Chemistry Reference
In-Depth Information
Figure 6.
Overview of carbohydrates. The most common form of monosaccharides in biology are 5- or
6- atom ring structures, which are termed furanose and pyranose, derived from the elemental organic
ring structures furan and pyridine. The structures presented here are in a common format referred to as
Haworth projections; other structural representations may also be used, and the reader is referred to
contemporary biochemistry texts for these alternate formats. The most important feature of any ring
monosaccharide is the presence of an anomeric carbon, which represents the site of O atom attachment
in the ring, and, is the starting point for numbering of ring carbon atoms. The ring structure can undergo
reversible opening and closure in solution; the anomeric carbon in the linear form is either an aldehyde
or ketone group, which undergoes nucleophilic attack by one of the monosaccharide
OH groups to
reform the ring structure. This reversible reaction involves the loss of a water molecule, which is not
shown above. The glycosidic bond is the covalent bond that joins the anomeric carbon of one
monosaccharide to the
−
OH or other strong nucleophilic group of another monosaccharide. The type of
glycosidic bond, a or b, is defined by the position of the glycosidic bond (above, beta; below, alpha)
with respect to the ring structure of the first monosaccharide. The covalent assemblage of
monosaccharides leads either to the formation of oligosaccharide or polysaccharide chains. These
chains can be purely linear, feature branching points for additional chain attachment, or, can possess
any combination of linear and branching bonds.
−
Oligosaccharides and polysaccharides are created via polymerization of individual
monosaccharide units, using linkages called glycosidic bonds (Fig. 6), which involve
bond formation between the anomeric carbon of a monosaccharide with the
OH group
of another monosaccharide, with loss of a water molecule. This is called an O-glycosidic
bond. A glycosidic bond is usually designated as being either
−
, depending on the
position of the glycosidic bond (up, down) with respect to the ring structure of the
monosaccharide bearing the anomeric carbon. The interesting feature of a glycosidic
bond is that the donor
α−
or
β−
OH group can be from nearly any position on the monosaccharide
ring, which can lead to linear chains or branching chains. Any number of branch points
can occur within an oligosaccharide or polysaccharide. Add to this the number of
−
