Biology Reference
In-Depth Information
FIGURE 7.3 The 9 carbohy-
drates found on membrane
glycolipids and glycoproteins.
CH 2 OH
CH 2 OH
CH 2 OH
Ο
Ο
Ο
α -D-galactose
α
-D-mannose
α -D-glucose
CH 2 OH
Ο
Ο
Ο
CH 3
HOH 2 C
α -L-arabinose
α -D-xylose
α
-L-fucose
CH 2 OH
O - -
CH 2 OH
O
CH 2 OH
Ο
Ο
C
HOHC
Ο
CHOH
Ο
Ο
Ο
NH C
CH 3
NH C
CH 3
NH C
CH 3
N-acetylglucosamine
N-acetylgalactosamine
sialic acid
One under-appreciated aspect of carbohydrates is their enormous potential for informa-
tion storage. Carbohydrates exist in either an aldehyde or a ketone series. In addition,
each carbon location that has a carbon with four different groups on it (positions 2 through
5 for glucose) comes in two orientations or isomers. An aldehyde 6-carbon (hexose) sugar
therefore has 4 asymmetric carbons, each with two isomers or 2 4 or 16 different possible
isomers. The potential for information storage is greatly enhanced when two or more sugars
are connected through what is known as a glycosidic bond. For example, linking two
glucoses together by removing water between carbon #1 of one glucose (referred to as the
anomeric carbon) to the alcohol at position number 4 of the second glucose produces the
disaccharide maltose ( Figure 7.4 ).
It is generally assumed that biological information storage resides within the primary
sequence of proteins and nucleic acids. However it is becoming more evident that polysac-
charides have far more information storage capacity than these other biopolymers. This
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