404
chapter 19
Lipids II: Phospholipids, Glycosphingolipids, and Cholesterol
O
O
H.C— o — c — R '
Il
I
R— C — O— CH
O
N H
3
H £ — O — P — O — C H — C H — C O O
O-
P h o s p h a tid y ls e r in e
B a s e - e x c h a n g e
e n z y m e
r.
E th a n o la m in e
C a 2+
S.,
S e rin e
O
P h o s p h a tid y ls e r in e
d e c a r b o x y la s e
^ -c o
2
O
CH— O— C— R'
R— C— O— ÇH
O
H £ — O — P — O — C H —
C H — N H
3
O-
P h o s p h a tid y le th a n o la m in e
FIGURE 19-3
Biosynthesis of phosphatidylethanolamine from phosphatidylserine. The
base-exchange enzyme on the cytosolic face of the endoplasmic reticulum
can interconvert these phospholipids in the presence of Ca2+ and the
alternate head group, serine or ethanolamine. The decarboxylase is
localized in the inner membrane of mitochondria and catalyzes the
nonequilibrium conversion of phosphatidylserine to
phosphatidylethanolamine.
Phosphatidylethanolamines can also be synthesized
by decarboxylation of phosphatidylserine and in mam-
mals principally through action of the Ca2+-mediated
base exchange enzyme (Figure 19-3). Phosphatidylser-
ine production in liver occurs at the cytosolic face of
the endoplasmic reticulum. In brain tissue, this phospho-
lipid accounts for up to 15% of the total phospholipid
content.
Phosphatidylinositols and phosphatidylglycerols are
synthesized via CDP-diacylglycerol (Figure 19-4). Phos-
phatidylinositols are enriched with arachidonic acids by
deacylation and reacylation. Two other phosphoinositides
are synthesized and degraded in the phosphoinositol cy-
cle (Chapter 30) by the enzymes on the plasma mem-
brane, endoplasmic reticulum, and cytosol of brain and
liver cells. These anionic phospholipids contain myo-
inositol, which can be derived from food or produced via
cyclization of glucose-
6
-phosphate. Phosphatidylinositol
4-phosphate and phosphatidylinositol 4,5-bisphosphate
are derived from phosphatidylinositol by ATP-dependent
phosphorylation of the 4- and 5-positions of the inos-
itol ring. Dephosphorylation reactions are carried out
by phosphomonoesterases. The exact proportion of the
three forms in cell membranes is unknown, although the
CH2OH
CHOH -
I
CHjOH
G ly c ero l
A T P
A D P
___
S
.
1
C H 2O H
H O C H
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,
C K O P O ?
R 'C O S C o A
V
R " C O S C o A
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Il
I
— ^ —» 2 — s p — R — C — 0 — Ç
C o A S H C o A S H
’
HjC—O— C— R'
P h o s p h a tid ic
a c id
P h o sp h a tid y lin o sito l
D ip h o sp h atid y lg ly ce ro l ■
(cardiolipin)
AMD
HjCOPQj
^ ♦ P P
0
In o sito l o
H C —
q —
q —
r
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»
I
R — C — O — ÇH
O
O
T
II
II
H
2
c — O — P — O — p — O — CH.
1
I
•
O-
O"
CDP-diacylglycerol
5
L G Iy ce ro I-3 -p h o sp h ate
l " - C M P
P h o sp h a tid y lg ly c e ry l p h o s p h a te
C D P -
C M P
d ia c y g ly ce ro l
^—
7
— /
P h o s p h a tid y lg ly c e ro l
FIGURE 19-4
Biosynthesis of phosphatidylinositols and phosphatidylglycerols. Enzymes: (1) glycerol kinase; (2) acyltransferases;
(3) phosphatidate cytidyltransferase; (4) CDP-diacylglycerol: inositol phosphatidate transferase; (5) CDP-
diacylglycerol: OT-glycerol-3-phosphate phosphatidate transferase; (
6
) phosphatase; (7) phosphatidylglycerol:
CDP-diacylglycerol phosphatidate transferase.
