exam
Final Mar. 23, 3:00pm-5:59pm
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Lecture 25
Lipids and Steroids Biosynthesis (II)
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Important concepts
1. Understand the regulation of cholesterol synthesis
Understand regulation on HMG-CoA reductase
Understand receptor mediated endocytosis
Transport of cholesterol and TAC by lipoproteins
LDL endocytosis
2. Understand the synthesis of bile acids and steroids
Understand the mechanism of cytochrome P450
3
HMG-CoA Reductase
Membrane protein of ER
Responsive to cholesterol levels
increase in HGM-CoA reductase levels
(200x)
Transcriptional regulation
Translational regulation
Regulatory degradation
Activity regulated by phosphorylation
(AMPK) dephosphorylated form more
active
4
Direct Effect of Cholesterol: transcription
Sterol regulatory
SRECBP cleavage element binding protein
activating protein
(SREBP)
(SCAP)
SCAP:
sensor
HMG CoAreductase
and ~20
other
cholesterol
biosynthetic
genes
In small vesicles
5
HMG-CoA Reductase
Transcriptional regulation
Translation of reductase mRNA:
inhibited by Mevalonate derivatives
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HMG-CoA Reductase
Reductase degradation:
Membrane domain of the reductase
sense [sterol] => proteolysis
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HMG-CoA Reductase
Transcriptional regulation
Translation of reductase mRNA:
inhibited by Mevalonate derivatives
Reductase degradation:
Membrane domain of the reductase
sense [sterol] => proteolysis
Ubiquitination under some conditions
Activity switched off by phosphorylation
(AMP-activated Kinase) cholesterol
synthesis stops at low [ATP]
All four mechanisms are modulated by receptors that sense
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cholesterol in the blood.
Reminder: Dietary Lipids are transported in chylomicrons
bloodstream
Degraded or stored
in adipose tissues &
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muscle
Lipoproteins transport cholesterol and triacylglycerols
LDL: cholesterol transport to peripheral tissues; regulates de
novo cholesterol synthesis
HDL: pick up cholesterol released into the plasma from dying
cells or degraded membranes (reverse cholesterol transport)
10
Lipoproteins transport cholesterol and triacylglycerols
LDL
Recognized by target cells
11
12
Lipoprotein particle metabolism
Lipoprotein lipases release TAG
Excess TAG and C
FFA=free fatty acids
13
High serum level of cholesterol
Cause disease or even death
LDL, bad cholesterol carry excess cholesterol
HDL, good cholesterol picks up excess cholesterol
Healthy person: HDL/LDL=3.5
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Receptor mediated endocytosis
Cells outside the liver and intestine obtain cholesterol from
the plasma
The primary source of cholesterol is LDL
LDL uptake is a paradigm for the uptake of many
molecules
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Receptor mediated endocytosis
LDL
This region invaginating
and fusing to form a
endocytic vesicle
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Receptor mediated endocytosis
Cholesterol
regulation
clathrin
Released unesterified cholesterol can be used for membrane biosynthesis
Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes re-esterifying 17
cholesterol for storage in the cell
Michael Brown, 1941Nobel Prize in Physiology or
Medicine, 1985
Joseph Goldstein, 1940Nobel Prize in Physiology or
Medicine, 1985
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Treatment of high cholesterol level in the blood
Increase LDL receptor gene
expression by inducing the cells
need for cholesterol
Inhibit reabsorption of bile salts
from the intestine, reduce
absorption of dietary cholesterol
and fats
Inhibit de novo synthesis of
cholesterol
Statin HMG-CoA
reductase
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Important inhibitor concepts
1. Understand the regulation of cholesterol synthesis
Understand regulation on HMG-CoA reductase
Understand receptor mediated endocytosis
Transport of cholesterol and TAC by lipoproteins
LDL endocytosis
2. Understand the synthesis of bile acids and steroids
Understand the mechanism of cytochrome P450
20
Cholesterol derivatives: bile salts
primary bile salt
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Cholesterol derivatives: bile salts
Bile salts are synthesized in liver, stored in gall bladder,
released into the small intestine
Bile salts are amphipathic (contain both polar and
nonpolar regions)
Bile salts solubilize dietary lipids
Increase surface of lipids for lipase digestion
Lipids are more readily absorbed by the intestine
22
Cholesterol derivatives: steroid hormones
These hormones play
critical roles in
regulating organism
functions
They bind to and
activate receptors =>
transcriptional
regulation
Ovum implantation to
uterus; preganancy
Male
Promote
gluconeogenesis and
form of glycogen
Increase of blood
volume and pressure
Secondary sex
characteristics
23
Female
Cholesterol derivatives: steroid hormones
Numbering scheme of cholesterol and derivatives:
Rings denoted by letters
Above plane- oriented; below plane- oriented
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Steroids are hydroxylated by cytochrome P450
Monooxygenase using NADPH and O2
Heme enzyme in ER of liver and small intestine
Require O2 (O2 activated by binding to the iron in heme)
Metabolizing drugs such as caffeine and ibuprofen, ethanol
May generate powerful carcinogens from harmless
compounds -> cancer
Toxic
Mechanism: Steroids hydroxylated by cytochrome
P450 Monooxygenase using NADPH and O2
Ferryl Fe=O,
highly reactive
Forms transient
free radical R
Rcaptures OH
Flavoprotein (FR)
reduces adrenodoxin,
non-heme iron protein
Reduce Fe in P450
from ferric to ferrous,
which allows binding
(activation) of O2 26
Steroid hormones: derived from cholesterol
C27
Hydroxylated at C20 and then C22
C21
Precursor
for many
steroids
Cleaved by desmolase
Consume 3 NADPH and 3 O2
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Steroid hormones: derived from cholesterol
C3: hydoxyl -> keto
5 double bond oxidized to 4
Hydroxylation at C11, C17, C21
Hydroxylation at C21 then C11
C18: angular CH3
oxidized to aldehyde
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Steroid hormones: Androgens and Estrogens
Cleave to form androgens
Reduction at C17
Testes and muscle
mass
C19
Hydroxylation at C17
Estrogens
C19: angular CH3 ->
aromatic ring
Reduction at C17
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Splitting rings in cholesterol by light -> Vitamin D
Spontaneous Isomerization
Provitamin D3
Hydroxylation in
liver and kidney
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hormone
Vitamin D
Control of calcium and phosphorus metabolism
Vitamin D binds to a receptor -> the complex functions as a
transcription factor
Natural foods have low content of vitamin D
Insufficient vitamin D: softening of the bones (rickets)
Fish-liver oil, fortified foods (milk), exposure to sunlight
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Important concepts
1. Understand the regulation of cholesterol synthesis
Understand regulation on HMG-CoA reductase
Understand receptor mediated endocytosis
Transport of cholesterol and TAC by lipoproteins
LDL endocytosis
2. Understand the synthesis of bile acids and steroids
Understand the mechanism of cytochrome P450
32
Reference slides
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Lipoproteins transport cholesterol and triacylglycerols
A part of a liver
cell (a vesicle is
releasing VLDL).
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Characteristic conformation:
Bile salts:
Buckled structure
Steroid hormones:
Planar structure
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