Page 50 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN006P-81 June 29, 2001 21:48

834 Glycoconjugates and Carbohydrates

proteoglycans. Has a linear structure that features a CHO CHO
repeating disaccharide unit comprised of glucosamine
H C OH HO C H
or galactosamine and a uronic acid or galactose. The
amino sugars are generally N-acetylated and contain CH 2 OH CH 2 OH
ester sulfate. The pattern of sulfation varies and may D-Glyceraldehyde L-Glyceraldehyde
confer specific biological properties.
Fischer projection formulas
Glycoside Acetal or ketal formed by substitution of the
anomeric hydroxyl with an aglycone. This reaction
fixes the configuration at the anomeric center. Link- CHO CHO
ages are generally acid labile. H C OH HO C H
Ketose (ulose) Polyhydroxy ketone (at carbon 2) of four
or more carbon atoms (see Aldose). CH 2 OH CH 2 OH
Mutarotation The change in optical rotation associated D-Glyceraldehyde L-Glyceraldehyde
with the equilibration, in solution, of the anomeric and Perspective formulas
ring forms of a saccharide beginning with a single crys-
FIGURE 1 Projection formulas of D- and L- glyceraldehyde.
talline form. Thus, dissolving α-D-glucopyranose in
water allows formation of the β-anomer as well as the
furanose forms. Since all have different optical rota- drates extends beyond the food industry to textiles, phar-
tions, the initial value changes until the equilibrium maceuticals, and a wide range of chemicals.
mixture is achieved. The common definition of carbohydrates is polyhy-
Oligosaccharide Sugars that contain 2–10 monosaccha- droxy aldehydes or ketones (the carbonyl group is gen-
ride units. These may be linear or branched. The most erally at C-2), their derivatives, oligomers and polymers.
common representatives are derived from aldoses and The term itself arises from the empirical formula of the
will contain a single reducing terminus (free, or po- compounds initially studied (glucose, for example) since
tential, aldehydo group) and one or more nonreducing that is represented by C(H 2 O) n .
termini (linked to other sugars only at their anomeric An appreciation of the diversity available in such struc-
hydroxyl group). tures began about 1900 with the work of van’t Hoff on
Polysaccharide Structure containing more than 10 the tetrahedral carbon atom, extrapolated by Emil Fischer
monosaccharide units. These may be the same (homo- in his elucidation of the stereochemistry of glucose. Dur-
polysaccharide) or different (heteropolysaccharide), ing that series of experiments, Fischer was able to deduce
and the structure may be linear or branched. Natural the relative positions of all of the hydroxyls at the asym-
diversity is very large. metric centers and could relate glucose to glyceraldehyde
Proteoglycan Special category of glycoproteins in which (Fig. 1). However, he was unable to provide an absolute
the saccharide chain is covalently linked to protein via stereo structure for glyceraldehyde and thus had to decide
an initiating xylose residue, a specific core oligosac- between mirror image structures for natural glucose. That
charide, and is followed by a glycosaminoglycan he chose the correct one may reflect little more than that
chain. he was right handed. He did synthesize all of the possible
Pyranose Six-membered ring form of a saccharide aldohexoses.
formed by reaction of the appropriate hydroxyl group
with the carbonyl carbon. Generally the most stable
structure in solution. The conformer resembles cyclo- I. MONOSACCHARIDES
hexane and is generally a chair form with axial and
equatorial substituents. It is generally accepted that monosaccharides, or simple
sugars, may have three to nine carbon atoms in a linear
chain. Thus the nomenclature aldotriose, aldotetrose, al-
THE UTILIZATION of carbohydrates as food sources, dopentose, etc., or ketotetrose, ketopentose, etc. The vast
sweetening agents, and clothing materials dates back sev- majority of naturally occurring sugars contain five or six
eral thousand years. Likewise, the production of beer and carbon atoms. These saccharides are classified as D-or L-
wine was known in ancient times although the relation- if they can be derived from D- or L-glyceraldehyde, respec-
ship to saccharides and fermentation was not appreciated tively, or the four carbon ketoses (Fig. 2). The determin-
then. Manufacture of sugar as a sweetener including re- ing asymmetric center is that farthest from the carbonyl
finements such as decolorization with charcoal also has group—projection of the hydroxyl group to the right is
ancient origins. The commercial importance of carbohy- D. As the chain length increases, new asymmetric centers