Biology Reference
In-Depth Information
PE (
Figure 5.11
), infrequently referred to as cephalin, from the word cephalic, meaning
pertaining to the head, is the second most prevalent phospholipid in man, but is the principal
phospholipid in bacteria. This is due to the ability of man to convert PE to PC while bacteria
cannot and so bacteria are relegated to having PE as their major membrane phospholipid. PE,
PC, and cholesterol are also the major components of egg yolk, a ready and cheap commercial
source of these lipids. In humans, PE is found particularly in nervous tissue including white
matter of brain, nerves, and in spinal cord. PE is a primary amine-containing phospholipid
and therefore has a highly reactive chemical handle that can be easily derivatized. This prop-
erty will be exploited in Chapter 9 for various membrane biochemical studies. Since the pKa
of the PE amine is ~8.5, at physiological pH the amine is mostly, but not completely, proton-
ated. With the fully dissociated phosphate, PEs have a slightly negative net charge. PEs are
therefore not quite fully zwitterions. The PE's primary role in membranes is as a major struc-
tural lipid. The PE's head group is not only chemically reactive but is small and poorly
hydrated. With unsaturated chains, PEs have a pyramid shape, with a wide base and narrow
head. Phospholipid shapes are discussed in Chapter 10. Unlike PCs, PEs can form hydrogen
bonds with neighboring polar groups. In the absence of other phospholipids, PEs prefer non-
lamellar phase including reverse hexagonal H
II
phase, an unusual membrane structure that is
discussed in Chapter 10.
Natural PCs (
Figure 5.12
) are often referred to by an old term, lecithin. Actually, lecithin is
a complex mixture of many lipids, the predominant one being mixed acyl chain PC. PC is the
major phospholipid found in animal membranes and is therefore the most important
membrane structural lipid component in man. PC is ideally suited to fit comfortably into
membranes. The molecule is can-shaped with its polar head group being about the same
width as the sum of its apolar tails (Chapter 10). In addition, PC is a true zwitterion, possess-
ing a formal positive and negative charge, but no net charge. Therefore PC does not exhibit
negative charge repulsion, a potential problem with most other phospholipids. In man PC
can be made by methylating the nitrogen of PE three times, a process missing in prokaryotes.
In addition, dietary lecithin is the major source of the essential biochemical choline.
H
O
N
+
O
P
O
2
CH2
H
O
-
H
pKa 2.1
FIGURE 5.11
PE
e
Phosphatidylethanolamine.
O
CH3
N
+
CH2
CH2
CH3
O
P
O
CH3
O
-
pKa 2.1
FIGURE 5.12
PC
e
Phosphatidylcholine.
