Biology Reference
In-Depth Information
erythrocyte plasma membrane and bear the antigenic determinants for the MN and Ss blood
groups. By weight, 60% of glycophorin is carbohydrate and the abundant sialic acid (see
Chapter 7) imparts a negative charge to the erythrocyte outer surface. The negative charges
on glycophorin cause the erythrocytes to repel one another preventing them from clumping
in the blood. With age, the loss of sugars triggers destruction of old erythrocytes.
Glycophorin a is a single span integral protein of 131 amino acids whose cartoon structure
is depicted in
Figure 6.7 [19]
. By weight, 60% of glycophorin's mass is carbohydrate, all of
which are attached to amino acids on the N-terminal, exterior segment. All of the carbohy-
drates are attached to only a few of the first 50 amino acids of glycophorin's 80 amino acid
exterior sequence. As with cytochrome b
5
, this extracellular segment can be cleaved by
trypsin. The attached sugar tetrasaccharides are O-linked to serine or threonine while the
larger oligosaccharides are N-linked to asparagines. O-linked serine and threonine and
N-linked asparagine are the usual way sugars are linked to membrane proteins (see Chapter
7). Glycophorin a is held in the membrane via a 22-amino acid single span trans-membrane
a
-helix
[20]
. Finally, glycophorin a has a 29 amino acid C-terminal segment that extends into
the aqueous space in the erythrocyte interior. This segment is totally devoid of sugars. Both
the N-terminal and C-terminal non-membrane domains have many hydrophilic amino acids
that are not found in the trans-membrane
-helix segment. Glycophorin a is locked in place
by an
a
-helix that is flanked by positively charged amino acids lysine and arginine. These
cationic amino acids interact with the negatively charged phospholipid head groups. In
the erythrocyte membrane glycophorin a probably exists as dimers that form a coiled struc-
ture involving the
a
-helices.
a
TYPE II. MULTIPLE TRANS-MEMBRANE SPAN BY
-HELICES: BACTERIORHODOPSIN
Proteins span membranes via two major structures,
a
-helices or
b
-barrels. The first of
these structures that was shown to be located inside membranes is the
a
-helix, a secondary
structure first formulated in 1948 by Linus Pauling while lying in bed, sick from a cold.
The legend states that boredom led Pauling to start playing with paper and scissors and
a
SUGARS
N-TER
α-HELIX
C-TER
GLYCOPHORIN
FIGURE 6.7
Cartoon structure of glycophorin showing extra-cellular (N-terminal), trans-membrane (
a
-helix)
and intracellular (C-terminal) domains
[19]
.
