section 9.1
Classification
147
serendipity berries, respectively. These two proteins are in-
tensely sweet and produce a perception of sweetness at a
concentration as low as 10
- 8
mol/L. Despite the similarity
in sweetness, thaumatin and monellin bear no significant
structural similarities with respect to amino acid sequence
or crystalline structure. However, they do exhibit immuno-
logical cross-reactivity suggesting a common chemical
and structural feature. Sweet substances may act as short-
term antidepressants, presumably by raising serotonin (a
metabolite of tryptophan) levels in the central nervous sys-
tem. This property of sweet-tasting carbohydrates may
unwittingly contribute to the development of obesity in
susceptible individuals.
Polysaccharides
Polysaccharides, also known as
glycans,
contain many
monosaccharide
units joined
together by
glycosidic
linkages. They may be homopolysaccharides (e.g., glyco-
gen, starch, and cellulose), which contain only one type
of monomeric residue, or heteropolysaccharides, which
consist of two or more different types of monosaccha-
ride units glycosidically joined in different ways. The het-
eropolysaccharides have complex structures, and they may
also be found covalently linked with proteins and lipids
(e.g., proteoglycans and glycosphingolipids).
Starch and glycogen are energy storage forms of car-
bohydrate and are thus known as
storage carbohydrates.
When the supply of carbohydrate exceeds the needs of the
cell, the excess is converted to storage forms. When the
situation is reversed, the storage forms are converted to
usable forms of carbohydrate. Therefore, a storage carbo-
hydrate should be capable of rapid synthesis as well as
breakdown in response to the energy requirements of the
cell. As monosaccharide accumulates in the cell, its rapid
conversion to insoluble, high-molecular-weight polysac-
charide prevents an osmotic imbalance and also maintains
a favorable concentration gradient between the intra- and
extracellular compartments, which facilitates sugar trans-
port. Starch, the storage polysaccharide of most plants and
particularly of tubers (e.g., potatoes) and seeds (corn and
rice), consists of a mixture of
amylose
and
amylopectin.
Amylose is an unbranched polymer of glucose in which
the glucosyl residues are linked in a(l -> 4) glycosidic
linkages (Figure 9-21). The conformation of amylose has
been elucidated by the use of stable amylose complexes
prepared by reacting amylose with iodine. X-ray diffrac-
tion studies of such complexes have revealed a helical
conformation with six glucose residues per turn of the
helix. The amylose-iodine complex has an intense blue
color, which provides the basis for the iodine test for
starch.
Amylose
Amylopectin
FIGURE 9-21
Structures of the starch polysaccharides amylose and amylopectin.
Amylopectin contains glucosyl units joined together in
both
a{\
4) and a(l ->
6
) linkages, the latter linkages
being responsible for branch points (Figure 9-21). Unlike
amylose, amylopectin is unable to assume a stable heli-
cal conformation because of the branching. Amylopectin
complexes with iodine to a much lesser extent than amy-
lose; therefore, the amylopectin-iodine complex has a red-
violet color that is much less intense than the blue of the
amylose-iodine complex. Starch from different sources
contains different amounts of amylose and amylopectin.
In most plants, amylopectin is the more abundant form
(about 75-80%). Virtually no amylose is found in starch
obtained from some waxy varieties of maize (com) and
rice. Starch is easily digested by humans (Chapter 12).
Glycogen is the animal equivalent of starch in plants and
functions as the main storage polysaccharide in humans. It
is a branched polysaccharide of D-glucose and, like amy-
lopectin, contains both a (l —» 4) and a(l —►
6
) linkages,
the latter forming branch points (Figures 9-22 and 9-23).
Each molecule of glycogen contains one reducing glucose
