Membrane Proteins - An Introduction to Biological Membranes - page 93

~8 - 10 Å

~3 Å

~27 - 32 Å

~3 A
~8 - 10 Å

FIGURE 6.6 Three sections of the membrane lipid bilayer. The inner hydrocarbon region is ~27 to 32 ˚ thick.
The very narrow boundary region between the hydrophobic inner core and the hydrophilic interfacial regions is
~3 ˚ . Finally, the polar head group region is ~8 to 10 ˚ thick.

spanned is composed of a lipid bilayer that can be divided into 3 sections ( Figure 6.6 ). The
inner hydrocarbon region is ~27 to 32 ˚ thick. The very narrow boundary region between
the hydrophobic inner core and the hydrophilic interfacial regions is ~ 3 ˚ . Finally, the polar
head group region is ~8 to 10 ˚ , although this may be wider in membranes that include large
amounts of carbohydrate-rich components.
All membrane proteins are 100% asymmetrically distributed with respect to the
membrane and do not flip-flop across the membrane. Therefore, trans-membrane proteins
are locked in place. A characteristic of many trans-membrane proteins is the presence of tyro-
sines and tryptophans at the aqueous interface [15] . These amino acids serve as interfacial
anchors that can interact simultaneously with the membrane hydrophobic interior and the
aqueous exterior. Also, for reasons not entirely clear, the cationic amino acids lysine, histi-
dine, and arginine occur more commonly on the cytoplasmic side of membranes (positive-
inside rule [16] ). The inner (cytoplasmic) leaflet is also the location of the majority of
membrane primary amine lipids, PE and PS [17] . Virtually all integral proteins have at least
one stretch of ~20 hydrophobic amino acids that implies the existence of a trans-membrane
a -helix. a -Helical domains are found in all types of biological membranes and so represent
a common membrane structural motif [18] . Since each amino acid in an a -helix spans
~1.5 ˚ , it takes approximately 20 amino acids to cross the ~30 ˚ of the hydrophobic interior.
As expected, the trans-membrane a -helices are generally devoid of polar amino acids while
non-membrane spanning portions of the protein are usually enriched in these amino acids.
a -Helices can cross the membrane a single time or multiple times.

a

TYPE I. SINGLE TRANS-MEMBRANE -HELIX: GLYCOPHORIN

Since erythrocytes, unlike other human cells, do not have any complicating internal
membranes, and are easily obtained in pure form from blood, they have served for decades
as the 'laboratory for membrane studies'. Many of the classic membrane biochemical and
biophysical studies discussed in later chapters were first worked out on erythrocytes, and
many of the membrane protein studies were first done on resident glycophorin. However,
despite the fact that glycophorin is one of the most studied of all membrane proteins, its bio-
logical function remains elusive. Glycophorins a and b are the major glycoproteins in the