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]
.