five hydroxyl groups. The carbon atoms, to which hydroxyl groups are
attached, are often chiral centers, and stereoisomerism is common among
monosaccharides. Simple monosaccharides with four, five, six, and seven carbon
atoms are called tetroses, pentoses, hexoses, and heptoses, respectively.
Because these molecules have multiple asymmetric carbons, they exist as
diastereoisomers, isomers that are not mirror images of each other, as well as
enantiomers. In regard to these monosaccharides, the symbols D and L designate
the absolute configuration of the asymmetric carbon farthest from the aldehyde
or keto group. D-Ribose, the carbohydrate component of RNA, is a fivecarbon
aldose. D-Glucose, D-mannose, and D-galactose are abundant six-carbon aldoses.
Disaccharides
A disaccharide consists of two
monosaccharides joined by an O-glycosidic bond. Disaccharides can be homo- and
heterodisaccharide. Three most abundant disaccharides are sucrose, lactose, and
maltose. In sucrose the anomeric carbon atoms of a glucose unit and a fructose
unit are joined. Lactose, the disaccharide of milk, consists of galactose
joined to glucose by a β (1→4) glycosidic linkage. In maltose, α (1→4)
glycosidic linkage joins two glucose units. Sucrose and lactose are
heterosaccharides and maltose is homosaccharide.
Oligosaccharide
An oligosaccharide is a saccharide
polymer containing a small number (typically three to ten) of component sugars,
and is also known as simple sugars. They are generally found either O- or
N-linked to compatible amino acid side chains in proteins or to lipid moieties.
They (homo-and hetero-oligosaccharides) are also liberated as intermediate
products of saccharification by action of glycosidases on polysaccharides.
Polysaccharides
Polysaccharides are relatively
complex carbohydrates. They are polymers made up of many monosaccharides joined
together by glycosidic bonds. They are, therefore, very large, often branched,
macromolecules. They tend to be amorphous, insoluble in water, and have no
sweet taste. When all the monosaccharides in a polysaccharide are of the same
type, the polysaccharide is called a homopolysaccharide and when more than one
type of monosaccharide is present, they are called heteropolysaccharides.
Examples include storage polysaccharides such as starch and glycogen and
structural polysaccharides such as cellulose, and chitin. Xylan a hemicellulose
is a heteropolysaccharide. Polysaccharides have a general formula of Cn(H2O)n-1
where n can be any number between 200 and 2500.
Structure of carbohydrates
Monosaccharides
Examples of monosaccharide include
glucose (dextrose), fructose, galactose, and ribose. Monosaccharides are the
building blocks of disaccharides like sucrose (common sugar) and
polysaccharides (such as cellulose and starch and hemicellulose). With few
exceptions (deoxyribose or dexyglucose), monosaccharides have the chemical
formula (CH2O)n+m with the chemical structure H (CHOH)nC=O(CHOH)mH.
If n or m is zero, it is an aldehyde and is termed an aldose; otherwise it is a
ketone and is termed a ketose. Monosaccharides contain a ketone or aldehyde
functional group, and hydroxyl groups on most or all of the non-carbonyl carbon
atoms. Most monosaccharides form cyclic structures, which predominate in
aqueous solution, by forming hemiacetals or hemiketals (depending on whether
they are aldoses or ketoses) between an alcohol and the carbonyl group of the
same sugar. Glucose, for example, readily forms a hemiacetal linkage between
its carbon-1 and the hydroxyl group of its carbon-5. Since such a reaction
introduces an additional stereogenic center, two anomers are formed (α-anomer
and β-anomer) from each distinct straight-chain monosaccharide.
Fischer projection: Structures of
monosaccharides are defined by Fischer projection where all horizontal
bonds project toward the viewer, while vertical bonds project away from the
viewer. Therefore, a Fischer projection cannot be rotated by (2n+1)×90° in the
plane of the page or the screen, as the orientation of bonds relative to one
another can change, converting a molecule to its enantiomer.
Haworth Projection: A Haworth projection
is a common way of representing the cyclic structure of monosaccharides with a
simple three-dimensional perspective. The Haworth projection was named after
the English chemist Sir Walter N. Haworth. A Haworth projection has the
following characteristics: ¾ Carbon is the implicit type of atom. ¾ Hydrogen
atoms on carbon are implicit.
There is another way of
representation of cyclic structure of saccharides; one is chair and another is
boat conformation. In chair conformation, the α-anomer has the OH- of the
anomeric carbon in an axial position, whereas the β-isomer has the OH- of the
anomeric carbon in equatorial position The lowest-energy chair conformation, 6
of the 12 hydrogens are in axial positions their C-H bonds are parallel to each
other and appear to stick up and down from the ring structure, the other 6 are
in equatorial positions. On the other hand, boat conformation has a higher
energy than the chair form due to steric strain resulting from the two axial
1,4-hydrogen atoms. The torsional strain in the boat conformation has a maximum
value because all the carbon bonds are eclipsed. The boat and envelope forms
are transition states between the twist forms and the twist and chair forms
respectively, and are impossible to isolate. The twist-boat conformation is
less stable than the chair conformation. The difference in energy between the
chair and the twist-boat conformation of monosaccharides can be measured
indirectly by taking the difference in activation energy for the conversion of
the chair to the twist-boat conformation and that of the reverse isomerization.
The axial hydrogens are those sigma bonds that are parallel to an imaginary
axis running through the ring structure. The equatorial hydrogens are those
whose sigma bonds are perpendicular to that axis.
Disaccharides
Disaccharides consist of two
monosaccharide units, linked together with glycosidic bonds in α or β orientation.
The most important of them are sucrose, lactose, and maltose. Sucrose is the
most abundant and consists of a molecule of α-glucose and β-fructose
linked together. Lactose is found in milk and dairy products and consists of
galactose and glucose linked by a β-1,4-glycosidic bond. Maltose is
mainly produced by partial hydrolysis of starch and consists of two glucose
units linked by an α-1,4-glycosidic bond.
Sucrose is the most important
disaccharide. It is popularly known as table sugar. Sucrose is
found in all photosynthetic plants. It is commercially obtained from sugarcane
and sugar beets via an industrial process. The molecular formula of
sucrose is C12H22O11. The chemical
structure of sucrose comprises of α form of glucose and β form of fructose.
Lactose is the primary ingredient
found in the milk of all mammals. Unlike the majority of saccharides, lactose
is not sweet to taste. Lactose consists of one galactose carbohydrate and one
glucose carbohydrate. These are bound together by a 1-4 glycosidic bond in a
beta orientation. Maltose has two monosaccharide glucose molecules bound
together. The link is between the first carbon atom of glucose and the
fourth carbon of another glucose molecule, know as one-four glycosidic linkage.
Polysaccharides
Polysaccharides are ubiquitous
biopolymers that occur widely in nature. These are polymers of simple sugars
that are monosaccharides linked together by glycosidic linkages. The sugars participating
in the bonds are called residues.
The glycosidic bond is a bridge between the two residues consisting of an
oxygen atom between two carbon rings. The glycosidic bond results from a
dehydration reaction. In the dehydration reaction a hydroxyl group is lost
from a carbon of one residue while a hydrogen is lost from a hydroxyl group
from another residue. A water molecule (H2O) is removed and the carbon of the first residue joins to the
oxygen from the second residue.
Specifically, the first carbon (carbon-1) of one residue and the
fourth carbon (carbon-4) of the other residue are linked by the oxygen, forming
the 1,4 glycosidic bond. There are two types of glycosidic bonds, based on the
stereochemistry of the carbon atoms. An α(1→4) glycosidic bond forms when the
two carbon atoms have the same stereochemistry or the OH on carbon-1 is below
the sugar's ring. A β(1→4) linkage forms when the two carbon atoms have different
stereochemistry or the OH group is above the plane. There are different types of polysaccharides.
Polysaccharides exhibit a molecular structure that can be linear or highly
branched. Polysaccharides may be homopolysaccharide or heteropolysaccharide.
A homopolysaccharide consists
of one sugar or sugar derivative. For example, cellulose, starch, and glycogen
are all composed of glucose subunits. Chitin consists of repeating
subunits of N-acetyl-D-glucosamine,
which is a glucose derivative.
A heteropolysaccharide contains more than one sugar or sugar derivative. Most heteropolysaccharides consist of two monosaccharides. They are often associated with proteins. A good example of a heteropolysaccharide is hyaluronic acid, which consists of N-acetyl-D-glucosamine linked to glucuronic acid.
- F
- Carbohydrates add on to the taste and appearance of food item, thus making the dish tempting and mouthwatering.
- Carbohydrates
aid in regulating blood glucose and also do good to our body by breaking
down fatty acids, thus preventing ketosis.
- The
structural diversity possible by linking the different, common sugar is
immense: theoretically far greater than that of proteins, which largely
consist of 22 amino acids linked by a single type of peptide bond.
- Linkages
between sugars can occur through a glycoside linkage between the anomeric,
first carbon of a sugar in either α or β configuration with any of a
variety of hydroxyl groups on the adjacent sugar.
- Insoluble
carbohydrate polymers serve as structural and protective elements in the
cell walls of bacteria and plants and in the connective tissues of
animals.
- Plant
cell walls are complex arrangements of cellulose, hemicellulose and
lignin. This contributes significantly to the overall digestibility of the
fiber. The proportion of each component depends on species and age of the
plant. Cellulose is the primary structural component of plants. It is
found primarily in the cells walls and is a primary fiber component of
animal feeds.
- Glycosaminoglycans
as polymers of derivatives of carbohydrates are of critical importance in
intercellular communication in organisms. They interact with a wide
variety of proteins, including growth factors and chemokines, which
regulate important physiological processes. The presence of
glycosaminoglycans on cell membranes and in the extracellular matrix also
has resulted in their exploitation by infectious pathogens to gain access
and entry into animal cells.
- Other
carbohydrate polymers lubricate skeletal joints and participate in
recognition and adhesion between cells.
- More
complex carbohydrate polymers covalently attached to proteins or lipids
act as signals that determine the intracellular location or metabolic fate
of these hybrid molecules, called glycoconjugates. Glycoconjugates carry
various important functions of cell.
- Glycoproteins
act as receptors and integral membrane proteins in membranes, cytoskeletal
proteins in cytoplasm, extracellular proteins such as antibodies,
hormones, collagen, enzymes (RNase, DNase, lipases, cholinesterase,
phosphatase, pepsinogen, glycosyltransferases) etc.