3. Cholesterol

1
3. Cholesterol lipoproteins
2
Cholesterol makes phospholipid membranes more
plastic
HO
cholesterol
3
Cholesterol associates with sphingolipids and
certain proteins to form rafts in the plasma
membrane
lipid raft
protein with glycosyl-phosphatidylinositol (GPI)
anchor at carboxyl end
sphingolipids other phospholipids
other phospholipids
cholesterol
Lipids probably are more ordered in rafts than
elsewhere in the membrane because sphingolipids
usually have long, saturated fatty acid side
chains
4
Cholesterol serves as a precursor for steroid
hormones bile acids
cholesterol
progesterone
testosterone
estradiol
cholate (a bile acid)
cortisone
5
Steroids are members of a large group of natural
products with structures based on isoprene
isoprene
cholesterol (27 carbons)
squalene (30 carbons)
ubiquinone-7 (35-carbon side chain)
b-carotene (40 carbons)
6
Experiments with radioactive tracers showed that
rats synthesize cholesterol and squalene from
acetate.
CH3CO2-
squalene
acetate
cholesterol
The labeling patterns in squalene and cholesterol
were similar, supporting the hypothesis that
squalene is an intermediate in cholesterol
biosynthesis.
7
A 5-carbon isoprenoid building block is
synthesized from acetate by way of a 6-carbon
intermediate, mevalonate
three molecules of acetate
mevalonate
isopentenyl pyrophosphate
8
Mevalonate is synthesized from three molecules of
acetylCoAvia b-hydroxy-b-methylglutarylCoA
(HMG-CoA)
HMG-CoA
acetoacetylCoA
2 acetylCoA
2 NADPH 2H
HMG-CoA reductase
2 NADP CoA-SH
key control point for cholesterol biosynthesis!
mevalonate
9
Formation of isopentenyl pyrophosphate from
mevalonate consumes three molecules of ATP
ATP
ATP
mevalonate
ATP
isopentenyl pyrophosphate
10
Isopentenyl pyrophosphate tautomerizes to
dimethylallyl pyrophosphate, which can form a
relatively stable carbonium ion
dimethylallyl pyrophosphate
isopentenyl pyrophosphate
allyl carbonium ion
11
Isopentenyl pyrophosphate and dimethylallyl
pyrophosphate combine to form geranyl
pyrophosphate
dimethylallyl- pyrophosphate
Isopentenyl pyrophosphate
geranyl pyrophosphate (10 carbons)
This creates a new allyl-pyrophosphate derivative
that can combine with a second molecule of
isopentenyl pyrophosphate to form farnesyl
pyrophosphate (15 carbons).
12
Two molecules of farnesyl pyrophosphate condense
head-to-head to form squalene
13
Formation of squalene epoxide requires O2 and
NADPH
squalene
squalene 2,3-epoxide
squalene monooxygenase
14
Cyclization of squalene epoxide zips up the
sterol rings
Protonation opens the epoxide and generates a
carbonium ion that reacts with the nearby CC
double bond, creating a new carbonium ion
theres more
15
The cyclase reaction continues
I dont expect you to remember these details.
16
Cholesterol biosynthesis is controlled primarily
by HMG-CoA reductase
ß-Hydroxy-ß-methyl-glutarylCoA
Phosphorylation by AMP-dependent protein kinase
inactivates HMG-CoA reductase dephosphorylation
activates it.
insulin
HMG-CoA reductase
glucagon
mevalonate
enzyme proteolysis
enzyme synthesis
Synthesis of the low-density lipoprotein (LDL)
receptor, which mediates uptake of lipoproteins
containing cholesterol, also is regulated.
Cholesterol or a derivative inhibits synthesis of
the receptor.
cholesterol
Cholesterol or a cholesterol derivative inhibits
synthesis and stimulates proteolysis of HMG-CoA
reductase.
17
Cholesterol prevents activation of transcription
of the HMG-CoA reductase gene
ER membrane
SREBP
to nucleus
Cholesterol binds reversibly to a protein that
holds the Sterol Regulatory Element Binding
Protein (SREBP) in the ER membrane
In the absence of cholesterol, the proteins
separate and SREBP is cleaved by proteases
A soluble fragment of SREBP diffuses to the
nucleus, where it activates transcription
18
Inhibitors of HMG-CoA reductase (statins) are
used clinically to decrease cholesterol
biosynthesis
mevalonate
In addition to decreasing LDL cholesterol,
statins decrease the level of C-reactive protein
in the blood. C-reactive protein is a marker of
acute inflamation. Its role in atherosclerosis is
unclear. Ridker et al., New Engl. J. Med. 325 20
(2005) Nissen et al., New Engl. J. Med. 352 29
(2005)
19
Lipoproteins carry cholesterol between the liver
and other tissues
plasma membrane
monolayer of phospholipids some free
cholesterol
core of triacylglycerols cholesterol esters
apolipoprotein
LDL receptor
LDL
This is a cartoon. The structures of lipoproteins
are not known.
blood
cytosol
20
Low-density lipoproteins (LDL and VLDL) carry
cholesterol from the liver to other tissues
high-density lipoproteins carry it in the other
direction
other tissues
intestine
liver
Triacylglycerols from VLDLs and chylomicrons are
hydrolyzed in capillaries of muscle and adipose
cells by lipoprotein lipase
The liver both synthesizes cholesterol and
removes it from the blood for conversion to bile
acids.
adipose tissue, muscle
21
Cells take up low-density lipoproteins by
receptor-mediated endocytosis
LDL particle
LDL receptor
Golgi
receptor resynthesis
endosome
ER
cholesterol
lysosome
Ebola virus uses parts of this pathway to enter
cells.
nucleus
22
High levels of cholesterol in the blood can
result in atherosclerosis -- deposition of
cholesterol and other materials in the inner
walls of blood vessels
atherosclerotic plaque
Coronary artery disease is the leading cause of
death in industrialized countries.

• Major risk factors
• high blood cholesterol (especially
• LDL cholesterol gt 100 mg/dL)
• smoking
• diabetes mellitus
• obesity
• physical inactivity

Blood clot
There usually are no symptoms until blood flow to
the heart is seriously compromised.
Individuals with defective LDL receptors have
exceptionally high plasma cholesterol (familial
hypercholesterolemia). Because cholesterol does
not enter their cells, HMGCoA reductase is not
regulated properly and cholesterol biosynthesis
remains switched on. If untreated, people with
this condition tend to die of atherosclerosis at
a young age.
23
Variant forms of apolipoproteins E and A are
associated with different risks of heart disease
Apo-E, a major component of chylomicrons and
VLDLs, has 3 common alleles Apo-E3 is the most
common form. Apo-E4 has been linked to
Alzheimers disease elevated risk of heart
disease. Apo-E2 binds poorly to LDL receptors,
is associated with hyperlipidemia.
High levels of HDLs generally are linked to lower
risk of heart disease Exercise increases
HDLs. But people with Apo-A1(Milano) have very
low levels of HDL and also have very low risk of
heart disease. Some drugs that increase HDLs have
unexpected adverse (off-target) effects.
24
Gene Therapy for Atherosclerosis?
R. Flynn et al. Expression of apolipoprotein A-1
in rabbit carotid endothelium protects against
atherosclerosis. Molecular Therapy 10, 1833-1841
(2011).