Biology Reference
In-Depth Information
FIGURE 5.14
Close relationship of
the head group alcohols of PE, PC, and
PS.
ETHANOLAMINE
CHOLINE
SERINE
CH
3
H
H
H
H
H
H
C
C
3
+
HO
+
NH
3
+
HO
C
C
CH
3
HO
C
C
C
H
H
H
H
H
CH
3
O
O
-
O
OH
P
O
O
OH
O
-
pKa 2.1
HO
HO
OH
FIGURE 5.15
PI
e
Phosphatidylinositol.
Since the phosphoinositides are highly anionic they are effective at enhancing non-specific
electrostatic interactions with cellular proteins, particularly to protein 'pH domains', at inner
leaflet plasma membrane interfaces. PIs are in essence an inert storage form of the potent bio-
logical signaling second messenger molecules, membrane-bound diacylglycerol (DAG) and
water-soluble inositol phosphates. These molecules are released upon hydrolysis of, for
example, PIP
2
by phospholipase C (
Figure 5.16
). In fact, PIs are the major source of DAG
in cells. DAG in turn affects the activity of a large, important family of signaling enzymes,
The PKCs. The water-soluble inositol phosphates affect the opening of channels for K
þ
,
Na
þ
,Ca
2
þ
and other ions. Stimulation of Ca
2
þ
release from the endoplasmic reticulum is
an important signaling event. PIP
2
has also been shown to affect cell shape, motility and other
processes by interacting with actin on the cytoskeleton. PIP
2
is also a cofactor for phospho-
lipase D that produces PA, yet another signaling molecule. As a component of the nucleus,
the phosphoinositides affect DNA repair, transcription and RNA dynamics. PI is also
involved in binding a variety of proteins to the membrane surface via a glycosyl bridge
[22]
. Proteins bound in this way are referred to as being glycosyl-phosphatidylinositol
(GPI)-anchored proteins (see Chapter 6). It is clear that the phospholipid PI plays an impor-
tant role in many aspects of cellular biochemistry in addition to being a component of the
membrane lipid bilayer.
PG (
Figure 5.17
)
[23]
is another anionic lipid found in mammalian membranes in low
amounts (1 to 2% of total phospholipids). PG therefore is also not a major membrane struc-
tural lipid. However, PG is the second most abundant lipid in lung surfactant, comprising
up to 11% of the surfactant lipids and its level is used to determine the maturity of a baby's
lung. PG differs from the other phospholipids already discussed in possessing 2 optically
active carbon centers (the
-2 position in the phosphatidyl group and the central carbon
of the alcohol glycerol) and in often having a more unsaturated chain occupying the
sn
-1
position. It appears that the major membrane role for PG in animals is as a precursor for
mitochondrial cardiolipin (CL,
Figure 5.18
). CL (discussed below) is essential for normal
sn
