1. Introduction
Carbohydrates are the most abundant biomolecules on
Earth.
Each year, photosynthesis converts more than 100
billion metric tons of CO2 and H2O into cellulose and
other plant products.
Certain carbohydrates (sugar and starch) are a dietary
staple in most parts of the world, and the oxidation of
carbohydrates is the central energy-yielding pathway in
most non-photosynthetic cells.
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2. Classification of Carbohydrates
There are three major size classes of carbohydrates
1. Monosaccharide (Greek, mono = one)
2. Oligosaccharides (Greek, oligo= few) 2-10
monosaccharide units.
3. Polysaccharides (Greek, Poly = many) >10
monosaccharide units.
The word “saccharide” is derived from the Greek
sakcharon, meaning “sugar”
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3. Monosaccharides
Monosaccharide consist of a single polyhydroxy
aldehyde or ketone unit. The most abundant
monosaccharide in nature is the six-carbon sugar D-
glucose, sometimes referred to as dextrose.
Monosaccharide has a backbone, which is un- branched,
single bonded carbon chain. One of the carbon atoms is
double bonded to an oxygen atom to form carbonyl
group. Each of the other carbon atoms has a hydroxyl
group.
Depending on the number of carbon atoms, the
monosaccharides are named trioses (C3), tetroses (C4),
pentoses (C5), hexoses (C6), heptoses (C7).
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4. Cont’d
There are two families of monosaccharides
1. Aldoses- Monosaccharides having aldehyde groups.
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6. Cyclization of monosaccharides
All monosaccharide's with five or more carbon atoms in the
backbone occur predominantly as cyclic (ring) structures in
which the carbonyl group has formed a covalent bond with
the oxygen of a hydroxyl group along the chain.
The formation of these ring structures is the result of a general
reaction between alcohols and aldehydes or ketones to form
derivatives called hemiacetals or hemiketals.
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7. Cont’d
These six-membered ring compounds are called
pyranoses because they resemble the six membered
ring compound pyran.
Aldohexoses also exist in cyclic forms having five-
membered rings, which, because they resemble the five
membered ring compound furan, are called furanoses.
Only aldoses having five or more carbon atoms can
form pyranose rings.
Reaction between the aldehyde group at C-1 and the
hydroxyl group at C-5 forms a hemiacetal linkage,
producing either of two stereoisomers, the anomers,
which differ only in the stereochemistry around the
hemiacetal carbon.
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8. Formation of the two cyclic forms of D-glucose
Pyranoses and furanoses
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9. Cont’d
The C-2 keto group in the open chain form of fructose
can react with the 5th carbon atom containing
hydroxyl group to form an intramolecular hemiketal.
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10. Physical properties of Monosaccharides
They are colorless, crystalline compounds, readily
soluble in water.
Their solutions are optically active and exhibit the
phenomenon of muta-rotation.
The interconversion of an anomers is called
mutarotation.
Carbohydrates spontaneously change between the and
β configuration.
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11. Stereoisomerism
Asymmetric carbon is a carbon that has four different
groups or atoms attached to it and having optically
activity in solution.
All the monosaccharide's except dihydroxyacetone
contain one or more asymmetric or chiral carbon atoms
and thus occur in optically active isomeric forms.
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12. Cont’d
The orientation of - OH and- H groups around the
carbon atom adjacent to the terminal primary alcohol
carbon determines its D or L form .
When the - OH group on this carbon is on the right, the
sugar is a member of the D-series, when it is on the left,
it is a member of the L-series. These D and L
configuration are also called Enantiomers.
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13. Optical Activity
The presence of asymmetric (chiral) carbon atom
causes optical activity. When a beam of plane-polarized
light is passed through a solution of carbohydrate it will
rotate the light either to right or to left.
Depending on the rotation, molecules are called
dextrorotatory (+) (D) or levorotatory (-) (L).
When equal amounts of D and L isomers are present,
the resulting mixture has no optical activity, since the
activities of each isomer cancel one another. Such a
mixture is called racemic or DL mixture.
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14. Cont’d
When sugars are different from one another, only in
configuration with regard to a single carbon atom
(around one carbon atom) they are called epimers of
each other.
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15. Cont’d
Isomeric forms of monosaccharides that differ only in their
configuration about the hemiacetal or hemiketal carbon atom
are called anomers. The hemiacetal (or carbonyl) carbon atom
is called the anomeric carbon.
The anomers of D-glucose interconvert in aqueous solution by a
process called mutarotation.
The two stereoisomers at the hemiacetal (anomeric) carbon are:
The alpha anomer: Where- OH group is down (Haworth)
The beta anomer: Where- OH group is up (Haworth)
Anomers are diastereomers (having different physical
properties)
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16. Reducing and non reducing sugar
Monosaccharides can be oxidized by relatively mild
oxidizing agents such as ferric (Fe3+) or cupric (Cu2+)
ion.
The carbonyl carbon is oxidized to a carboxyl group.
Glucose and other sugars capable of reducing ferric or
cupric ion are called reducing sugars.
The oxidation of a sugar’s anomeric carbon by cupric or
ferric ion occurs only with the linear form, which exists
in equilibrium with the cyclic form(s).
When the anomeric carbon is involved in a glycosidic
bond, that sugar residue cannot take the linear form
and therefore becomes a non-reducing sugar.
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17. Cont’d
In describing disaccharides or polysaccharides, the
end of a chain with a free anomeric carbon (one not
involved in a glycosidic bond) is commonly called the
reducing end.
By convention, the name describes the compound with
its non reducing end to the left, and we can “build up”
the name in the following order.
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18. Cont’d
1. Give the configuration ( ) at the anomeric carbon joining
the first monosaccharide unit (on the left) to the second.
2. Name the nonreducing residue; to distinguish five- and
six-membered ring structures, insert “furano” or
“pyrano” into the name.
3. Indicate in parentheses the two carbon atoms joined by the
glycosidic bond, with an arrow connecting the two
numbers; for example, (1 4) shows that C-1 of the first-
named sugar residue is joined to C-4 of the second.
4. Name the second residue. If there is a third residue,
describe the second glycosidic bond by the same
conventions.
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19. Cont’d
Non reducing disaccharides are named as glycosides; in
this case, the positions joined are the anomeric carbons.
In the abbreviated nomenclature, a double-headed
arrow connects the symbols specifying the anomeric
carbons and their configurations.
In contrast to maltose and lactose, sucrose contains no
free anomeric carbon atom; the anomeric carbons of
both monosaccharide units are involved in the
glycosidic bond.
Sucrose is therefore a non reducing sugar.
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20. 3.2. Oligosaccharides
Oligosaccharides contain 2 to 10 monosaccharide units,
joined by characteristic linkages called glycosidic bonds.
The most abundant oligosaccharides found in nature are
the disaccharides with two monosaccharide units.
When two monosaccharide's are covalently bonded
together by glycosidic linkages a disaccharide is formed.
Glycosidic bond is formed when the hydroxyl group
on one of the sugars reacts with the anomeric carbon
on the second sugar.
Biologically important disaccharides are sucrose,
maltose, and lactose.
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21. 1. Maltose
Maltose contains two D glucose residues joined by a
glycosidic linkage between OH at the first carbon atom
of the first glucose residues and OH at the fourth carbon
atom of the second glucose forming a -(1,4) glycosidic
linkage as shown in Figure below.
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22. 2. Lactose
Lactose is a disaccharide of -D galactose and -D-
glucose which are linked by -(1,4) glycosidic linkage.
Lactose acts as a reducing substance since it has a free
carbonyl group on the glucose. It is found exclusively in
milk of mammals (Milk sugar).
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23. 3. Sucrose (Cane sugar)
Sucrose is a disaccharide of - D- glucose and -D-
fructose. It is obtained from cane sugar. It is also
present in various fruits.
In contrast to other disaccharides sucrose contains no
free anomeric carbon atom. Since the anomeric
carbons of both its component monosaccharide units are
linked to each other. For this reason sucrose is non
reducing sugar.
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24. 3.3. Polysaccharides
The polysaccharides are sugar polymers containing
more than 20 or so monosaccharide units, and some
have hundreds or thousands of units.
Most of the carbohydrates found in nature occur in the
form of high molecular polymers called
polysaccharides.
There are two types of polysaccharides:-
1. Homopolysaccharides:- that contain only one type of
monosaccharide building blocks. Examples: Starch,
glycogen, Cellulose and dextrins.
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25. Starch
It is one of the most important storage polysaccharide in
plant cells. It is especially abundant in tubers, such as
potatoes and in seeds such as cereals.
Starch consists of two polymeric units made of glucose
called Amylose and Amylopectin but they differ in
molecular architecture.
I. Amylose is unbranched with 250 to 300 D-Glucose
units linked by -(1, 4) linkages.
II. Amylopectin consists of long branched glucose
residue (units) with higher molecular weight.
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26. Glycogen
Glycogen is the main storage polysaccharide of animal
cells. It is present in liver and in skeletal muscle.
Like amylopectin glycogen is a branched polysaccharide
of D-glucose units in - (1, 4) linkages, but it is highly
branched.
The branches are formed by -(1,6) glycosidic linkage
that occurs after every 8 -12 residues.
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27. Cellulose
Cellulose is the most abundant structural
polysaccharide in plants. It is fibrous, tough, water
insoluble.
Cellulose is a linear unbranched homopolysaccharides
of 10,000 or more D- glucose units connected by -(1,
4) glycosidic bonds.
Humans cannot use cellulose because they lack of
enzyme (cellulase) to hydrolyze the -( 1-4) linkages.
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28. Cont’d
iii. Glycoproteins (Mucoproteins)
Glycoprotiens are proteins to which oligosaccharides are
covalently attached.
They differ from the glycosaminoglycans in that the length
of the glycoproteins carbohydrate chain is relatively short.
The glycoprotein carbohydrate chains are often branched
instead of linear and may or may not be negatively
charged.
iv. Proteoglycans
When glycosamnoglycans are attached to a protein
molecule the compound is called proteoglycan.
[proteoglycans = Glycosaminoglycans + proteins]
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