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Carbohydrates and
Classification
Carbohydrates are the most abundant biomolecules on
earth. They have either an aldehyde or a keto group in
addition to many hydroxyl groups. Dihydroxyacetone is
the simplest carbohydrate with a keto group.
Carbohydrates are defined as the polyhydroxy aldehydes
or polyhydroxy ketones or substances that yield such
compounds on hydrolysis. The empirical formula of
carbohydrates is (CH2O) n. Carbohydrates serve as energy
stores, structural elements and they are precursors for
many organic compounds like fats and amino acids.
Carbohydrates are classified into four categories.
Monosaccharides (simple sugars) containing a single
polyhydroxy aldehyde or ketone unit. Ex: Glucose.
Disaccharides consist of two monosaccharides linked by
glycosidic linkage. Ex: Maltose. Oligosaccharides
consist of three to twelve monosaccharides. Ex:
Maltotriose which is a trisaccharide, made up of three
glucose units. Polysaccharides consist of more than
twelve monosaccharides. Ex: Cellulose.
Monosaccharides
Monosaccharides with an aldehyde group are called an aldose
sugar. Ex: Glucose and with keto group called ketose sugar.
Ex: Fructose. On the basis of number of carbon atoms,
monosaccharides are classified as trioses, tetroses,
pentoses, hexoses and heptoses. Glyceraldehyde is an
aldrotriose with a single asymmetric carbon atom. Fischer
projection of a molecule with asymmetric carbon is
discussed. D and L glyceraldehdye are the mirror images of
each other, also called enantiomers. Monosaccharides exhibit
isomerism. Glucose and fructose are aldose-ketose isomers of
one another. Glucose and mannose are epimers. They differ in
configuration at the 2nd carbon of these aldohexoses. Mostly
monosaccharides occur in aqueous solution as aqueous ring.
In the process, carbonyl group (-CHO/-C=O) forms a covalent
bond with oxygen of a hydroxyl group along the chain. The
ring structure hemiacetal is formed by the combination of an
aldehyde and an alcohol group. The ring has six members and
is called pyranose ring. Ex: α-D-Glucopyranose. With the
ketose group, hemiketal is formed. The ring contains five
members and is called furanose ring. Ex: α-D-Fructofuranose.
Anomerism is the process in which there is a creation of
asymmetric carbon, called anomeric carbon. Ex: α-D-Glucopyranose
and β-D-Glucopyranose. Mutarotation is the process in which
cyclic α and β anomers produce equilibrium in a solution.
A constant source of blood glucose is compulsory for human
life. Glucose is the preferred energy source of the brain.
Glycogen stored in muscle releases hexose units for
glycolysis within the muscle itself. Glycogen in the liver
is concerned with export of hexose units for maintenance of
the blood glucose, particularly between meals.
Monosaccharides are degraded by a process called glycolysis
which takes place in cytosol and a series of reactions in
citric acid cycle in mitochondria. When one molecule of
glucose is oxidized to carbon dioxide and water, 38 ATP
molecules are generated. Reducing action of monosaccharides
depends on the free anomeric carbon in their molecules,
which are capable of reducing cupric to cuprous ions and
themselves get oxidized to sugar acids. Oxidation of glucose
gives rise to glucuronic acid. Two monosaccharides join
covalently (glycosidic bond) when hydroxyl group of one
reacts with anomeric carbon of the other forming a
glycoside. Deoxy sugars, amino sugars, sugar acids and sugar
alcohols are derivatives of monosaccharides.
Disaccharides
Maltose is a reducing disaccharide. It is made up of 2
glucose units linked by α (1-4) glycosidic linkage. Anomeric
carbon of second glucose residue is free. It is produced
during starch digestion and it exhibits mutarotation.
Lactose is also a reducing disaccharide made up of one
glucose and one galactose, linked by β (1-4) glycosidic
linkage. It is present in milk and exhibits mutarotation.
Sucrose is a non-reducing sugar. It is composed of one
glucose and one fructose linked by α1- β2 glycosidic
linkage. Anomeric carbon units are involved in linkage;
hence no free anomeric carbons are available.
Polysaccharides
Consist of repeating units of monosaccharides or their
derivatives held together by glycosidic bonds. Ex: Starch,
Hyaluronic acid, homopolysaccharides contain only a single
type of monosaccharide. Ex: Starch, glycogen, cellulose,
chitin. Starch is found in cereals, legumes, potatoes. Here,
glucose is the monosaccharide unit. They are made up of
amylase and amylopectin with α (1-4) linage in linear
structure of amylase and α (1-4) and α (1-6) linkage in
branched structure amylopectin. In cellulose, polymer of
glucose is linked by β (1-4) glycosidic linkage. Cellulose
is present in plant cell wall and functions as a dietary
fiber. Glycogen is the main storage polysaccharide in
animals and is abundant in the liver. Glycogen is more
extensively branched. Heteropolysaccharides are composed of
different types of monosaccharides or their derivatives.
Mucopolysaccharides are known as glycosaminoglycans. They
are negatively charged large complexes. Ex: Hyaluronic acid,
heparin, chondroitin sulfate, dermatan sulfate, keratin
sulfate etc. Glycoproteins mostly contain oligosaccharides
which are tightly bound to proteins. Ex: Immunoglobulins,
mucin of stomach. They function as enzymes, hormones,
receptors, structural proteins, transport proteins. In
proteoglycans, proteins are covalently bound with
mucopolysaccharides. Found in the bone, elastin and
collagen.
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