Structure of saccharides
Polysaccharides
It is estimated that approximately 4x1011 tons of carbohydrates are biosynthesized each year
on earth by plants and photosynthesizing bacteria. The majority of these carbohydrates are
produced as polysaccharides. Polysaccharides are macromolecules consisting of a large
number of monosaccharide residues. They are sometimes also called glycans. (The term
'glycan' is also used for the saccharide component of a glycoprotein even though the chain
length may not be large; cf. chapter 6). For polysaccharides which contain a substantial
proportion of amino sugar residues the term glycosaminoglycan is a common one.
Polysaccharides which consist of only one kind of monosaccharide are called
homopolysaccharides (homoglycans); when they are built up of two or more different
monomeric units they are named heteropolysaccharides (heteroglycans). In the latter type,
the monosaccharide units are usually linked to each other in a definite pattern, rather than
randomly. Certain sequences of monomeric building blocks are often found to be regularly
repeated as so–called repeating units. Homo - as well as heteropolysaccharides can be linear
or branched.
For polysaccharides, as for every polymer, it is not possible to attribute one distinct
molecular weight as they are polydisperse molecules, which are characterized by an average
molecular weight. The number of monosaccharide units in a polysaccharide is termed degree
of polymerization or d.p. The size of a polysaccharide varies between approximately 16,000
and 16,000,000 daltons (Da).
Polysaccharides exist in an enormous structural diversity as they are produced by a geat
variety of species, including microbes, algae, plants and animals. Among these are fructans,
xanthans, fucans, bacterial gel polysaccharides, capsule polysaccharides of bacteria, or
agar, which is a mixture of two polysacccharides and is obtained from red–purple seaweeds.
The most well–known polysaccharides are starch, glycogen, cellulose and chitin.
Starch
Starch is a mixture of two glucans (polysaccharides built from glucose), which are called α–
amylose and amylopectin. It is synthesized by plants as their principal food reserve and
deposited in the plant cell cytoplasm as insoluble granules.
α–Amylose is a linear polymer of several thousand glucose residues, α–(1,4)–
glycosidically linked. This polymer adopts an irregularly aggregating helically coiled
conformation containing regular left–handed helix regions. Amylopectin on the other hand,
carries α–(1,6)–connected branches every 24 to 30 glucose residues of the α–(1,4)–linked
chain, resulting in a tree– or brush–like structure. It contains up to a million glucose
residues which makes it among the largest molecules occurring in nature.
Starch is degraded by enzymes called amylases, which randomly hydrolyze the α–(1,4)–
glycosidic bonds digesting the polysaccharide into oligosaccharide fragments, such as
maltose and maltotriose as well as oligosaccharides containing α–(1,6)–branches, the latter
being called dextrins. The oligosaccharides produced by amylase digestion are further
hydrolyzed to glucose by specific glucosidases and by debranching enzymes, which
remove the α–(1,6)–branches.
Glycogen
