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Carbohydrates.pdf
1. CARBOHYDRATES
• Introduction:
• Carbohydrates are widely distributed in nature. They are found
abundantly in plants and in animals. In plants, carbohydrates may be
found as sucrose, cellulose, starch and the like. In animals, they are
very important sources of food. They may be transformed to simpler
forms in the animal digestive tract. In the liver they are transformed
into glycogen also known as animal starch. When broken down to
glucose, they provide the main source of energy for living organisms.
Carbohydrates are likewise found in the connective tissues of animals
where they serve as lubricants in joints.
2. • The simplest form of carbohydrates are the monosaccharides. When
2 of these are linked together the resulting carbohydrate is called a
disaccharide. Both monosaccharides and disaccharides are called
sugars .Linking together of many thousands of monosaccharides lead
to the formation of larger carbohydrates known as polysaccharides.
3. CHEMISTRY OF CARBOHYDRATES
1. Definition
Carbohydrates – also known as saccharides, are polyhydroxy aldehydes or
ketones and their derivatives with at least three carbon atoms that possess a
carbonyl group.
- most abundant organic molecules
- represented by the simple stoichiometric formula (CH2O)n hence
used to be known as “hydrated carbon”.
- functions: provide significant fraction of the energy in the diet
of most organisms
storage form of energy in the body; used as a primary cellular
fuel
serve as cell membrane components that mediate some forms of
intercellular communication
serve as structural components of many organisms
4. 2. CLASSIFICATION
Classification of carbohydrates is based on their ability to be hydrolyzed
to smaller compounds, by the number of carbon atoms, by the direction
of rotation of polarized light, and by the structural relationships to the
three-carbon sugar glyceraldehyde.
2.1 Monosaccharides are simple, monomeric sugars that can no longer
be hydrolyzed to smaller compounds. They consist of a single poly-
hydroxy aldehyde or ketone unit.
2.1.1 They may be further classified, depending on the number of
carbon atoms present into:
5. - Trioses: the smallest molecules composed of 3 carbon atoms. There
are two trioses, the glyceraldehyde (aldose) and the
dihydroxyacetone (ketose), which have the same atomic composition
but differ in the location of their hydrogen and double bonds
(tautomers).
- Tetroses: composed of 4 carbon atoms, (CH2O)4, include threose,
erythrose, and ketose.
- Pentoses: of major biological importance consisting of 5 carbon
atoms. Examples are ribose, xylose, arabinose and rhamnose
- Hexoses: 6 carbon atoms like glucose, fructose, mannose and
galactose
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6. 2.1.2 Depending on the kind of functional group present,
monosaccharides can be classified into:
a. Aldoses - contain the aldehyde group, -(H)C=O
b. Ketoses - possess the ketone group, - C=O
c. Derived monosaccharides contain functional groups other than
carbonyl or hydroxyl
2.2 Disaccharides are groups of carbohydrates which on hydrolysis yield
two monosaccharides which may be the same, or different from each
other .Examples are sucrose, lactose, maltose and iso-maltose.
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7. They may be distinguished from one another from:
a. the two specific sugar monomers involved, and their
stereo configurations
b. the carbons involved in the linkage
c. the order of 2 monomer units, if they are of different kinds
d. anomeric configuration of the hydroxyl group on carbon 1 of
each residue
2.3 Oligosaccharides contain three to twelve units of monosaccharides
joined by glycosidic linkage. Maltotriose and raffinose belong to
this group.
2.4 Polysaccharides are polymers of more than 12 monosaccharide
units joined in long linear or branched chains. Of interest and
importance in this group are starch, glycogen and cellulose.
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8. 3. MONOSACCHARIDES
3.1 These are polyhydroxy aldehydes and polyhydroxy ketones or their
derivatives which have the empirical formula (CH2O)n where n=3
or some large number. A characteristic of the simple sugar is the
presence of the carbonyl ketone group (-C=O), or the carbonyl
aldehyde group (-CHO).
- If the carbonyl group is at the end of the chain, the monosaccharide
is an aldehyde derivative and called an aldose.
The simplest aldose is glyceraldehyde or glycerose.
- If the carbonyl group is at any other position, it is a ketone derivative
and called a ketose, the simplest of which is dihydroxy acetone.
9. 3.2 Monosaccharides are represented using the Fischer projection
formulas given below:
H-C=O CH2-OH
H-C-OH C=O
CH2OH CH2-OH
Glyceraldehyde Dihydroxyacetone
The carbon atoms are identified by corresponding numbers, with
carbon 1(C1) being the carbon of the carbonyl group (functional group)
in glyceraldehyde and C2 in dihydroxyacetone.
10. 3.3 Isomerism
Isomers are compounds that have the same structural or
chemical formula but differ in spatial configuration. The presence of
asymmetric carbon atoms to which four different groups or radicals are
joined allows the formation of isomers. The number of possible
isomers of a sugar depends upon the number of asymmetric carbon in
the molecule, expressed as 2n where n is the number of asymmetric
carbon.
3.3.1 D and L configuration. It the OH group on the asymmetric
carbon farthest from the carbonyl group points to the right (dextro),
the sugar is the D isomer. If the OH group points to the left (levo), it is
the L isomer.
11. The D and L isomers are mirror images of each other and are known
as enantiomers. Most simple sugars that occur in mammals are of the
D configuration.
H-C=O H-C=O
H-C-OH HO-C-H
CH2OH CH2OH
D-Glyceraldehyde L-Glyceraldehyde
3.3.2 Epimers. These are isomers that differ only in the configuration
around one specific carbon atom. Glucose and mannose are
epimers with respect to carbon atom 2, while glucose and
galactose are epimers with respect to carbon 4.
12. 3.3.3 Optical activity. When a beam of polarized light is allowed to pass
through a solution of an optical isomer, it will be rotated either to the
right, dextrorotatory (+), or to the left, levorotatory (-), D and L- sugars
are not necessarily d-or l-, respectively.
3.3.4 Cyclization of monosaccharides (the ring structure). Most
monosaccharides with five or more carbon atoms in the chains give
these compounds the potential to form very stable ring structures.
Where the aldehyde or ketone group reacts with an alcohol group on
the same sugar, they form a hemiacetal or hemiketal ring, respectively.
The hydrogen of the hydroxyl group on the penultimate carbon shifts to
the carbonyl group with the formation of an oxygen (O). If the resulting
ring has six members (5 carbons and 1 oxygen), it is a pyranose ring, and
if five-membered (4 carbons and 1 oxygen), it is called a furanose ring.
14. 3.3.5 Anomeric carbon. With the ring formation, the first carbon
becomes asymmetric resulting in the creation of an anomeric
carbon designated as alpha (ɑ) or beta (ß) anomer. When the OH
group on the new asymmetric carbon (anomeric carbon ) is on
the same side of the oxygen ring, the sugar is an alpha anomer,
and ß anomer if on the opposite side of the oxygen ring. The
cyclic alpha or ß anomers of sugars in solution are in equilibrium
with each other, and can readily undergo spontaneous
interconversion using the open-chain structure leading to the
process of mutarotation.
15. 3.3.6 Pyranose and furanose ring structures. (Haworth projection). The
stable ring structure of monosaccharides resemble the ring structures
of either pyran or furan, hence glucopyranose, glucofuranose,
fructopyranose and fructofuranose.
CH2OH
HO-C-H H O OH
H-C-OH H
HO H H
HO-C-H O OH
H-C-OH H OH
H-C
CH2OH
ß-D-Glucopyranose ß-D-Glucopyranose
17. In the Haworth projection, the alpha (ɑ) designation is used if the
OH group of the anomeric carbon is below the plane of the ring and ß if
the OH group is above the plane of the ring.
3.3.7 Chair and boat conformation. The actual conformation of the
pyranose and furanose rings are not planar. Instead, the pyranose
ring exists in the more stable “chair” and the less favored “boat” forms.
The OH groups exist in either axial (vertical) or equatorial (non-vertical)
position depending on whether they are approximately parallel or
perpendicular to the axial, which can be defined perpendicular to the
central plane of the molecule.
19. 4. BIOLOGICALLY IMPORTANT SUGARS (Derivatives of Monosaccharides)
4.1 Glycosides- asymmetric mixed acetals formed by the reaction of the
anomeric carbon atom of the intermolecular hemiacetal or pyranose form
of the aldohexose with a hydroxyl group furnished by an alcohol. The
anomeric carbon in glycosides do not interconvert by mutarotation in the
absence of an acid catalyst.
The glycosidic linkage is also formed by the reaction of the
anomeric carbon of the monosaccharide with a hydroxyl group of another
monosaccharide to yield a disaccharide.
They are also hydrolyzed by enzymes called glycosidase which
differ in their specificity according to the type of glycosidic bond (ɑ or ß ),
structure of the monosaccharide unit (s); and the structure of the alcohol.
20. 4.2 Neutral Sugars
CH2OH
H O H HOCH2 O
H CH2OH
OH H H HO
HO OH H OH
H OH HO H
ɑ-D-Galactopyranose ɑ-D-Fructofuranose
21. CH2OH CH2OH
HO O H H O H
H H
OH H OH HO
H OH HO OH
H OH H H
ɑ-D-Galactopyranose ɑ-D-Mannopyranose
22. 4.3 Sugar Acids
The oxidation of the terminal group/s (either the aldehyde group
or the alcohol group, or both) to carboxylic group /s produces three
different acid derivatives of the aldoses.
4.3.1 Aldonic acid is formed from the oxidation of the aldehyde
group.
4.3.2 Uronic acid is produced by oxidation of the terminal carbon
(primary alcohol group) to a carboxyl group.
4.3.3 Aldaric acid results from the oxidation of both aldehyde group
and the terminal carbon (primary hydroxyl group) to carboxyl group,
also called saccharic acids.
23. COOH CHO COOH
(CHOH)n (CHOH)n (CHOH)n
CH2OH COOH COOH
Aldonic acid Uronic acid Aldaric acid
4.4 Amino Sugars
Hexosamines are formed through substitution of a hydroxyl group
by an amino group on the sugar ring. The amino group is usually
acetylated (N-acetylglucosamine and N-acetylgalactosamine) or may
also be sulfated (heparin). Sialic acid is a ketose that is an acetylated
derivative of neuraminic acid. Other examples are muramic acid and N-
glycolyl derivaties of hexosamines.
24. CH2OH CH2OH
H O H H O H
H H
OH H OH H
HO OH HO CH
O
H NH2 H H-N C-ON
alpha-D-Glucosamine N-acetyl-alpha-D-glucosamine
26. 4.5 Pentoses
The major pentoses are D- ribose, L-arabinose, D-xylose and
L-xylulose.
O CHO
HOCH2 OH HO-C-H
H-C-OH
H H O H-C-OH
H CH2OH
OH OH
ß-D-Ribofuranose D-Arabinose
27. 4.6 Deoxysugars
These are formed thru selective reduction, when a hydroxyl group
attached to the ring structure has been replaced by a hydrogen atom.
The most important is 2-deoxy-D-ribofuranose.
O
HOCH2 OH
H H O
H
OH H
2-deoxy ß-D-Ribofuranose
28. 4.7 Phosphoric Acid Esters of Monosaccharides
These are formed by the reaction of the sugar with a phosphate group which usually comes
from a high-energy compound such as adenosine triphosphate (ATP).
O
CH2OH CH2-C-P-OH
O
O O
H H H H
O
HO OH H O-P-OH HO OH H OH
O
H OH H OH
Glucose-1-phosphate D Glucose-6-phosphate
29. 4.8 Sugar Alcohols
Polyols result from the reduction of the carbonyl group of the sugar
to alcohol.
glyceraldehyde – glycerol
glucose – sorbitol or glucitol
mannose- mannitol
galactose – dulcitol or galactitol
ribose – ribitol
31. 5. OLIIGOSACCHARIDES
5.1 Homo-oligosaccharides yield only one kind of sugar upon
hydrolysis.
Maltose (D-glucopyranosyl-ɑ-1,4-ɑ-D-glucopyranose)
CH2OH CH2OH
H O H H O H -formed as an intermediate
OH H O OH product of the action of
HO OH amylase on starch
H OH H OH - contains 2-D- glucose
residues
Ma-Is- Cell
34. 5.2 Hetero-oligosaccharides yield more than one kind of sugar units
upon hydrolysis.
5.2.1 Lactose (D-galactopyranosyl-ß-1, 4-ɑ-D-glucopyranose)
CH2OH CH2OH
HO O O -has a free anomeric carbon
on the glucose residue
O hence in reducing sugar
OH OH OH
OH OH
35. 5.2.2 Sucrose (D-glucopyranosyl-ɑ-1, 2- D –fructofuranose)
H O HOCH2 O H
OH H
HO O CH2OH
H OH 1, 2 OH
- cane sugar or table sugar
- no free anomeric carbon hence
does not undergo mutarotation
- hydrolysis-inversion
36. 5.2.3 Glycoproteins are carbohydrates linked to protein by covalent
combination. Protein linkage may either be N-linked or O-linked.
N-acetylglucosamine-asparagine linkage
CH2OH O
H O H N NH
N - C - CH2 - CH
C=O N-linked
OH
HO
O
NH - C - CH3
38. Galactose - hydroxylysine linkage
CH2OH NH2
HO O CH2
OH H O CH
H H CH2 O-linked
H OH CH2
CH-NH2
C=O
Xylose-serine linkage
(Glycans), on complete hydrolysis with acid or specific enzymes
monosaccharides and/or simple monosaccharide derivatives.
39. 6. POLYSACCHARIDES
5.1 Homoglycans are polysaccharides that consist of only one type
of repeating monosaccharide unit.
- Amylose is a linear chain of alpha-D glucose units linked via
alpha-1, 4 glycosidic bonds. The repeating disaccharide of
this unbranched polymer is maltose.
- Amylopectin is a homoglycan consisting of a branched chain
structure composed of alpha-D-glucose units in
40. alpha-1, 4 linkages and alpha-1, 6 linkages at branching points.
• - Starch is a mixture of amylose and amylopectin.
• - Glycogen resembls amylopectin in having alpha-1, 6 branches from
an alpha-1, 4 chain, although it is more branched.
- Cellulose consists of a long, unbranched chain of D-glucose units with
B-1, 4 linkages. The repeating unit is B-cellobiose.
6.2 Heteroglycans are composed of two or more different mono-
saccharide residues.
- Pectins consist of D-galacturonic acid units with alpha-1, 4
glycosidic linkages, with some of the carboxyl groups present as
methyl esters.
41. • - Mucopolysaccharides are built up from more than one type of
monosaccharide units. These components may be aminated,
sulfated, or N-acetylated.
- Hyaluronic acid is an acidic mucopolysaccharide made up of
repeating disaccharide units of glucuronic acid joined to
N-acetylglucosamine.
- Heparin is a sulfated, acidic mucopolysaccharide that consists
of an O-sulfated glucuronic acid bound to an N-sulfated
glucosamine that has a second O-sulfate group.
- Chondroitin sulfate contains the repeating disaccharide unit of
glucuronic acid joined to N-acetylgalactosamine sulfate.
42. References:
Biochemistry by: Campbell
Harper’s Illustrated Biochemistry by: Murray, Robert K., et al.
Lehninger’s Principles of Biochemistry by: Cox, et al.
UPOU Manual in Biochemistry
On-Line references
