Biology Reference
In-Depth Information
CH
2
OH
CH
2
OH
Ο
Ο
OH
OH
OH
Ο
HO
OH
OH
α
-D-glucose
β
-D-glucose
MALTOSE
FIGURE 7.4
The di-saccharide maltose. Two
a
-D-glucose sugars are attached through a glycosidic bond from
the anomeric carbon (C-1) of the left glucose to C-4 of the right glucose.
has led to a search for the elusive 'carbohydrate code'. Many different sugar links are
possible, even for disaccharides, and this number increases logarithmically upon each
additional linked sugar in the polysaccharide. In a 1997 paper entitled
Information capacity
of the carbohydrate code
, R.A. Laine calculated the possible number of ways that sugars
could be linked
[11]
. For a linear tri-saccharide, there are 12 possible linkage sites (4 on
each sugar) that can be arranged into 6,000,000 different structures. This is in sharp
contrast to a tri-peptide composed of the 20 different common amino acids that can be
arranged in only 8,000 (20
3
) different ways. Therefore, in this simple example, carbohy-
drates have a greater information storage capacity, by an order of magnitude of three,
compared to proteins.
Of the nine sugars commonly found in membranes (
Figure 7.3
), one is highly unusual
and supports a vast array of functions. Sialic acid
[12,13]
is a 9-carbon carboxylic acid that
is an anion under physiological conditions. Actually sialic acid is a family of similar
sugars, the most common being N-acetylneuraminic acid. Sialic acid was named by
Gunnar Blix in 1952 after the Greek word for saliva. By the time Blix gave the name to sialic
acid, he had been working on this family of compounds for more than 15 years. Sialic acid
is widely found in gangliosides and glycoproteins of animal plasma membranes, but is
also found in most other organisms including plants, fungi, and bacteria. Glycoproteins
of cancer cells that can metastasize are particularly rich in sialic acid. The negative charge
on sialic acid has many diverse functions, some advantageous, some not. For example,
anionic sialic acid helps keep erythrocytes from clumping in the blood stream but is
also the binding site and entry port for the Human Influenza Virus. Sialic acid-rich oligo-
saccharides help retain waters close to the cell surface and thus are involved in water
uptake. Sialic acid also 'hides' mannose units from incorrectly reacting with components
of complement. Erythrocytes have a 120-day life span after which they are targeted for
destruction. Sialic acid plays an important role in this process. Aged erythrocytes are
known to lose sialic acid from their membranes, exposing the remaining neutral sugars
to host antibodies. After antibody binding, the senescent cell is removed from circulation
