Membrane Polar Lipids - An Introduction to Biological Membranes - page 66

TABLE 5.2 Melting Points (T m s) for PCs and PEs where both the sn-1 and sn-2
Acyl Chains are Identical.

diPC ( o C) diPE ( o C)

Acyl chain

Myristic 23.6 51

Palmitic 41.3 63

Stearic 58 82

Oleic 22 15

Elaidic 5 41

PEs have higher T m s by more than 20 o C than do homologous PCs.

If we examine the alcohol structures of PE, PC, and PS, it is clear how closely related the
three phospholipids are and how easy it might be to interconvert one to the other
( Figure 5.14 ). PE can be converted to PC by simply methylating the ethanolamine nitrogen
three times. This step is not possible in bacteria (as the enzyme is missing) and so bacteria
lack PC. PS can be converted to PE by simple decarboxylation.
PI ( Figure 5.15 ) is another anionic lipid at physiological pH and so contributes to the lipid
bilayer and to the surface negative charge density of membranes. In fact PIs are considered to
be one of the most acidic of the phospholipids. Its low concentration (PI is generally even less
abundant than PS, Table 5.1 ) indicates that this phospholipid plays only a small structural
role in membranes. However, it has been appreciated for some time that PI does have several
crucial roles in cell function [19] . Although PI is present in all tissues and cell types, it is espe-
cially abundant in brain where it can account for 10% of total phospholipids. Consistent with
the other phospholipids, PI has a saturated fatty acid in the

sn -1 chain and an unsaturated

fatty acid in the -2 chain. What is unusual about PI is the highly specific nature of its chains.

Most PIs are the single molecular species, 18:0,20:4 PI.
Phosphoinositides hold a central role in cell signaling and regulation [20,21] . Often PI is
phosphorylated at various positions on the inositol chain, most commonly positions 4 and
5, although position 3 can also be phosphorylated ( Figure 5.16 ). Positions 2 and 6 are not
phosphorylated due to steric hindrance. Phosphorylated PIs are referred to as phosphatidy-
linositol phosphates or polyphosphoinositides. They are usually present at low levels, typi-
cally at about 0.5 to 1% of the total lipids of the inner leaflet of the plasma membrane. The
major phosphoinositide is PI 4,5-bisphosphate (PIP 2 ).

sn

NH3 +

pKa 9.1

O

O CH2 HC

O

P

pKa 2.2

O - C

O -
pKa 2.1

O

FIGURE 5.13 PS e Phosphatidylserine.