1. CHOLESTEROL
SYNTHESIS

2. CHOLESTEROL
“Cholesterol is the most highly decorated small molecule in biology. Thirteen Nobel Prizes have been awarded to scientists who devoted major parts of their careers to cholesterol. Ever since it was isolated from gallstones in 1784, cholesterol has exerted an almost hypnotic fascination for scientists from the most diverse areas of science and medicine…. Cholesterol is a Janus-faced molecule. The very property that makes it useful in cell membranes, namely its absolute insolubility in water, also makes it lethal.”
Michael Brown and Joseph Goldstein
Nobel Lectures (1985)

3. CHOLESTEROL

4. CHOLESTEROL

5. Biosynthesis of membrane lipids and steroids 1
Cholesterol is synthesized by virtually all tissues in humans, although liver, intestine, adrenal cortex, and reproductive tissues, including ovaries, testes, and placenta, make the largest contributions to the body’s cholesterol pool.
As with fatty acids, ALL THE CARBON ATOMS IN CHOLESTEROL ARE PROVIDED BY ACETATE, and NADPH provides the reducing equivalents.
Biosynthesis of membrane lipids and steroids 1

6. Biosynthesis of membrane lipids and steroids 1
Synthesis requires enzymes in both the cytosol and the membrane of the smooth endoplasmic reticulum (ER).
The pathway is responsive to changes in cholesterol concentration, and regulatory mechanisms exist to balance the rate of cholesterol synthesis within the body against the rate of cholesterol excretion.
An imbalance in this regulation can lead to an elevation in circulating levels of plasma cholesterol, with the potential for
vascular disease.
Biosynthesis of membrane lipids and steroids 1

7. B. The Biosynthesis of Cholesterol
All 27 carbon atoms of cholesterol are derived from acetyl CoA:
Modified from Nelson & Cox, Lehninger Principles of Biochemistry, 3rd Edition, Fig
Reproduced from Stryer, Biochemistry, 4th Edition, p. 692.

8. Biosynthesis of membrane lipids and steroids 1
The first two reactions in the cholesterol synthetic pathway are similar to those in the pathway that produces ketone bodies (see Figure 16.22, p. 196). They result in the production of HMG CoA (Figure 18.3). First, two acetyl CoA molecules condense to form acetoacetyl CoA. Next, a third molecule of acetyl CoA is added, producing HMG CoA, a six-carbon compound. [Note: Liver parenchymal cells contain two isoenzymes of HMG CoA synthase. The cytosolic enzyme participates in cholesterol synthesis, whereas the mitochondrial enzyme functions in the pathway for ketone body synthesis.]
Biosynthesis of membrane lipids and steroids 1

9. Biosynthesis of membrane lipids and steroids 1
SYNTHESIS OF CHOLESTEROL
Acetyl-CoA à 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA)
HMG-CoA à mevalonate
Mevalonate à isopentenyl-PP
6 isopentenyl-pyrophosphate à squalene
Cyclisation of squalene à cholesterol
Biosynthesis of membrane lipids and steroids 1

10. The first stage in cholesterol synthesis is the formation
of isopentenyl pyrophosphate
from acetyl CoA.
a) Formation of 3-hydroxy-3-methylglutaryl CoA
(i.e., HMG CoA) from acetyl CoA and acetoacetyl CoA.

11. The next step, the reduction of HMG CoA to mevalonate, is catalyzed by HMG CoA reductase, and is the rate-limiting and key regulated
step in cholesterol synthesis. It occurs in the cytosol, uses two molecules of NADPH as the reducing agent, and releases CoA, making the reaction irreversible (Figure 18.4).

12. Mevalonate formation is the first stage of cholesterol synthesis (summary so far)

13. [1] Mevalonate is converted to 5-pyrophosphomevalonate in two
steps, each of which transfers a phosphate group from ATP.
[2] A five-carbon isoprene unit—isopentenyl pyrophosphate (IPP)— is formed by the decarboxylation of 5-pyrophosphomevalonate.
The reaction requires ATP.
[3] IPP is isomerized to 3,3-dimethylallyl pyrophosphate (DPP).

14. [4] IPP and DPP condense to form ten-carbon geranyl pyro -
phosphate (GPP).
[5] A second molecule of IPP then condenses with GPP to form 15-
carbon farnesyl pyrophosphate (FPP). [Note: Covalent attachment
of farnesyl to proteins, a process known as “prenylation,” is
one mechanism for anchoring proteins to plasma membranes.]
[6] Two molecules of FPP combine, releasing pyrophosphate, and
are reduced, forming the 30-carbon compound squalene. [Note:
Squalene is formed from six isoprenoid units. Because three ATP
are hydrolyzed per mevalonate residue converted to IPP, a total
of 18 ATP are required to make the polyisoprenoid squalene.]

15. [7] Squalene is converted to the sterol lanosterol by a sequence of
reactions catalyzed by ER-associated enzymes that use molecular
oxygen and NADPH. The hydroxylation of squalene triggers
the cyclization of the structure to lanosterol.
[8] The conversion of lanosterol to cholesterol is a multistep process,
resulting in the shortening of the carbon chain from 30 to
27 carbons, removal of the two methyl groups at carbon 4, migration
of the double bond from carbon 8 to carbon 5, and reduction
of the double bond between carbon 24 and carbon 25. [Note:
This ER-associated pathway includes several different enzymatic
reactions. Smith-Lemli-Opitz syndrome (SLOS), a relatively common
autosomal recessive disorder of cholesterol biosynthesis, is
caused by a partial deficiency in 7-dehy dro cholesterol-7-reductase—
an enzyme involved in the migration of the double bond.
SLOS is one of several multisystem, embryonic malformation
syndromes associated with impaired cholesterol synthesis.]

17. Dimethylallyl-PP + isopentenyl-PP = Geranyl-PP (5 + 5) = 10
Geranyl –PP + isopentenyl-PP = farnesyl-PP (10 + 5) = 15
Farnesyl-PP + Farnesyl-PP =
Squalene ( )= 30
PPi release renders rx. irreversible. C5
C10
C15
C30

18. Steps of cholesterol biosynthesis IV5 5
Dimethylallyl-PP + isopentenyl-PP = Geranyl-PP (5 + 5) = 10
Geranyl –PP + isopentenyl-PP = farnesyl-PP (10 + 5) = 15
Farnesyl-PP + Farnesyl-PP = Squalene ( )= 30
NADPH + H+
PPi
Squalene -syntase – Endoplasmic reticulum 10 5 15 15
Biosynthesis of membrane lipids and steroids 1 30

19. 4. The final stage of cholesterol synthesis
consists of the following steps:
In a reaction utilizing O2 and NADPH
squalene epoxide is formed. The
enzyme is squalene monooxygenase.
The epoxide is cyclized to lanosterol
by a cyclase with formation of 4 rings
and 4 new C-C bonds.
Finally, lanosterol is converted to
cholesterol via a series of rxs. resulting
in: removal of 3 methyl grps., reduction
of 1 double bond by NADPH, and the
migration of another double bond.

20. Steps of cholesterol biosynthesis V
Cyclization
Monooxygenase
C2,3 epoxide
Transfer of Methylgroups
14-13, 8 – 14
Sterol carrier protein 1 (SCP1)
Squalene
Sterol carrier protein 2 (SCP2)
Lanosterol
Biosynthesis of membrane lipids and steroids 1

21. Steps of cholesterol biosynthesis V
1 molecule of cholesterol
18 acetyl-CoA
36 ATP
16 NADPH
Biosynthesis of membrane lipids and steroids 1