General Index
Oligosaccharides
Index
Disaccharides
Holosides formed from two monosaccharides are called Disaccharides. The union can take place in two different ways:
1. The anomeric carbon of one with al alcoholic group of the other. In this case, the second monosaccharide is the aglycone. Its
anomeric carbon remains free and maintains its reducing properties. We then talk of Reducing Disaccharides.
2. The union can also be formed between both anomeric carbons. In this case, the anomeric carbons looses their reducing properties
and the resulting compound is a Non-reducing Disaccharide.
We present now the structure of three reducing disaccharides, Maltose, Cellobiose and Lactose; and two
non-reducing disaccharides, Trehalose and Sucrose.
This disaccharide results from the union of two D-glucoses in such a way that the anomeric carbon of one forms a glycosidic bond with
the -OH group in the carbon 4 of the other, remaining the first in the a- configuration. Thus, the systematic name of
the maltose is a-D-glucopiranosyl- (1,4)-D-glucopyranose. The free anomeric carbon of the other shows reducing properties
and anomeric forms a- and b-. Maltose is obtained by hydrolysis of starch.
The axis of both pyranose rings form an angle of approximately 120 degrees:
The a-glucosyl residue:
Cellobiose has the same composition as maltose, with the exception of the glycosidic bond is in b-.
Then, the systematic name of cellobiose is b-D-glucopiranosyl (1,4) D-glucopyranose. As in the case of maltose, cellobiose
has reducing properties and anomeric forms. Cellobiose is obtained from hydrolysis of cellulose.
In the case of cellobiose, the main axis od the pyranose rings are more or less parallel:
The b-glucosyl residue:
This fact has important structural implications. if we unite in a linear chain six glucoses in a (1,4)
bonds (like in maltose), we obtain Maltohexaose. If the union is in b (1,4) bond, we obtain Cellohexaose. Compare
both structures:
It can be seen that maltohexaose presents a helicoidal structure, while cellohexaose has a linear, fibrous structure.
Other reducing disaccharide is Lactose:
Lactose is a reducing disaccharide whose systematic name is b-D-galactopiranosyl (1,4) D-glucopyranose.
It results of the union of a galactose residue with a glucose through a (1,4) bond. It is the main carbohydrate of milk. We can see the two monosaccharides,
b-D-galactopyranosyl:
And the glucose residue (aglycone):
The free anomeric carbon is marked as CA.
When the union of monosaccharides takes place between both anomeric carbons, we obtain
Non-reducing Disaccharides. An example is Trehalose:
Trehalose is a disaccharide formed by two glucoses bound by their anomeric carbons. Having
no free anomeric carbons, its systematic name will be a-D-glucopiranosyl -a-D-glucopyranoside.
Note the suffix -oside that indicates the non reducing character of the sugar. Note also that in the
systematic name it becomes necessary to mention the anomeric character of both residues (a- and a-
in this case).
Other non-reducing disaccharide is Sucrose (cane sugar, common sugar), formed by the union of D-glucose and D-fructose.
Sucrose is a non-reducing disaccharide formed by the union of a D-glucose residue in
a- to a D-fructose in b-. Its systematic name is
a-D-glucopyranosyl-b-D-fructofuranoside.
The residue a-glucoside is:
And the b-fructoside:
Other Oligosaccharides
Oligosaccharides bound to proteins and lipids have many important functions in the cell,
mainly related to surface recognition signals. Thus, (1) They act as labels for the destination of proteins in
the cell (secretion, organelles, etc.); (2) Membrane proteins and lipids present in the external leaflet
of the cell membrane acquire oligosaccharide labels for the recognition of many chemical signals, like
growth factors, lectins, antibodies, viruses, etc.
Oligosaccharides bound to lipids (glycolipids) and to proteins (glycoproteins) present
in their structure many monosaccharide derivatives such as N-acetylglucosamine, N-acetylgalactosmine,
sialic acid, Fucose, etc., whose structure was seen in the page Monosaccharide Derivatives.
Proteins synthetized in the rough endoplasmic reticulum of the cell pass to its lumen
where they are bound to a complex oligosaccharide, called Nuclear Oligosaccharide, that undergoes further
transformations to give the different "labels" present in the proteins:
Several enzyme system act on the nuclear oligosaccharide producing different labels
according to the different destinations. Thus, those proteins targeted to the lysosome are labeled by
Lysosomal destination oligosaccharide:
While those proteins targeted for secretion out of the cell are labeled by the Secretion
Oligosaccharide: