Page 287 - Color Atlas of Biochemistry

278 Tissues and organs
Lipoproteins
Most lipids are barely soluble in water, and
many have amphipathic properties. In the
blood, free triacylglycerols would coalesce
into drops that could cause fat embolisms.
By contrast, amphipathic lipids would be de-
positedin the bloodcells’membranes and
would dissolve them. Special precautions are
therefore needed for lipid transport in the
blood. While long-chain fatty acids are bound
to albumin and short-chain ones are dissolved
in the plasma (see p. 276), other lipids are
transported in lipoprotein complexes,of
which there several types in the blood
plasma, with different sizes and composition.
A. Composition of lipoprotein complexes
Lipoproteins are spherical or discoid aggre-
gates of lipids and apoproteins.They consist
of a nucleus of apolar lipids (triacylglycerols bind LDL through an interaction between
and cholesterol esters) surrounded by a sin-
gle-layered shell approximately 2 nm thick of
amphipathic lipids (phospholipids and cho-
lesterol; the example shown here is LDL).
The shell, in which the apoproteins are also
deposited, gives the surfaces of the particles
polar properties and thereby prevents them
from aggregating into large particles. The
larger the lipid nucleus of a lipoprotein
is—i. e., the larger the number of apolar lipids is reused. After fusion of the vesicle with ly-
it contains—the lower its density is.
Lipoproteins are classified into five groups. (see p. 234), and cholesterol and other lipids
In order of decreasing size and increasing
density, these are: chylomicrons, VLDLs
(very-low-density lipoproteins), IDLs (inter-
mediate-density lipoproteins), LDLs (low-
density lipoproteins), and HDLs (high-density
lipoproteins). The proportions of apoproteins
range from 1% in chylomicrons to over 50% in
HDLs. These proteins serve less for solubility
purposes, but rather function as recognition
molecules for the membrane receptors and
enzymes that are involved in lipid exchange.
B. Transport functions
The classes of lipoproteins differ not only in
their composition, but also in the ways in
which they originate and function.
The chylomicrons take care of the transport
of triacylglycerols from the intestine to the
tissues. They are formed in the intestinal mu-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.
cosa and reach the blood via the lymphatic
system (see p. 266). In the peripheral vessel-
s—particularly in muscle and adipose tis-
sue—lipoprotein lipase [1] on the surface of
the vascular endothelia hydrolyzes most of
the triacylglycerols. Chylomicron breakdown
is activated by the transfer of apoproteins E
and C from HDL. While the fatty acids released
and the glycerol are taken up by the cells, the
chylomicrons gradually become converted
into chylomicron remnants,which areulti-
mately removed from the blood by the liver.
VLDLs, IDLs,and LDLs are closely related to
one another. VLDLs formed in the liver (see
p. 312) transport triacylglycerols, cholesterol,
and phospholipids to other tissues. Like chy-
lomicrons, they are gradually converted into
IDL and LDL under the influence of lipoprotein
lipase [1]. This process is also stimulated by
HDL. Cells that have a demand for cholesterol
their LDL receptor and ApoB-100, and then
take up the complete particle through recep-
tor-mediated endocytosis. This type of trans-
port is mediated by depressions in the mem-
brane (“coated pits”), the interior of which is
lined with the protein clathrin. After LDL
binding, clathrin promotes invagination of
the pits and pinching off of vesicles (“coated
vesicles”). The clathrin then dissociates off and
sosomes, the LDL particles are broken down
are used by the cells.
The HDLs also originate in the liver. They
return the excess cholesterol formed in the
tissues to the liver. While it is being trans-
ported, cholesterol is acylated by lecithin cho-
lesterol acyltransferase (LCAT). The cholesterol
esters formed are no longer amphipathic and
can betransported in thecoreof the lipopro-
teins. In addition, HDLs promote chylomicron
and VLDL turnover by exchanging lipids and
apoproteins with them (see above).