Introduction
Carbohydrates are carbon compounds that contain large
quantities of hydroxyl groups. The simplest carbohydrates also contain either an
aldehyde moiety (these are termed polyhydroxyaldehydes) or a ketone
moiety (polyhydroxyketones). All carbohydrates can
be classified as either monosaccharides,
oligosaccharides or polysaccharides.
Anywhere from two to ten monosaccharide units, linked by glycosidic bonds, make
up an oligosaccharide. Polysaccharides are much larger, containing hundreds of
monosaccharide units. The presence of the hydroxyl groups allows carbohydrates
to interact with the aqueous environment and to participate in hydrogen bonding,
both within and between chains. Derivatives of the carbohydrates can contain
nitrogens, phosphates and sulfur compounds. Carbohydrates also can combine with
lipid to form glycolipids
or with protein to form glycoproteins.
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Carbohydrate Nomenclature
The predominant carbohydrates encountered in the body are structurally
related to the aldotriose glyceraldehyde and to
the ketotriose dihydroxyacetone. All carbohydrates
contain at least one asymmetrical (chiral) carbon and are, therefore, optically
active. In addition, carbohydrates can exist in either of two conformations, as
determined by the orientation of the hydroxyl group about the asymmetric carbon
farthest from the carbonyl. With a few exceptions, those carbohydrates that are
of physiological significance exist in the D-conformation. The mirror-image conformations, called
enantiomers, are in the L-conformation.
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| Structures of
Glyceraldehyde
Enantiomers | back to the
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Monosaccharides
The monosaccharides commonly found in humans are classified according to the
number of carbons they contain in their backbone structures. The major
monosaccharides contain four to six carbon atoms.
Carbohydrate Classifications
| # Carbons |
Category
Name |
Relevant
examples |
| 3 |
Triose |
Glyceraldehyde, Dihydroxyacetone |
| 4 |
Tetrose |
Erythrose |
| 5 |
Pentose |
Ribose, Ribulose, Xylulose |
| 6 |
Hexose |
Glucose, Galactose, Mannose,
Fructose |
| 7 |
Heptose |
Sedoheptulose |
| 9 |
Nonose |
Neuraminic acid
also called sialic
acid |
The aldehyde and ketone moieties of the carbohydrates with five and six
carbons will spontaneously react with alcohol groups present in neighboring
carbons to produce intramolecular hemiacetals or
hemiketals, respectively. This results in the
formation of five- or six-membered rings. Because the five-membered ring
structure resembles the organic molecule furan,
derivatives with this structure are termed furanoses. Those with six-membered rings resemble the
organic molecule pyran and are termed pyranoses.
Such structures can be depicted by either Fischer or Haworth style
diagrams. The numbering of the carbons in carbohydrates proceeds from the
carbonyl carbon, for aldoses, or the carbon nearest the carbonyl, for ketoses.
 |
 |
| Cyclic Fischer
Projection of a-D-Glucose |
Haworth Projection of
a-D-Glucose |
The rings can open and re-close, allowing rotation to occur about the carbon
bearing the reactive carbonyl yielding two distinct configurations (a and b) of the hemiacetals and
hemiketals. The carbon about which this rotation occurs is the anomeric carbon and the two forms are termed anomers.
Carbohydrates can change spontaneously between the a
and b configurations-- a process known as mutarotation. When drawn in the Fischer projection, the
a configuration places the hydroxyl attached to the
anomeric carbon to the right, towards the ring. When drawn in the Haworth
projection, the a configuration places the hydroxyl
downward.
The spatial relationships of the atoms of the furanose and pyranose ring
structures are more correctly described by the two conformations identified as
the chair form and the boat
form. The chair form is the more stable of the two. Constituents of
the ring that project above or below the plane of the ring are axial and those that project parallel to the plane are
equatorial. In the chair conformation, the
orientation of the hydroxyl group about the anomeric carbon of a-D-glucose is axial and equatorial in
b-D-glucose.
 |
| Chair form of a-D-Glucose |
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Disaccharides
Covalent bonds between the anomeric hydroxyl of a cyclic sugar and the
hydroxyl of a second sugar (or another alcohol containing compound) are termed
glycosidic bonds, and the resultant molecules are
glycosides. The linkage of two monosaccharides to
form disaccharides involves a glycosidic bond. Several physiogically important
disaccharides are sucrose, lactose and maltose.
- Sucrose: prevalent in sugar cane and sugar
beets, is composed of glucose and fructose through an a-(1,2)b-glycosidic bond.
 |
| Sucrose |
- Lactose: is found exclusively in the milk of
mammals and consists of galactose and glucose in a b-(1,4) glycosidic bond.
 |
| Lactose |
- Maltose: the major degradation product of
starch, is composed of 2 glucose monomers in an a-(1,4) glycosidic bond.
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| Maltose | back to the
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Polysaccharides
Most of the carbohydrates found in nature occur in the form of high
molecular weight polymers called polysaccharides.
The monomeric building blocks used to generate polysaccharides can be varied; in
all cases, however, the predominant monosaccharide found in polysaccharides is
D-glucose. When polysaccharides are composed of a single monosaccharide building
block, they are termed homopolysaccharides.
Polysaccharides composed of more than one type of monosaccharide are termed
heteropolysaccharides.
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Glycogen
Glycogen is the major form of stored carbohydrate in animals. This crucial
molecule is a homopolymer of glucose in a-(1,4)
linkage; it is also highly branched, with a-(1,6)
branch linkages occurring every 8-10 residues. Glycogen is a very compact
structure that results from the coiling of the polymer chains. This compactness
allows large amounts of carbon energy to be stored in a small volume, with
little effect on cellular osmolarity.
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Starch
Starch is the major form of stored carbohydrate in plant cells. Its
structure is identical to glycogen, except for a much lower degree of branching
(about every 20-30 residues). Unbranched starch is called amylose; branched starch is called amylopectin.
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This article has been modified by Dr. M. Javed Abbas.
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20:35 21/12/2002
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