Biology Reference
In-Depth Information
PE (
Table 5.1
). These
lipids will predominantly dictate many membrane properties including 'fluidity'. Although
'fluidity' is to a large extent dictated by the length and degree of unsaturation of the acyl
chains, the nature of the polar head group is also important. As demonstrated below, PEs
with identical chains generally have higher T
m
s than PCs by ~20
o
Cto40
o
C and therefore
at a fixed temperature are less 'fluid' than PCs (
Table 5.2
).
While PC and PE are predominantly structural lipids responsible for maintaining
the lipid bilayer matrix of membranes, PS (
Figure 5.13
) has many additional functions
[13,14]
. In fact PS, along with PI, are the most diverse and dynamic of all of the phos-
pholipids. Although PS is an essential lipid in all human cells, its major functions are
associated with the brain where it is far more abundant than in other organs. Many clin-
ical trials have suggested that PS supports a variety of brain functions that diminish
with age
[15,16]
. Dietary supplementation with PS can alleviate, ameliorate, or some-
times even reverse age-related decline of memory, learning, concentration, word skills,
and mood and may also improve capacities to cope with stress and maintain internal
circadian rhythms. From this very brief list of general physiological processes, it is clear
that PS must serve a number of distinct molecular functions in addition to just being
a component of the lipid bilayer.
PS is involved in both activating and anchoring a variety of proteins to membranes,
including ATPases, kinases, receptors, and the crucial signal transducing proteins, protein
kinase C and adenylate cyclase
[17,18]
. PS is also involved in cell
e
cell recognition and
communication processes.
Normally PS is located almost exclusively in the membrane inner leaflet. It is quite
unusual for any other phospholipid to be ~100% asymmetrically distributed across
a membrane (Chapter 9). This condition is maintained by ATP-dependent amino-PL trans-
locases (also known as flipases). If PS accumulates in the outer membrane leaflet, as it
does with age, it signals that the cell should be recycled. In addition, under physiological
conditions, PS is an anion and so, similar to PA, interacts strongly with divalent metals
like Ca
2
รพ
.
The major structural phospholipids in animal membranes are PC
>
TABLE 5.1
Average Lipid Composition of
a Mammalian Liver Cell.
Lipid
Mol (%)
PC
45
e
55
PE
15
e
25
PI
10
e
15
PS
5
e
10
PA
1
e
2
2
e
5
CL
SM
5
e
10
Cholesterol
10
e
20
