CHAPTER
Lipids III:
Plasma Lipoproteins
Lipids, by virtue of their immiscibility with aqueous solu-
tions, depend on protein carriers for transport in the blood-
stream and extracellular fluids. Fat-soluble vitamins and
free fatty acids are transported as noncovalent complexes.
Vitamin A is carried by retinol-binding protein and free
fatty acids on plasma albumin. However, the bulk of the
body’s lipid transport occurs in elaborate molecular com-
plexes called
lipoproteins.
20.1
Structure and Composition
Lipoproteins are often called pseudomicellar because
their outer shell is in part composed of amphipathic
phospholipid molecules. Unlike simple micelles, lipopro-
teins contain
apolipoproteins,
or
apoproteins,
in their
outer shell and a hydrophobic core of
triacylglycerol
and
cholesteryl esters.
Unesterified, or free, cholesterol, which
contains a polar group, can be found as a surface com-
ponent and in the region between the core and surface
(Figure 20-1). Most lipoproteins are spherical. However,
newly secreted high-density lipoproteins (HDLs) from
the liver or intestine are discoidal and require the action
of lecithin-cholesterol acyltransferase (LCAT) in plasma
to expand their core of neutral lipid and become spheri-
cal. The hydrophobic core of the low-density lipoprotein
(LDL) molecule may contain two concentric layers: one
of triacylglycerol and another of cholesteryl ester.
The apoproteins are distinct physically, chemically,
and immunochemically and have important roles in
lipid transport and metabolism (Table 20-1). In keep-
ing with their individual metabolic functions, they have
specific structural domains. Amino acid substitutions or
deletions in critical domains result in functional ab-
normalities. The apoproteins share a common struc-
ture in the form of an amphipathic helix, in which the
amino acid residues have hydrophobic side chains on
one face of the helix and hydrophilic polar residues on
the other. The hydrophilic face is believed to interact
with the polar head groups of the phospholipids, while
the hydrophobic residues interact with their fatty acid
portions.
The laws of mass action govern the interactions of lipids
and most apoproteins in lipoproteins, so that as the affini-
ties between surface components change during lipopro-
tein metabolism, apoproteins may dissociate from one par-
ticle and bind to another. In fact, all of the apoproteins,
with the possible exception of apoprotein B (apo B), can
change their lipoprotein associations. The reason for the
unique behavior of apo B remains a mystery. On the basis
of their principal transport function, lipoproteins may be
divided into two classes according to the composition of
their major core lipids. The principal triacylglycerol car-
riers are chylomicrons and very-low-density lipoproteins
(VLDLs), whereas most cholesterol transport occurs via
LDLs and HDLs.
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