Lecture #15a
Synthesis of Membrane Lipids and
Cholesterol
Slide 1. Synthesis of glycerol phosphate derivatives. Phosphoglyerol
derivatives contain a number of bonds that can only be synthesized with the
net input of energy. These bonds include:
-the ester linkages between the fatty acids and the glycerol and also,
-the phosphodiester linkages that connect the central phosphate to glycerol
and an organic alcohol
Slide 2. Biosynthesis of phosphatidic acid. The pathway of phosphatidic
acid synthesis from L-glycerol-3 phosphate involves the formation of two
fatty acyl CoA derivatives which then condense with the two hydroxyl
groups of glycerol 3-phosphate to form ester linkages. One ATP is cleaved
to AMP and inorganic pyrophosphate for each ester bond synthesized.
Slide 3. Metabolic fates of phosphatidic acid. The Phosphatidic acid can
serve as a metabolic precursor of both triacylglycerols and phosphoglycerol
derivatives
-In the synthesis of triacylglycerols the phosphate group is first cleaved from
the 3-position of phosphatidic acid yielding 1,2-diacylglycerol. A third fatty
acyl group is then added using an acyl CoA derivative as the donor
molecule.
-In the synthesis of phosphoglycerol derivatives, various head groups can be
added to the phosphate group of phosphatidic acid. These reactions require
energy input to forge a phosphoester linkage between the phosphate group
and the organic alcohol.
Slide 4. Formation of CDP-diacylglycerol. One method for activating the
phosphate group of phosphatidic acid is to form an activated CDPdiacylglycerol derivative. In this reaction phosphatidic acid reacts with
CTP. The CMP component of CTP becomes attached to the phosphate
group of phosphatidic acid and a pyrophosphate group is released. The
reaction is driven by the subsequent energy releasing cleavage of the
pyrophosphate to two molecules of inorganic phosphate (not shown).
Slide 5. Two pathways for the synthesis of glycerolphosphate
derivatives. The figure 21-24 shows two different general pathways (both
energetically favorable) for synthesizing glycerolphosphate derivatives:
-In the first pathway (shown on the left,) CDP-diacylglycerol is condensed
with the hydroxyl group of an alcohol with the release of CMP.
-In the alternative pathway (shown on the right), glycerolphosphate
derivatives are formed when 1,2-diacylglycerol reacts with a CDP-alcohol
derivative with the release of CMP.
Slide 6. Synthesis of cardiolipin and phosphatidylinositol. The
activated intermediate, CDP-diacylglycerol is the precursor of both
cardiolipin (diphosphatidylglycerol) and phosphatidylinositol. In both
reaction sequences CMP is released from CDP-diacylglycerol and a
phosphoester linkage is formed with a hydroxyl group of the alcohol
substrate.
Slide 7. Interconversion of phosphatidylserine and
phosphatidylethanolamine. The phosphotidylglycerol derivatives,
phosphatidylserine and phosphatidylethanolamine, can be interconverted
by a transferase reaction that does not require any energy input.
Slide 8. Pathway for phosphatidylcholine synthesis. The synthesis of
phosphatidylcholine (lecithin) involves the alternate type of pathway in
which 1,2-diacylglycerol reacts with a CDP-alcohol derivative (in this case
CDP-choline) with the release of CMP.
Slide 9. Origin of the carbon atoms of cholesterol. We now turn to a
consideration of cholesterol which is the good guy/bad guy of lipids.
Cholesterol has many necessary functions including serving as an essential
membrane component and as a precursor of steroid hormones and bile acids.
On the other hand cholesterol esters that accumulate in the arteries of old
guys like me are the villains which cause atherosclerosis leading to heart
attacks and strokes.
The diagram shows that all of the carbons of cholesterol are derived from
acetylCoA—the red carbons from the carboxyl group and the black carbons
from the methyl group. Cholesterol has three six-membered rings and one
five-membered ring, none of which are aromatic.
Slide 10. Summary of cholesterol biosynthesis. This diagram breaks the
pathway of cholesterol biosynthesis into four arbitrary stages. Although it is
not shown here, each of those stages involves multiple steps catalyzed by
many different enzymes. The first stage begins with the two carbon
compound, acetylCoA and ends with a six carbon intermediate, mevalonate.
The second stage begins with mevalonate and ends with a five carbon
activated isoprene analog, isopentenyl-pyrophosphate. Next, in stage three,
six isopentenyl-pyrophosphate units are polymerized to give a 30-carbon
intermediate, squalene. And finally in stage four, squalene is converted to
cholesterol by a series of rearrangements and ring closure reactions.
Slide 11. Pathway from AcetylCoA to Mevalonate. We mentioned before
that acetylCoA occurs at a branch point in metabolism. In this case it serves
as the precursor of cholesterol. In the first reaction, two molecules of
acetylCoA condense to form acetoacetylCoA. The acetoacetylCoA can then
react with a third molecule of acetylCoA to give 3-hydroxy-3methylglutraryl-CoA. The thioester carbonyl group of 3-hydroxy-3methylglutraryl-CoA is then reduced to an alcohol yielding mevalonate.
Notice that it takes two molecules of NADPH to convert the thioester (which
is the equivalent of a carboxyl group) to and alcohol. That is a four electron
reduction, hence the need for two molecules of NADPH. In the process a
CoA molecule is released. The driving force for this series of reactions is
the release of two CoA units and the conversion of two NADPH molecules
to NADP+.
Slide 12. Conversion of Mevalonate to Geranyl Pyrophosphate. In the
next series of reactions the mevalonate is first converted to a pyrophosphate
derivative. That takes two ATP’s going to ADP. The next reaction
involves an ATP dependent dehydration and decarboxylation. Wow! All
that stuff happening in one reaction—I am not going to burden you with the
mechanism, primarily because I don’t know how it works—else I would
happily burden you, and it would have very little effect on my conscience.
The end result is a nice little five-carbon compound isopentenyl
pyrophosphate which has a carbon-carbon double bond at one end. Next
there is an isomerization of that new double bond. At that point two fivecarbon dimethylallyl pyrophosphate molecules condense to form a ten-
carbon product, geranyl pyrophosphate with the release of an inorganic
pyrophosphate. This series of reactions is energized by the cleavage of three
molecules of ATP, a decarboxylation reaction, and the release of a
pyrophosphate. I am sure that the pyrophosphate is promptly cleaved to two
inorganic phosphates to give the pathway a further boost.
Slide 13. Formation of Geranyl Pyrophosphate. In the next reaction a ten
carbon geranyl pyrophosphate condenses with a five-carbon isopentenyl
pyrophosphate to give a fifteen-carbon farnesyl pyrophosphate. An
inorganic pyrophosphate molecule is release in the process, and its cleavage
would again promote the forward reaction.
Slide 14. Conversion of Farnesyl Pyrophosphate to Cholesterol. This
slide summarizes the rest of the pathway, and thank goodness, most of the
action takes place off the slide. I am really sick of this pathway by this
point. In essence two fifteen-carbon farnesyl pyrophosphate molecules
condense to give a thirty-carbon product, squalene. Then there are a series
of reactions which bond various carbon atoms together to form three fused
six-membered rings and a fused five membered ring. Somewhere along the
line three carbons are lost as carbon dioxide, a double bond is introduced
into one of the six-membered rings and one of the rings is hydroxylated. We
are left with the final 27 carbons of cholesterol—three fused six membered
rings, one fused five membered ring, and an eight-carbon hydrocarbon side
chain. This final phase of the pathway is energized by NADPH oxidation,
pyrophosphte release, and some other stuff which will remain anonymous.
Slide 15. Converison of squalene to cholesterol. A series of “magic”
enzyme reactions convert the linear compound squalene into the condensed
steroid nucleus of cholesterol. Note again that cholesterol is not an aromatic
compound and that many of its carbon atoms exhibit chirality.
Slide 16. Steroid hormones derived from cholesterol. Here we see some
of the good guy features of cholesterol. It is a necessary precursor of steroid
hormones including progesterone, testosterone, and estradiol.
Slide 17. Structure of low density lipoprotein (LDL). Here we see the
structure of the low density lipoprotein particle (LDL). Cholesterol,
cholesterol esters, phosphoglycerols, and triacylglycerols are transported in
body fluids in the form of lipoprotein particles. Each particle consists of a
core of hydrophobic lipids surrounded by a shell of more polar lipids and
proteins. The protein components of these lipoprotein particles are called
apoproteins and they have two roles:
-one role is to solubilize hydrophobic lipids
-the other is to act as cell-targeting signals
Slide 18. Electron micrograph of lipoprotein particles. Lipoprotein
particles vary in size, composition and function. Chylomicrons are the
largest and high density lipoproteins (HDL) are the smallest. Very low
density lipoproteins (VLDL) and low density lipoproteins (LDL) are
intermediate in size.
Slide 19. Major classes of human plasma lipoproteins. This table gives
the biochemical composition of the major lipoprotein particles.
Chylomicons have the highest percentage of triacylglycerols and the lowest
percentage of protein. Conversely, HDL particles have the lowest
percentage of triacylglycerols and the highest percentage of protein.
The primary functions of these lipoprotein particles are as follows:
-Chylomicrons carry dietary lipids (primarily triacylglycerols) from the
intestines to various tissues
-VLDL particles carry excess triacylglycerols and cholesterol from the liver
-IDL particles are synthesized from VLDL particles when triacylglycerols in
the IDL particles are hydrolyzed on capillary surfaces
-LDL particles are formed from IDL particles upon further hydrolysis of
triacylglycerols. The major role of LDL particles is to transport cholesterol
to peripheral tissues and to regulate de novo cholesterol synthesis at these
tissues.
-HDL particles pick up cholesterol released into the plasma by dying cells
and from membranes undergoing turnover. The cholesterol is esterified in
the HDL particles and returned to the liver.
Slide 20. Uptake of cholesterol by receptor mediated endocytosis. The
figure shows how LDL particles are taken up into tissues by receptormediated endocytosis. Subsequent to uptake the various LDL constituents
are broken down into their component parts which are then used for
biosynthetic reactions.
Slide 21. Relationship between cholesterol and atherosclerosis. The
relationship between cholesterol and atherosclerosis is well established:
-The lipid plaques in occluded blood vessels, which cause atherosclerosis,
have high concentrations of cholesterol esters.
-Occluded blood vessels result in heart attacks and strokes.
-The occurrence of atherosclerosis is linked to high blood cholesterol levels
-There is a positive correlation between high LDL levels and the occurrence
of atherosclerosis.
-There is a negative correlation between high HDL levels and the occurrence
of atherosclerosis.
Slide 22. Dietary effects on cholesterol levels and on atherosclerosis.
The secret to avoiding atherosclerosis is to eat a diet which is low in total
fat, saturated fat, cholesterol, (also low free sugar) and to choose parents
who are not prone to atherosclerosis.
Slide 23. Biosynthesis of cholesterol esters. This slide reminds us that the
major components of atherosclerotic plaques are cholesterol esters. These
esters are synthesized in the body from cholesterol and fatty acyl CoA’s.
Slide 24. An artery partially occluded with a fatty plaque. Here is an
artery that is about 50% occluded with cholesterol ester material. This is not
good.
Slide 25. An artery more occluded with a fatty plaque. Here is an artery
that is about 75% occluded with cholesterol ester material. This is even more
not good.
Slide 26. An artery totally occluded with fatty plaque and blood clot.
This artery is about 85% occluded with fat and the remaining area has been
blocked with a blood clot. This is what happens in heart attacks and strokes.
Needless to say you do not want this to happen in your arteries. On the
positive side, this is more likely to be me than you at this time. Oh! To be
young again!
Slide 27. Human genetic disorders promote atherosclerosis. Speaking of
choosing your parents:
-Familiar hypercholesterolemia is a hereditary condition in which there is
a defect in LDL receptors which reduces or prevents the ability to carry out
receptor mediated endocytosis
-Familial HDL deficiency results from a lack of a protein ABC1. In cells
lacking the ABC1 protein there is a lack of uptake of cholesterol by HDL
particles, and the cholesterol poor HDL particles are rapidly destroyed.
Slide 28. Regulation of cholesterol biosynthesis. The synthesis, uptake
and metabolism of cholesterol are regulated by a number of physiological
mechanisms. Cholesterol inhibits its own uptake and biosynthesis. In
addition there is both stimulation and inhibition of cholesterol biosynthesis
by various hormones.
Slide 29. Inhibitors of HMG-CoA reductase. HMG-CoA Reductase is a
key reaction in the biosynthesis of cholesterol. There are a number of drugs
available that act as inhibitors of HMG-CoA Reductase. Components of
these drugs seem to mimic the structure of mevalonate.