section 10.3
Cell-Surface Glycoproteins
161
FIGURE 10-6
A lipid bilayer that contains both integral and peripheral proteins.
10.3 Cell-Surface Glycoproteins
Glycoproteins play major roles in antigen-antibody reac-
tions, hormone function, enzyme catalysis, and cell-cell
interactions. Membrane glycoproteins have domains of
hydrophilic and hydrophobic sequences and are amphi-
pathic molecules. The carbohydrate moieties of glycopro-
teins are distributed asymmetrically in cell membranes,
cluster near one end of the protein molecule (Figure 10-7),
and constitute a hydrophilic domain of amino acid residues
(Chapter 21) as well as carbohydrates. The hydrophobic
domain of the molecule interacts with the lipid bilayer.
FIGURE 10-7
Membrane asymmetry with respect to location of glycoproteins and
glycolipids. These carbohydrate-containing molecules are exclusively
present on the external membrane.
Phospholipid molecule
Fatty alkyl chains
Head group
Monolayer
-
m
m
m
u
Water
Bilayer
Water
-
u
m
m
u
- '-
Water
Liposome vesicle
D C
> •
Water
2 5 0
FIGURE 10-8
Interactions of phospholipids in an aqueous medium and the formation of
liposome vesicle. Phospholipids spontaneously form lipid bilayers in
which the polar head groups interact with water, whereas the hydrophobic
tails interact among themselves to form an environment that excludes
water. The lipid bilayers are stabilized by noncovalent interactions.
The role of glycoproteins in cell-cell interaction is co-
ordination and regulation of adhesion, growth, differenti-
ation of cells, and cell size. Disruption of these processes
may lead to loss of control of cell division and growth, a
property characteristic of cancer cells. Normal cells grown
in tissue culture show
contact inhibition.
When cells are
allowed to grow in a tissue culture medium under opti-
mal conditions on a surface, such as that of a Petri dish,
they grow and divide until the surface is covered with
a monolayer and further growth is inhibited. However,
when these cells are transformed by treatment with certain
viruses or carcinogenic chemicals, they lose contact in-
hibition and continue to grow beyond the monolayer to
form multilayered masses of cells. Cancer cells (malignant
neoplastic cells), like transformed cells, show continued
growth and invasiveness (spreading) in tissue culture. In
both transformed and cancer cells, cell-cell interaction has
