This document discusses cholesterol absorption, synthesis, and metabolism. It covers: 1) Cholesterol is obtained through diet or synthesized in the liver, with acetyl CoA as the precursor. HMG-CoA reductase, the rate-limiting enzyme in synthesis, is regulated by feedback and degradation. 2) Cholesterol is transported by lipoproteins like chylomicrons, VLDL, and HDL. VLDL delivers cholesterol to tissues and is converted to LDL, while HDL performs reverse cholesterol transport. 3) Excess cholesterol is stored as esters or converted to bile salts to aid absorption. Oxidized LDL taken up by macrophages leads to foam cell formation and atherosclerosis.

1. Cholesterol
Absorption,
Synthesis, &
Metabolism
By
Meroka A,
Med Biochemistry Dept,
KEMU

2. Cholesterol Background
• Atherosclerotic vascular
disease
• Stabilizes cell
membrane
• Precursor to bile salts
and steroid hormones
• Cholesterol precursors
converted to ubiquinone,
dolichol, & vitamin D

3. Cholesterol Background
Synthesis
• Obtained through diet
or synthesis
• Synthesized in many
cells, but mostly in the
liver and intestine
• Acetyl CoA is the
precursor to
cholesterol synthesis –
raw material

4. Cholesterol Background
(Transport)
• Chylomicrons & VLDL transport cholesterol to other cells
through the bloodstream
• Chylomicrons package cholesterol in intestine, while VLDL
package in liver
• Triacylglycerols are also transported by Chylomicrons and
VLDL
• HDL – reverse cholesterol transport

5. Student Learning Outcomes
• Describe the rate-limiting step in cholesterol
synthesis and how the HMG-CoA reductase is
regulated
• Briefly describe the fates of cholesterol
• Describe the aspects of Atherosclerosis

6. Cholesterol Synthesis
• Perhydrocyclopentanophenanthrene structure
consists of four fused rings
• Cholesterol contains a hydroxyl group at C3,
double bond between C5 & C6, eight-membered
hydrocarbon chain at C17, & methyl groups at
C10 & C13
Cholesterol
Perhydrocyclopentanophenanthrene
Fig. 1
Fig.2

7. Cholesterol Synthesis
Stage I: Acetyl CoA to Mevalonate
A. B. C.
Rate limiting step
Fig.3

8. Cholesterol Synthesis
Stage I: Transcription Control
• Feedback regulatory system
• Rate of HMG-CoA reductase mRNA synthesis
controlled by sterol regulatory element binding
protein (SREBP)
• Once in the Golgi, SREBP is cleaved twice by S1p
& S2P to release the transcription factor
Fig. 4A

9. Cholesterol Synthesis
Stage I: Proteolytic Degradation of HMG-CoA
Reductase
• When sterol present, enzyme undergoes sterol
accelerated ERAD (ER associated degradation)
• HMG-CoA is ubiquitinated and extracted from
membrane where it is then degraded by proteosomes
Fig. 4B

10. Cholesterol Synthesis
Stage I: Regulation by Covalent Modification
• Short-term regulation by
phosphorylation &
dephosphorylation
• Adenosine monophosphate
(AMP) activated kinase
phosphorylates HMG-CoA
• Glucagon, sterols,
glucocorticoids & low ATP
levels inactivate HMG-
CoA
• Insulin, thyroid hormone,
high ATP levels activate
enzyme
Fig. 4C

11. Cholesterol Synthesis Stage 2:
Mevalonate to 2 Activated Isoprenes
• Transfer 3 ATP to
Mevalonate in order to
activate C5 & OH-group
of C3
• Phosphate group at C3 &
Carboxyl group of C1
leave, which produces a
double bound
• This allows for two
active isoprenes
Fig.
5

12. Cholesterol Synthesis Stage 3: Condensation
of Isoprenes to form Squalene
• 1) Head to tail attachment
of isoprenes to form
Geranyl pyrophosphate
• 2) Head to tail condensation
of Geranyl pyrophosphate
and isopentenyl
pyrophosphate to form
Farnesyl pyrophosphate
• 3) Head to head fusion of
two Farnesyl pyrophosphate
to form squalene
Fig.6

13. Cholesterol Synthesis Stage 4:
Squalene to Four-Ring Steroid Nucleus
• Squalene monooxygenase adds oxygen to form an
epoxide
• Unsaturated carbons (double bonds) are aligned to
allow cyclization and formation of lanosterol
• After many reaction get cholesterol
Fig. 7

14. Fates of Cholesterol
• Membranes
• Cholesterol Ester
• Biliary Cholesterol
• Bile Acids
STOP !!

15. Cholesterol Esters
• Acyl-CoA: cholesterol
acyl transferase
(ACAT) is an ER
membrane protein
• ACAT transfers fatty
acid of CoA to C3
hydroxyl group of
cholesterol
• Excess cholesterol is
stored as cholesterol
esters in cytosolic
lipid droplets
Fig. 8

16. Bile Salts
• Bile acids & salts are effective detergents
• Synthesized in the liver
• Stored & concentrated in the gallbladder
• Discharged into gut and aides in absorption of
intra-luminal lipids, cholesterol, & fat soluble
vitamins
• Bile acid refers to the protonated form while
bile salts refers to the ionized form
– The pH of the intestine is 7 and the pKa of bile
salts is 6, which means that 50% are protonated
• These terms are sometimes used
interchangeably

17. Synthesis of Bile Salts
• Rate-limiting step performed by the 7α-hydroxylase (CYP7A1) and
is regulated by bile salt concentration
• End product: Cholic acid series & Chenocholic acid series
• Bile salts can be conjugated & become better detergents
Fig. 9 Fig. 10

18. Fate of Bile Salts
Fig. 12

19. Cholesterol Transport by Blood
Lipoproteins
• Cholesterol, cholesterol esters, triacylglycerols, &
phospholipids are insoluble and must travel via lipoproteins

20. VLDL to LDL
• The TG, free & esterified cholesterol, FA, & apoB-100 are packaged into
nascent VLDL
• Nascent VLDL are secreted to bloodstream and acquire apoCII & apoE from
HDL to form a mature VLDL
• Hepatic triglyceride lipase (HTGL) hydrolyzes additional triglycerides to
produce LDL
• 40% of LDL transported to extrahepatic tissues
• Excess LDL is taken up by macrophages
Fig. 14

21. Reverse Cholesterol Transport (RCT)
• HDL removes cholesterol from cells and returns it to the liver
• ABC1 transport protein uses ATP hydrolysis to move cholesterol
from inner leaflet to outer leaflet of membrane
• HDL receives cholesterol and uses the LCAT enzyme to modify &
trap the cholesterol
Oram, JF & Vaughan, AM. (2000) ABCA1-mediated transport of cellular cholesterol &
phospholipids to HDL apolipoproteins. Curr Opin Lipidol. June;11(3):253-60

22. Fate of HDL
• HDL can bind to specific hepatic receptors, but primary HDL
clearance occurs through uptake by scavenger receptor SR-B1
• Present on many cells
• SR-B1 can be upregulated in cells that require more cholesterol
• SR-B1 is not downregulated when cholesterol levels are high
HDL binds SR-B1 receptor
Transfers cholesterol &
cholesterol ester to cell
Depleted HDL dissociates &
re-enters circulation

23. HDL Interactions with Other Particles
• HDL transfers apoE & apoCII to Chylomicrons & VLDL
• HDL either transfers cholesterol & cholesterol esters directly to
liver or by means of CETP to VLDL (or other TG-rich lipoproteins)
• In exchange, HDL receives triacylglyceroles
• Prior to CETP mature HDL particles are HDL3, post CETP they
become larger and are called HDL2
Fig. 16
Fig. 17

24. Receptor-Mediated Endocytosis of
Lipoproteins
• LDL receptor are located at
coated pits, which also
contain clathrin
• Vesicles fuse with lysosome
where cholesterol esters are
hydrolyzed into cholesterol &
re-esterified by ACAT
• This avoids damaging effects
of high concentrations of
free cholesterol on membrane
• Unlike cholesterol esters of
LDL, these cholesterol esters
are monosaturated
Fig. 18

25. Feedback Regulation of
Receptors
• Regulation by SREBP or its cofactor
• Low levels of cholesterol leads to up
regulation of receptor genes
– Increase amount of cholesterol in cells
• High levels suppress expression of
receptor genes
– Reduces amount of cholesterol that enters
cells

26. Lipoprotein Receptors
• LDL receptor most well
characterized &
contains 6 different
regions
• LDL receptor-related
proteins are structurally
related but recognize
more ligands
• Macrophage scavenger
receptor : SR-AI & SR-
A2
– Take up oxidatively
modified LDL
– When engorged with
lipids macrophages
become foam cells

27. Anatomical & Biochemical
Aspects of Atherosclerosis
• Initial step is formation of fatty streak (foam cells) in
subintimal space
• Foam cells separate endothelial cells exposing them to blood,
which leads to plaques & thrombin at these sites
• When plaque content exposed to procoagulant elements in
circulation, acute thrombus formation occurs
• Further thrombus formation leads to complete occlusion of
lumen & eventually AMI or CVA
Fig 21. Layers of arterial wall

28. Key Concepts
• HMG-CoA conversion to mevalonate is the rate
limiting step of cholesterol synthesis
– HMG-CoA reductase regulated by feedback,
degradation, modification
• Cholesterol fate: membranes, esters, biliary
cholesterol, bile salts
– Bile salts aide in absorption of lipids
• Hydrolysis of VLDL leads to LDL, which transport
TG & CE to peripheral cells & macrophages
• HDL involved in RCT & apoprotein/lipid exchange
• LDL enters cells via receptor-mediated endocytosis
• Excess LDL taken up by macrophage leads to the
formation of foam cells, which is the beginning of
atherosclerosis