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Biochemistry lec# 3
Dr. faisal al-khateeb
27/4/2011 Wednesday
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Chapter 18
*Slide (1)  Chole/ster/OL, from its name ster it’s a
steroid contain steroid nucleus, OL alcohol, chole gall
bladder (bile) because it was first isolated from the gall bladder
almost 200 years ago so it was named according to the gall
bladder, its an alcohol steroid from the gall bladder.
*Slide (2)  This is a steroid nucleus consists of 4 rings, 3
six-member rings and the fourth is a five-member ring. Total
number of carbon is 17C.
(We have to know the numbering of carbons in the steroid
nucleus and how to draw it).
*Slide (3)  This is the complete structure of cholesterol.
That’s consisting of steroid nucleus and a hydrocarbon tail.
Features of the cholesterol structure:1) It contains the hydroxyl group at carbon
number 3, that’s why it's named cholesterol.
(we have to know its location)
2) Double bond between carbon number 5 and
carbon number 6.
3) 2 methyl groups attached to carbon 10 (carbon
19 of the first methyl group) and carbon 13
(carbon 18 of the second methyl group).
4) Side chain, that is consist of 8 carbons
attached to carbon 17
* This makes the total number of carbon atoms in cholesterol
equal to 27.
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*Slide (4)  These are the gall bladder stones, they are
made of cholesterol so cholesterol may precipitate in the gall
bladder forming cholesterol stones that’s because its is poorly
or insoluble in water and its secreted in the bile from where it
was isolated first. It should be in the soluble form, if the process
of making it soluble is defected, stones will be formed.
* Note cholesterol ester; fatty acid is esterifies to
cholesterol by eliminating the hydroxyl group at carbon 3 making
the cholesterol ester non polar compound.
*Slide (5)  Sources of cholesterol:1) Cholesterol is synthesized in the liver, small
intestine, adrenal cortex and number of
tissues at a rate of 1000 mg/day.
** Cholesterol is needed in all animal cells and all animal cells
are capable of producing cholesterol, which shows its importance
for the cells.
**As cholesterol is mainly produced by the liver and small
intestine it's then transported to different cells. Although cells have
the ability to make it they get it ready made.
2) The dietary cholesterol present about 300
mg/day in the case of low cholesterol diet, it
may be more than that, even if we want to
reduce the amount of cholesterol with normal
diet, we cant reduce it below 300 mg/day
because we are consuming meat, egg, milk or
any product that contain cholesterol.
**Cholesterol is found in animal product because all animal cells
contain cholesterol. On the other hand plant cells don’t produce
cholesterol, any food derived from plants or vegetable oil doesn’t
contain cholesterol.
**Although cholesterol is produced by all cells, it's not degraded
by these cells.
**Cholesterol is eliminated from the body either as it's in the
bile (cholesterol) or it is converted to bile salts and bile acids. Little
amount of cholesterol is converted to steroid hormones.
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*Slide (6)  This is an Ergosterol (which differs a little bit
from cholesterol)
-It's found in plant cells, so plant cells produce sterols other
than cholesterol.
-Plant steroids are poorly absorbed by humans, if they are
absorbed they are excreted again into the small intestine along
with cholesterol. So plan sterols are helpful to reduce the amount
of cholesterol in the body because their elimination or excretion is
accompanied with cholesterol.
*Slide (7)  Homeostasis of cholesterol;
These are the three major roots representing the source of
cholesterol in the liver. It's obtained from 1) diet through
chylomicron remnants then cholesterol is transferred to the liver, 2)
other part is synthesized by the liver, 3) cholesterol synthesized
and transported from the tissues to the liver through HDL.
**cholesterol is distributed from the liver to 1) tissues in the
form of VLDL, 2) free cholesterol is secreted in the bile,3) half of
the cholesterol is converted to bile salts or bile acids to be excreted
in the small intestine.
**Eggs are the major source of dietary cholesterol on daily
bases, one egg contains 200 mg of cholesterol which is almost the
daily requirement so if we want to reduce cholesterol intake one
egg is sufficient ( low cholesterol diet).
*Slide (8)  Cholesterol synthesis requires:1) Carbon source: acetyl CoA (produced from Boxidation, pyruvate dehydrogenase), all the 27
carbon atoms of cholesterol are produced from
acetyl CoA. The binding occur by labeling the
acetyl groups in acetyl CoA (like using C 13 ) , it
was found that all cholesterol atoms (27) comes
from the two carbon atoms of acetyl CoA.
2) Energy: in the form of ATP (in order to join
these molecules together).
3) Reducing power: NADPH (as in the case of
fatty acid synthesis).
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**Acetyl group is much more oxidized than the final product
cholesterol which is reduced so that’s why we need a reducing
agent.
4) One atom of oxygen (O2) for producing the
oxygen of cholesterol.
*Slide (9)  Stages of cholesterol synthesis.
- Cholesterol synthesis pathway is a long path
consists of more than 20 steps. But we will
study specific stages of the pathway.
- Over view of cholesterol synthesis starts with
Acetyl coA (two carbons compound) then it is
converted to Mevalonate (6 carbon
compound) which is decarboxylated to
Isoprene units (hydrocarbon units of 5
carbon atoms " isopentenyl") six of these
isoprene units condense together in several
steps to produce hydrocarbon compound
called Squalene (30 carbon compound) then
its converted to sterole, Lanosterole( 30
carbon compound) after many steps its
converted to Cholesterol (27 carbon
compound).
*Slide (10)  First step of cholesterol synthesis: acetyl CoA
condensation.
Two Acetyl CoA gives Acetoacetyl CoA then the third
acetyl CoA condenses to produce HMG CoA
(hydroxymethylglutaryl CoA) an intermediate in ketone bodies
synthesis. The enzyme required to catalyze this step is HMG CoA
synthase. These two steps are similar to steps of ketone bodies
synthesis but ketone bodies synthesis occurs in the mitochondria
while cholesterol synthesis occurs in the cytosol or cytoplasm.
*Slide (11)  The third step is reduction of carboxyl group
linked to coenzyme A. First reduction step produces an aldehyde
then the second reduction step produces an alcohol so two
reduction steps will convert HMG CoA into Mevalonate or
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mevalonic acid ( it’s a 6 carbon compound with a hydroxyl group
attached to carbon 3 and a carboxyl group).
*Slide (12)  Now how Mevalonic acid is condensed to
produce hydrocarbon compound squalene?
-Hydroxyl group at carbon number 1 in mevalonic acid is
phosphorylated, two steps of phosphorylation using two ATP
molecules produces 5-Pyrophosphomevalonic acid (2
phosphate groups are added to activate and produce isoprene
units). The next step is decarboxilation of the hydroxyl group at
carbon 3 and elimination of H2O by the usage of ATP gives up
Isopentenyl pyrophosphate, then isopentenyl pyrophosphate is
isomerized into 3,3-Dimethylallylpyrophosphate. Both
isopentenyl pyrophosphate and its isomer condensate to form (10
carbon) Geranyl pyrophosphate, one more condensation gives
Farnesyl pyrophosphate (15 carbon compound).
** The idea is activation then several condensations, to convert
isopentenyl group into 10 carbon compound then 15 carbon
compound.
*Slide (13)  Two farnesyl pyrophosphate condense head to
head so two pyrophosphate groups will be removed producing
squalene( hydrocarbon branched compound consist of hydrogen
and carbon, contain 30 carbon)
** Pathway of squalene synthesis is similar to many compounds
that are formed from number of isoprene units like coenzyme Q,
some vitamins, and rubber in plants.
*Slide (14)  A hydrocarbon chain formed from 30 carbons:
squalene.
It's drawn in this way to show the similarity to cholesterol structure.
*Slide (15)  squalene is converted to squalene 2, 3
epoxide. Oxygen now is needed and it is added to carbon 2, 3 in
the form of epoxide which is considered as a very unstable
compound that rapidly change to form lanosterol (first sterole, has
about 30 carbon), next steps contain several modification to
convert lanosterol into cholesterol.
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*Slide (16)  The last step in the synthesis pathway
7-dehydrocholesterol is the substrate for production of vitamin D
in the skin by the exposure to sun light (vitamin D3).
*Slide (17)  Overall steps for cholesterol synthesis with the
intermediate.
(We should be familiar with it!)
*Slide (18)  Synthesis of bile acid.
The conversion of cholesterol to cholic acid (intermediate in
the TAG digestion) involve hydroxylation at carbon 7 and 12, this
first step in bile acids synthesis is catalyzed by an enzyme that
adds hydroxyl group to carbon 7 called 7-a-hydroxylose. It is a
rate-limiting step and its regulated, activated by cholesterol and
inhibition by cholic acid. So it’s a feed-back inhibition, bile acids
inhibit their own synthesis. If we decrease the amount of cholic
acid in the liver, this will help in converting more cholesterol into
bile acids and getting rid of cholesterol (stimulating the
conversion).
*Slide (19)  Synthesis of bile acids and their circulation
between the liver and the small intestine.
First step: cholesterol is converted to primary bile acids (cholic
acids) then these cholic acids are conjugated with glycine to
produce conjugated bile acids, these are secreted by the liver to
the bile then to small intestine, in the small intestine they are
conjugated Again and the primary bile acids are converted to
secondary bile acids by removing some hydroxyl groups.
-Primary: - they are produced in the liver
-Secondary:-they are modified in the small intestine and
reabsorbed after doing their job (which is making the fat soluble),
then they are added again to the liver to be execrated again .So
everyday 30 g of bile salt are secreted, 90% are reabsorb and
execrated. This enterohepatic cycle occur many times per day.
Some cholesterol or bile acids escape from reabsorbing. So there
is net loss about of 0.5 g per day of bile acids and this represent
the amount that should be synthesized by the liver per day.
** Everyday 0.5g cholesterol is lost in fesses and 0.5 g
produced by the liver.
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**Any substance that binds to bile acids and prevent
reabsorption will help in decreasing the amount of bile acids
absorbed in the liver so it will increase the amount of cholesterol
converted to bile acids. If we cut this circle we are forcing more
cholesterol to be converted into bile acid and this is away to reduce
the amount of cholesterol in the body.
*Slide (20)  Lowering cholesterol level.
- That's in order to prevent and protect our bodies from
atheroscterosis.
- it can be achieved;
1. Dietary:* first step- is to reduce the cholesterol
intake to less than 300 mg per day. But
unfortunately even if the cholesterol intake is
reduced the synthesis will increase. If we reduce the
cholesterol intake into zero, the cholesterol
percentage will decrease only 10% not more than
that.
*Second step-increase polyunsaturated fatty
acid to saturated fatty acid (like w-3/ w-6/w-9) will
decease the cholesterol level.
*Third step- increasing the fiber in the food
(dietary fiber) achieve 5-10% in lowering cholesterol
level but we can't make all our food fibers.
*Forth step-ingestion of plant steroid esters.
2. Inhibition of synthesis may be the only possible way to
reduce the cholesterol into the target level.
3. Decrease enterohypatic circulation of bile acid by
some drugs that bind to bile acid and prevent their reabsorbtion.
*Slide (21)  This is the structure of HMG.
Conversion of HMG to mevalonate is a rate limiting step in
cholesterol synthesis. The enzyme HMG coA reductase can be
inhibited by drugs so the rate of synthesis will be decreased.
Simvastatis is a very common drug; it has a structure (ring) that is
similar to HMG. It binds to enzyme HMG CoA reductase and
inhibits it.
*Slide (22)  A very common drug (lipitor)
- Is used to reduce cholesterol synthesis.
- HMG CoA reductase inhibitor.
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*Slides (23)  Esterefication of cholesterol in the cells into
cholesterol ester occur by the addition of fatty acid to hydroxyl
group at carbon 3. Source of fatty acid is fatty acyl CoA
- type of enzymes  acyl transferase ( transfer fatty acid
from acyl CoA to the cholesterol ), its whole name acyl
CoA : cholesterol acyltranferase (ACAT).
*Slide (24) Esterefication of cholesterol in the plasma
-There is NO acyl CoA in the plasma. Cholesterol in
the plasma is found in the lipoproteins and HDL.
In the lipoproteins there is phospholipids lecithin
which is the donor of fatty acid, after donating fatty acid
it becomes lycolycitin, the enzyme required is lecithin:
cholesterol acyl transferase (LCAT).
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