Page 45 - Color Atlas of Biochemistry
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36 Biomolecules
Chemistry of sugars 5. Esterification. The hydroxyl groups of
monosaccharides can form esters with acids.
In metabolism, phosphoric acid esters such as
A. Reactions of the monosaccharides
glucose 6-phosphate and glucose 1-phosphate
The sugars (monosaccharides) occur in the (6) are particularly important.
metabolism in many forms (derivatives).
Only a few important conversion reactions
B. Polarimetry, mutarotation
are discussed here, using D-glucose as an ex-
ample. Sugar solutions can be analyzed by polarim-
1. Mutarotation. In the cyclic form, as op- etry, a method based on the interaction be-
posed to the open-chain form, aldoses have a tween chiral centers and linearly polarized
chiral center at C-1 (see p. 34). The corre- light—i. e., light that oscillates in only one
sponding isomeric forms are called anomers. plane. It can be produced by passing normal
In the β-anomer (center left), the OH group at light through a special filter (a polarizer). A
C-1 (the anomeric OH group) and the CH 2 OH second polarizing filter of the same type (the
group lie on the same side of the ring. In the α- analyzer), placed behind the first, only lets the
anomer (right), they are on different sides. polarized light pass through when the polar-
The reaction that interconverts anomers into izer and the analyzer are in alignment. In this
each other is known as mutarotation (B). case, the field of view appears bright when
2. Glycoside formation. When the anome- one looks through the analyzer (1). Solutions
ric OH group of a sugar reacts with an alcohol, of chiral substances rotate the plane of polar-
with elimination of water, it yields an ized light by an angle α either to the left or to
O–glycoside (in the case shown, α –methylglu- the right. When a solution of this type is
coside). The glycosidic bond is not a normal placed between the polarizer and the ana-
ether bond, because the OH group at C-1 has a lyzer, the field of view appears darker (2).
hemiacetal quality. Oligosaccharides and pol- The angle of rotation, α,is determined by
ysaccharides also contain O-glycosidic bonds. turning the analyzer until the field of view
Reaction of the anomeric OH group with an becomes bright again (3). A solution’s optical
NH 2 or NH group yields an N-glycoside (not rotation depends on the type of chiral com-
shown). N-glycosidic bonds occur in nucleo- pound, its concentration, and the thickness of
tides (see p. 80) and in glycoproteins (see the layer of the solution. This method makes it
p. 44), for example. possible to determine the sugar content of
3. Reduction and oxidation. Reduction of wines, for example.
the anomeric center at C-1 of glucose (2) pro- Certain procedures make it possible to ob-
duces the sugar alcohol sorbitol.Oxidation of tain the α and β anomers of glucose in pure
thealdehydegroup at C-1 gives the intramo- form. A 1-molar solution of α-D-glucose has a
lecular ester (lactone) of gluconic acid (a gly- rotation value [α] D of +112°, while a corre-
conic acid). Phosphorylated gluconolactone is sponding solution of β-D-glucose has a value
an intermediate of the pentose phosphate of +19°. These values change spontaneously,
pathway (seep.152). When glucoseis oxi- however, and after a certain time reach the
dized at C-6, glucuronic acid (a glycuronic same end point of +52°. The reason for this is
acid) is formed. The strongly polar glucuronic that, in solution, mutarotation leads to an
acid plays an important role in biotransforma- equilibrium between the α and β forms in
tions in the liver (see pp.194, 316). which, independently of the starting condi-
4. Epimerization. In weakly alkaline solu- tions, 62% of the molecules are present in the
tions, glucose is in equilibrium with the β form and 38% in the α form.
ketohexose D-fructose and the aldohexose D-
mannose, via an enediol intermediate (not
shown). The only difference between glucose
and mannose is the configuration at C-2. Pairs
of sugars of this type are referred to as epi-
mers, and their interconversion is called epi-
merization.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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