CHAPTER
19
Lipids II: Phospholipids,
Glycosphingolipids, and
Cholesterol
Phospholipids and glycosphingolipids are amphipathic
lipid constituents of membranes (Chapter 10). They play
an essential role in the synthesis of plasma lipoproteins
(Chapter 20) and eicosanoids (Chapter 18). They function
in transduction of messages from cell surface receptors to
second messengers that control cellular processes (Chap-
ter 30) and as surfactants. Cholesterol is mainly of animal
origin and is an essential constituent of biomembranes
(Chapter 10). In plasma, cholesterol is associated with
lipoproteins (Chapter 20). Cholesterol is a precursor of bile
acids formed in the liver; of steroid hormones secreted by
adrenals, gonads, and placenta; and 7-dehydrocholesterol
of vitamin D formed in the skin. In tissues, cholesterol ex-
ists primarily in the unesterified form (e.g., brain and ery-
throcytes), although appreciable quantities are esterified
with fatty acids in liver, skin, adrenal cortex, and plasma
lipoproteins.
19.1
Phospholipids
Phospholipids can be
glycerolipids
or
sphingolipids.
Examples of glycerolipids are phosphatidylcholine, phos-
phatidylethanolamine, phosphatidylserine, phosphatidyli-
nositol, and phosphatidylglycerol (Figure 19-1). To dis-
tinguish between the two primary alcohol-carbon atoms
of asymmetrically substituted glycerol derivatives, the
glycerol carbon atoms are numbered 1 through 3 from
top to bottom and the C
2
hydroxyl group is written to the
left. This system is known as the stereochemical number-
ing (
sn
) convention. Thus, the structural formula for
sn-
1,
2
-diacylglycerol is
O
1
II
H2C — O— C— R'
I
R"— C — O— C2— H
I
H2C3OH
Phosphatidylcholines
Phosphatidylcholines, or
lecithins,
are zwitterionic over
a wide pH range because of the presence of a quater-
nary ammonium group and a phosphate moiety. Phos-
phatidylcholines are the most abundant phospholipids in
animal tissues and typically contain palmitic, stearic, oleic,
linoleic, or arachidonic acid, usually with saturated fatty
acids in the
sn-
1
position and unsaturated fatty acids at
sn-
2.
The
de novo
pathways for phospholipid synthesis use
cytidine triphosphate (CTP) for activation of interme-
diate species (analogous to the role of UTP in glyco-
gen biosynthesis; Chapter 15). The principal pathway of
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