1.
Biochemistry
Sixth Edition
Chapter 26
The Biosynthesis of Membrane
Lipids and Steroids
Copyright © 2007 by W. H. Freeman and Company
Berg • Tymoczko • Stryer

2.
Cholesterol is made from Acetyl CoA
 27 C atoms of cholesterol are derived from
AcetylCoA in a 3 stage synthetic process.
Stage 1: Synthesis of isopentenyl pyroP, an activated isoprene
unit that is the key building block of cholesterol
Stage 2: Condensation of 6 molecules of isopentenyl pyroP to
form squalene
Stage 3: Squalene cyclizes and the tetracyclic product is
subsequently converted into cholesterol

3.
The synthesis of mevalonate, which is activated
as isopentenyl pyrophosphate, initiates the
synthesis of cholesterol
Stage 1: Making isopentenyl pyrophosphate from
acetylCoA
The synthesis of mevalonate is the committed step in
cholesterol formation
• Enzyme: HMG-CoA reductase
• Mevalonate is converted into 3-isopentenyl pyroP in 3
consecutive reactions requiring ATP.

4.
Squalene (C30) is synthesized from 6 molecules
of isopentenyl pyroP (C5)
Stage 2: Squalene is synthesized from isopentenyl
pyrophosphate
 Reaction series: C5  C10  C15  C30
1. Isopentenyl pyrophosphate isomerization
2. Dimethylallyl pyroP and isopentenyl pyroP condenses to form geranyl
pyroP.
3. The same kind of attack takes place again
– Geranyl pyroP is converted into an allylic carbonium ion and attacked by
isopentenyl pyroP, resulting in C15 farsenyl pyroP
– Geranyl transferase catalyzes each of these reactions.
 The last step in the synthesis of squalene is a reductive tail-to-
tail condensation of two molecules of farnesyl pyroP catalyzed
by the ER enzyme squalene synthase

5.
Squalene cyclizes to form cholesterol
Stage 3: The final stage of cholesterol biosynthesis
starts with the cyclization of squalene
• Squalene is activated by conversion into squalene epoxide.
• Squalene epoxide is then cyclized to lanosterol by
oxidosqualene cyclase.
• Lanosterol is converted to cholesterol in a multistep process by
– removal of the 3 methyl groups
– the reduction of 1 double bond by NADPH
– the migration of the other double bond.

6.
Cholesterol Synthesis Broken
down into Five Steps
1. Synthesis of mevalonic acid from Acetyl-CoA
• Conversion of HMG-CoA into mevalonate by HMG-CoA
reductase is the rate limiting step!
2. Formation of isoprenoid units from mevalonic acid.
3. Six activated isoprene units undergo condensation
to form Squalene.
4. Squalene is converted into Lanosterol (in animals).
5. Cholesterol is formed from lanosterol after several
further steps that includes the loss of three methyl
groups.

7.
Regulation of cholesterol biosynthesis
 In mammals, it is regulated by intracellular cholesterol
concentration and by the hormones glucagon and insulin.
• The rate limiting step is the conversion of HMG-CoA into mevalonate;
the enzyme is HMG-CoA reductase.
– It is allosterically inhibited by cholesterol and mevalonate.
• Insulin favors cholesterol synthesis.
• Glucagon inhibits cholesterol synthesis.
 High intracellular cholesterol
– activates ACAT (acyl CoA-cholesterol acyl transferase), increasing
esterification of cholesterol for storage.
– causes reduced production of the LDL receptor, slowing the uptake of
cholesterol from the blood.

8.
HMG-CoA reductase

9.
Regulation of
cholesterol
biosynthesis

10.
Translational control of cholesterol
Feedback regulation is primarily mediated by the amount of
HMG-CoA reductase activity. This enzyme is controlled in
multiple ways.
1. The rate of synthesis of reductase mRNA is controlled by the sterol
regulatory element binding protein (SREBP).
• SREBP is a transcription factor and binds to a specific site on DNA called
SRE (sterol regulatory element).
• SREBP is attached to ER or nuclear membrane when cholesterol level is
normal.
• When cholesterol is low, SRBEP is released by proteolytic cleavage and
moves to DNA and binds to SRE to start making HMG-CoA reductase.
• When cholesterol is high, the proteolytic release of the SREBP is blocked,
and the SREBP in the nucleus is rapidly degraded.

11.
Regulation continue
2. Nonsterol metabolites derived from mevalonate inhibit
translation of reductase mRNA
3. Degradation of reductase carefully controlled.
4. Phosphorylation decreases the activity of the reductase.
This enzyme, like Acetyl CoA carboxylase, is turned off by
an AMP-activated protein kinase. Therefore, cholesterol
synthesis stops when the ATP level is low.

12.
Lipoproteins
 Cholesterol is carried in the blood plasma by plasma lipoproteins.
 They are molecular aggregates of specific carrier proteins called
“apolipoproteins”
 Different combinations of lipids and proteins produce particles of
different densities, ranging from VLDL to HDL which may be
separated by ultracentrifugation and visualized by EM.
 At least 9 different apoproteins are found.
 The protein component of lipoprotein has a specific function
determined by its point of synthesis, lipid composition, and
apolipoprotein content.

13.
Composition of serum lipoproteins
CM
85% TAG
2% protein
8% phospholipid
5% Cholesterol-esters
VLDL
60% TAG
10% protein
15% phospholipid
15% Cholesterol
LDL
8% TAG
22% protein
20% phospholipid
50% Cholesterol
HDL
3% TAG
50% protein
30% phospholipid
17% Cholesterol

14.
The blood levels of certain lipoproteins can
serve diagnostic purposes
 Bad cholesterol (LDL)
 Good cholesterol (HDL)
– HDL functions as a shuttle that moves cholesterol
throughout the body.
– HDL binds and esterifies cholesterol released from the
peripheral tissues and then transfers cholesteryl esters to
the liver or to tissues that use cholesterol to synthesize
steroid hormones.
– HDL protects us from heart attacks. Why??
For a healthy person LDL/HDL ratio is 3.5

15.
Serum lipoproteins
 They are complexes of lipids and specific
proteins called “apoproteins”
 Classified according to increasing density
– CM (chylomicron)
– VLDL (very low density)
– LDL (low density), IDL (intermediary density)
– HDL (high density)
 They function to keep lipids soluble as they
transport them in the serum.

16.
LDL plays a central role in
cholesterol metabolism
 Cells outside the liver and intestine obtain
cholesterol from the plasma
 Specifically, their primary source of
cholesterol is the LDL
 The process of LDL uptake is called
receptor mediated endocytosis.

17.
Uptake of
cholesterol by
receptor mediated
endocytosis

18.
LDL receptor
Domain 1 – Amino-terminal region
• Cys rich sequence of about 40 residues that is
repeated , with some variation, 7times
• Site of LDL binding
• Ca also binds here.
Domain 4
• Very rich in Ser and Thr
• Contains O-linked sugars that may function as
struts to keep the receptor extended from the
membrane so that the LDL-binding domain is
accessible to LCL
Domain 2
• Homologous to EGF
• Repeated 3 times , and in between the second
and third repeat is the third domain
Domain 5
• 22 hydrophobic residues
• Spans the plasma membrane
Domain 3
• Similar to the blades of the transducin β
subunit
• Exposure to the low-pH environment of the
lysosomes causes the propeller-like structures
to interact with the LDL-binding domain. This
interaction displaces the LDL, which is then
digested by the lysosome.
Domain 6
• Has 50 residues
• Emerges on the cytoplasmic side of the
membrane
• Controls the interaction of the receptor with
coated pits and participates in endocytosis
 115 kd protein
 6 domains

19.
LDL
receptor

20.
The absence of the LDH receptor leads
to hypercholesteremia
 In familial hypercholesterolemia:
– The total concentration of cholesterol and LDL in the blood plasma is
markedly elevated in this genetic disorder.
– The result of a mutation at a single autosomal locus.
 The desirable cholesterol level is <200 mg/dL. But the levels for those
with the genetic disorder are:
– 680 mg/dL in homozygotes
– 300 mg/dL in heterozygotes
 Cholesterol is deposited in various tissues because of the increased
concentration of LDL cholesterol in plasma.
– Xanthomas, nodules of cholesterol, are prominent in the skin and tendons.
– LDL can be oxidized to form oxLDL that can be taken up by immune system
cells, called macrophages. The engorged macrophages form foam cells that
are trapped in the walls of the blood vessels and contribute to the formation
of atherosclerotic plaques.

21.
atherosclerosis
 Foam cells are trapped in the blood vessel wall
making it thick, contributing to the formation of
atherosclerosis plaques.
 Most homozygotes die of CAD in childhood.
 Why HDL is good cholesterol?
– Possibly, the HDL-associated protein destroys the oxLDL.
 Molecular defect: NO LDL RECEPTOR...

22.
Atherosclerotic plaque

23.
The clinical management of cholesterol levels
can be understood at a biochemical level
 Homozygous familial hypercholesterolemia can be treated
only by a liver transplant!
 With heterozygous familial hypercholesterolemia, the goal
is to increase the gene expression so that more LDL
receptors are made.
– If the cells are deprived of cholesterol, mRNA production for the
LDL receptor would increase.
– How do we deprive cells of cholesterol?
1. By inhibiting intestinal reabsorption of bile salts. Bile salts are
cholesterol derivatives that increase the absorption of dietary
cholesterol and dietary fats (achieved by positively charged polymers).
2. Blocking de novo synthesis of cholesterol with statins, like lovastatin.
These drugs are competitive inhibitors of HMG-CoA reductase.

24.
Lovastatin

25.
Important derivatives of cholesterol
 Bile salts
• Highly effective detergents
• Made in the liver
• Stored in the gallbladder
 Building block for 5 major classes of
steroid hormones
• Progesterone
• Androgen
• Estrogen
• Glucocorticoids
• Mineralocorticoids
 Vit D

26.
Nomenclature of steroid hormones
 The rings in steroids are denoted by the letters A,
B, C and D
 Cholesterol has 2 angular methyl groups
– The C-19 methyl group is attached to C-10.
– The C-18 methyl group is attached to C-13.
 C18 and C19 methyl groups lie above the plane
containing the 4 rings
– A substituent above the plane is termed b oriented.
– A substituent below the plane is termed a oriented.

27.
How do we make steroid hormones?
 Cholesterol has 27 carbon whereas steroid hormones contain 21 or
fewer carbon.
 Need to remove 6C unit from cholesterol to form pregnenolone
– Cholesterol side chain is hydroxylated at C-20 and C-22
– The bond between these carbon atoms is subsequently cleaved by
desmolase
– Rate limiting step
 Pregnenolone is next oxidized and then isomerized to progesterone.
 Progesterone is further modified by a series of hydroxylation reactions
to other steroid hormones. These enzymes are mixed-function oxidases
requiring NADPH and oxygen.
desmolase
cholesterol pregnenolone

28.
Steroid hormones
 All steroid hormones are derived from cholesterol.
– Mineralocorticoids
• Control mineral absorption
• Example: aldosterone
– Glucocorticoids
• Regulate gluconeogenesis and decrease inflammatory response
• Example: cortizon
– Androgens
• Sex hormones
– These hormones are effective at very low concentrations and are,
therefore, synthesized in relatively small quantities.
 Adrenal cortex has three histological zones that are
exclusive steroid producers.
– Together, the zones can produce all classes of steroid hormones.
– Each zone has cells that make different steroid hormones.

29.
Steroid hormone synthesis
Cholesterol (C27)
pregnenolone
desmolase
3-b-OHsteroid
dehydrogenase
Deficiency
No gluco, mineralo or sex h
Early death
progesteron
17-a-OHprogesteron
testosteron
estradiol
11-deoxycortisol
cortisol
21-a-OHlase
1-deoxycorticosterone
Aldosteron
11-b-OHlase
17-a-OHlase
Common form of CAH
Aldosteron and cortisol
sex hormones
Cortisol, aldosteron
sex hormones
Congenital Adrenal
Hyperplasia (CAH)

30.
IMPORTANT
 A defect in the activity or amount of an enzyme in
steroid hormone synthesis pathway can lead to
BOTH
– a deficiency in the synthesis of hormones beyond the
affected step
AND
– an excess in the hormones or metabolites before that
step.
 Therefore severe metabolic imbalances may
occur.

31.
Secretion of adrenal steroid hormones
 Adrenal cortical hormone secretion is controlled by the
hypothalamus, to which the pituitary gland is attached.
 When the body is stressed, released factors travel to
the anterior lobe that produce and secrete ACTH
(adrenocorticotropic hormone).
 ACTH is often called “stress hormone”
• Stimulates adrenal cortex to make mineralocorticoids and
glucocorticoids (collectively called corticosteroids)
 Corticosteroids bind to their specific receptors and do
their action

32.
Actions of corticosteroids
 Aldosteron
• Stimulates reabsorption of Na and excretion of K
 Cortisol
• Increased gluconeogenesis
• Anti-inflammatory action
 Estrogens
• Controls menstrual cycle
• Promotes female secondary sex characteristics
 Progesteron
• Maturation of fertilized ovum
 Testosteron
• Stimulates spermatogenesis
• Promotes male secondary sex characteristics

33.
Vit D is derived from cholesterol
by the ring-splitting activity of light
 Cholesterol is also Vit D precursor.
 Vit D plays an important role in Ca and phosphorous
metabolism
 7-dehydrocholesterol (provitD3) is photolyzed by the UV
light of sunlight to preVit D3. This spontaneously
isomerized to D3, D3 is then converted to Calcitriol, the
active hormone in the liver and kidney.
 Vit D deficiency in children causes rickets
 Recommended daily intake of Vit D is 400 iu.
 In adults Vit D deficiency causes osteomalacia (softening
of bones)

34.
Biochemistry
Seventh Edition
CHAPTER 26
The Biosynthesis of Membrane
Lipids and Steroids
Copyright © 2012 by W. H. Freeman and Company
Berg • Tymoczko • Stryer
Q.
Mice were divided into four groups, two of which were fed a normal diet and two of which were
fed a cholesterol-rich diet. HMG-CoA reductase mRNA and protein from liver were then isolated
And quantified. Graph A shows the results of the mRNA isolation.
a. What is the effect of cholesterol feeding on the amount of HMG-CoA reductase mRNA?
b. What is the purpose of also isolating the mRNA for the protein actin?
HMG-CoA reductase protein was isolated by precipitation with a monoclonal antibody to
HMG-CoA reductase. The amount of HMG-CoA protein in each group is shown in graph B.
c. What is the effect of the cholesterol diet on the amount of HMG-CoA reductase protein?
d. Why is this result surprising in light of the results in graph A?
e. Suggest possible explanations for the results in graph B.