Biology Reference
In-Depth Information
CH 2 OH
O
OH
OH
HO
OH
α -D-glucose
FIGURE 7.1
a
-D-Glucose.
The amount of carbohydrate on a membrane protein varies tremendously from 0 in some
multiple span trans-membrane proteins to 60% by weight for glycophorin and
85% for
some blood group substances. In fact some plasma membranes have so many surface carbo-
hydrates that in cross-section they resemble a hairbrush, with a lot of sugar on the outer surface
and none on the cytoplasmic inner surface. The term used to describe these membranes is gly-
cocalyx or 'sugar coat' [5] . A glycocalyx is commonly found on the outer surface of bacteria
and mammalian epithelial cells ( Figure 7.2 ). Membrane sugars are attached to either lipids
(glycolipids [6
>
8] ) or proteins (glycoproteins, [9,10] ). There are nine basic sugars commonly
found attached to membranes:
e
a
-D-glucose;
a
-D-galactose;
a
-D-mannose;
a
-L-fucose;
a
-D-xylose; N-acetyl glucosamine; N-acetyl galactosamine; and N-acetyl neu-
raminic acid (sialic acid) ( Figure 7.3 ). The carbohydrates can be either single sugars or short,
<
-L-arabinose;
a
15 unit, polysaccharides, and are always located on the extra-cellular membrane leaflet.
Microvilli
Glycocalyx
FIGURE 7.2 Glycocalyx. Electron micrograph of a cross section of microvilli in the human digestive track. The
glycocalyx can be seen at the top of the columnar microvilli. Reproduced with permission [23] .
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