The ability of the anomeric hydroxyl group of a sugar to
be substituted with another sugar (via one of its hydroxyl
groups), and for this process to be iterated leads to formation
of di- and higher saccharides. Because several hydroxyls
are available for such linkages and the anomeric configuration
can vary as well, the number of possible structures
grows exponentially even for oligomers derived from the
same sugar. This may explain, in part, why many biological
recognition events involve saccharides as a ligand.
Two disaccharides are widely distributed in nature:
sucrose and lactose. Sucrose (α-
D-glucopyranosy
L-β-Dfructofuranoside)
is the table sugar of commerce and a
major industrial product (Fig. 18); primary sources are
sugar cane and beets. This saccharide and its source material
(cane molasses) serve as the basis for rum production;
cruder precursors are important medium additives for the
production of antibiotics. The sweetness of sucrose, and
other simple sugars, is a major aspect of their commercial
importance. The fact that sucrose is a nonreducing sugar
contributes to its stability and market value.
Lactose (β-
D-galactopyranosy
L-1-4-
D-glucopyranose)
is the major sugar of milk (Fig. 19). It is of interest that
the ability to utilize either sucrose or lactose as a source of
calories is dependent on their enzymatic hydrolysis to the
constituent monosaccharides in the intestine. About onethird
of the oriental population lacks the requisite galactosidase
and is thus lactose intolerant; this same problem
occurs in some infants (developmentally related) and results
in a “colicky” baby.
Other naturally occurring disaccharides of interest are
maltose (α-
D-glucopyranosy
L-1-4-
D-glucopyranose), an
intermediate in the digestion of starches, and trehalose
(α-
D-glucopyranosy
L-α-
D-glucopyranoside), the major
sugar of insect blood (Fig. 20).
Naturally occurring tri- and higher oligosaccharides
(less than 10 sugar units) are rare. Raffinose (3), stachyose
(4), and verbascose (5) are all assembled on a sucrose core
with substituents originating on the glucose moiety.
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| Figure 19 Structure of lactose, the major sugar present in milk. |
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| Figure 20 Structure of trehalose, the predominant sugar
present in the blood of insects. |
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