CARBOHYDRATES
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Carbohydrates are a major energy source
for living organisms
Carbohydrates always have a 1:2:1 ratio of
carbon, hydrogen, and oxygen.
Mitochondria break down carbohydrates
and use the chemical energy released from
its bonds for its live activities
CLASSFICATION OF
CARBOHYDRATES
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MONOSACCHARIDES – simple sugars
classified according to the number of carbon
atoms they contain:
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3 carbons: trioses (glyceraldehyde)
4 C: tetroses (erythrose)
5 C: pentoses (ribose)
6 C: hexoses (glucose)
7 C: heptoses (sedoheptulose)
9 C: nonoses (neuraminic acid)
ALDOSES AND KETOSES
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Carbohydrates with an aldehyde (-CHO)
as their most oxidized functional group are
called aldoses e.g. glyceraldehyde
(CH2OH-CHOH-CHO);
Those with a keto group (-C=O) as their
most oxidized functional group are ketoses
e.g.dihydroxyacetone
(CH2OH-C=O-CH2OH)
Aldose carbohydrates normally have a
suffix “-ose” and ketoses end with suffix “ulose”.
GLYCOSIDES
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Monosaccharides can be linked by glycosidic bonds (joining
of 2 hydroxyl groups of sugars by splitting out water molecule)
to create larger structures.
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Disaccharides contain 2 monosaccharides e.g. lactose
(galactose+glucose); maltose (glucose+glucose);
sucrose (glucose+fructose)
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Oligosaccharides – 3 to 12 monosaccharides units e.g.
glycoproteins
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Polysaccharides – more than 12 monosaccharides units e.g.
glycogen (homopolysaccharide) having hundreds of sugar
units; glycosaminoglycans (heteropolysaccharides) containing
a number of different monosaccharides species.
SIMPLE SUGARS
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The monomers of carbohydrates are called
monosaccharides and are also called simple
sugars.
They are usually ring-like and are composed of five
or six carbons.
They are either a polyhydroxy aldehyde or a
polyhydroxy ketone, which means they have more
than one hydroxide group (-OH) and one carbonyl
group (C=O).
Some popular monosaccharides are glucose,
fructose, and galactose
STRUCTURES OF SUGARS
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ISOMERS- compounds that have the same chemical formula e.g.
fructose, glucose, mannose, and galactose are isomers of each other
having formula C6H12O6.
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EPIMERS- refer to sugars whose configuration differ around one
specific carbon atom e.g. glucose and galactose are C-4 epimers and
glucose and mannose are C-2 epimers.
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ENANTIOMERS- a special type of isomerism found in pairs of
structures that are mirror images of each other. The mirror images
are termed as enantiomers and the two members are designated as
D- and L- sugar. The vast majority of sugars in humans are D-sugars.
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CYCLIZATION OF SUGARS- most monosaccharides with 5 or more
carbon atoms are predominately found in a ring form, where the
aldehyde or ketone group has reacted with an alcoholic group on the
same sugar group to form a hemiacetal or hemiketal ring.
Pyranose ring- if the ring has 5 carbons and 1 oxygen.
Furanose ring- if the ring is 5-membered (4 carbons and 1 oxygen).
STRUCTURES Cotnd.
ANOMERIC CARBON- formation of
hemiacetal or hemiketal ring results in the
creation of an anomeric carbon at carbon 1
of aldose and carbon 2 of a ketose. These
structures are termed as α or β
configurations of the sugar e.g. α-D-glucose
(C5 H10 O5 - H-C-OH) or β-D- glucose
(C5 H10 O5 - OH-C-H). Enzymes are able to
distinguish between these two structures
and use one the other preferentially.
STRUCTURES Cotnd.
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MUTAROTATION- the cyclic α and β anomers of a sugar
in solution are in equilibrium with each other, and can readily
be interconverted. This process requires the ring structure
open to linear form during the interconversion and is resulted
due to the characteristic changes in the optical rotation of
polarized light.
REPRESENTATION OF SUGAR CONFORMATION. The
hemiacetal ring of sugar is called a ‘Fisher projection’. It also
can be represented by the cyclic form of ‘Haworth projection’.
A 3-dimensional conformation of sugars in nature can be
represented in a ‘boat’ and ‘chair forms.
D- and L- enantiomers
FISHER PROJECTION
GLUCOSE - HAWORTH
PROJECTION
FRUCTOSE AND RIBOSE
REACTIONS INVOLVING
MONOSACCHARIDES
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Reducing sugars- if the oxygen on the anomeric carbon (the
carbonyl group) of a sugar is not attached to any other
structure, that sugar is a reducing sugar. A reducing sugar can
react with chemical reagents (e.g. Benedict’s solution) and
reduce the reactive component. The anomeric carbon itself
becomes oxidized.
Oxidation-reduction reactions- oxidation of the –CH2-OH
group at the carbon 6 produces a – uronic acid. Reduction of
the carbonyl carbon (aldehyde or keto group) produces a new
alcohol group (polyols) e.g. glucose is reduced to sorbitol,
fructose to sorbitol and mannitol, galactose to galactitol.
Reduction of hydroxyl groups produce deoxysugars e.g.
ribose is converted to 2-deoxyribose, an essential component
of DNA.
Reactions cotnd.
Carbohydrates can also be attached by glycosidic
bonds to non-carbohydrate structures to form
‘complex carbohydrates’. The non-carbohydrate
portion of the molecule is called the aglycone and
the entire molecule can be called by the generic
name glycoside. The aldose, the carbon 1 or
ketose, the carbon 2, which participates in the
glycosidic link is called the a glycosyl residue. E.g.
if the anomeric carbon of glucose participates in
such a bond, that sugar is called a glucosyl residue.
If the identity of the attached sugar is known, its
name can be used – glucoside or galatoside.
OLIGOSACCHARIDES
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These contain 2-10 units of
monosaccharides (monomers) linked to
each other via glycosidic linkages, e.g.
sucrose, lactose, maltose, raffinose,
stachyose etc.
Glycosidic bonds: The anomeric hydroxyl
group and a hydroxyl group of another
sugar or some other compound can join
together, splitting out water to form a
glycosidic bond.
Glycosidic bond diagram
Glucosidic bond cont.
SUCROSE
LACTOSE
MALTOSE
POLYSACCHARIDES
Carbohydrates which are composed of
more than then monosaccharide units
linked to each other through glycosidic
bonds are referred to as
polysaccharides. These include
carbohydrates like cellulose, starch,
glycogen, fructans, arabinans etc.
CELLULOSE
GLYCOGEN