Cholesterol Biosynthesis Flashcards
Properties of cholesterol (5)
- steroid nucleus
- very hydrophobic
- OH group that can H bond with membrane phospholipids
- precursor for steroid hormones, bile salts, vitamin D
- component of membranes
major site of cholesterol biosynthesis
Liver
Requirements for cholesterol biosynthesis (3)
And why need each
- Acetyl CoA- hydrolysis of high energy thioester bonds
- ATP- for formation of activated isoprene units (activation step)
- NADPH
Where does the NADPH come from?
Pentose phosphate pathway
STEP 1
Joining 3 acetyl CoA molecules
Acetyl CoA + Acetyl CoA —> Acetoacetyl CoA (enzyme: thiolase)
Acetoacetyl CoA + Acetyl CoA —> HMG-CoA (enzyme: HMG-CoA synthase)
STEP 2
RATE LIMITING STEP
HMG-CoA + 2 NADPH + 2 H+—> Mevalonate + 2 NADP+ + CoA
Enzyme: HMG-CoA reductase
Step 4
Formation of Activated Isoprene Units
Mevalonate + 3 ATP —> 3-isopentenyl pyrophosphate + Pi + CO2
Isoprene units = _____ carbons
5
_____ ATP consumed to make 1 activated 5-C isoprene unit
3
What is the significance of forming the activated isoprene units?
Shortcut to making steroid nucleus
What contains isoprene units in the ETC?
CoQ
Drug ppl take to lower cholesterol
Statin drugs
Have to take CoQ supplement to combat muscle weakness
Statin mechanism
Competitive inhibitor of HMG-CoA reductase
Cannot make isoprene units therefore cannot make cholesterol
Step 4
3-isopentenyl pyrophosphate dimethylallyl pyrophosphate
*Both isomers are used in next step
Step 5
Condensation reactions:
- Start building the steroid nucleus by joining activated 5-C units —>
End product: 30-C squalene
What drives the condensation reactions?
Hydrolysis of PPi molecules
5-C units =
Dimethylallyl pyrophosphate
Isopentenyl pyrophosphate
10-C unit =
Geranyl pyrophosphate
15-C unit
Farnesyl pyrophosphate
Step 6
- Where?
- Requires?
- Doing what?
Cyclization & Hydroxylation Reactions:
- Occurs in smooth ER of liver
- Requires O2 for OH group and NADPH
- Completing the ring structure
Major regulated enzyme in cholesterol biosynthesis
HMG-CoA Reductase
__________ regulates the degradation and synthesis of HMG-CoA reductase
CHOLESTEROL
3 mechanisms of HMG-CoA regulation
- Hormonal
- Degradation
- Transcription and translation
In fasted state, HMG-CoA reductase is ___________ and ____________
phosphorylated and INACTIVE
In fasted state, glucagon and sterols activate ____________ which:
activate AMPK (also activated by AMP)
—> phosphorylates HMG-CoA reductase to inactivate it
In fed state, HMG-CoA reductase is _________ and ________
Dephosphorylated and ACTIVE
In fed state, insulin stimulates _____________ which:
A phosphatase
—> removes the phosphate and activates HMG-CoA reductase
Degradation of HMG-CoA reductase depends on?
Cholesterol level
If high cholesterol level —> degrade HMG-CoA reductase
Transcription and translation of HMG-CoA reductase is controlled by?
Cholesterol levels
If high cholesterol levels —> complex does not move to Golgi
Within the ER there are ?
2 integral membrane proteins
SCAP
SREBP
SCAP
Senses levels of cholesterol in liver
2 domains of SREBP
- Regulatory - normally interacts with SCAP
- DNA binding - binds to DNA
* functions as transcription factor
If cholesterol levels fall —>
Entire SCAP + SREBP complex migrates to Golgi
2 proteolytic enzymes in the Golgi
- Serine protease
2. Metalloprotease
Serine protease
Cuts loop on SREBP the spans into the lumen
—> separates regulatory and DNA binding domain
Metalloprotease
- requires zinc
Cuts at intersection of alpha-helix and loop that connects the DNA binding domain
—> frees up DNA binding domain to travel to the nucleus
What happens in the nucleus?
DNA binding domain binds to SRE (sterol response element)
—> increases transcription of HMG-CoA reductase gene
Lovastatin (mevinolin)
- Blocks HMG-CoA reductase and prevent cholesterol synthesis
- Is an (inactive) lactone
- In body, lactone is hydrolyzed to mevinolinic acid which is a competitive inhibitor of HMG-CoA reductase because it looks very similar to Mevalonate
How is cholesterol transported to tissues in the body?
LDL
LDL is from?
VLDL
When newly synthesized TG are unloaded —> VLDL becomes a little heavier —> VLDL becomes IDL —> eventually LDL
HDL function?
Reverse cholesterol transport
- removes excess/damaged cholesterol from cells and returns to liver
Triglyceride and protein content of carrier molecules
(MOST TG) Chylomicron —> VLDL —> IDL —> LDL —> HDL (LEAST TG)
Reverse for protein
With age, total cholesterol _____ and HDL levels _______
- Cholesterol rises
- HDL levels may fall
Regular exercise is known to
Raise HDL levels
Low fat diet
Reduces serum cholesterol levels
Trans-fat
Raises LDL and lowers HDL
Saturated fat
Raises LDL and does not affect HDL
2 ways that cholesterol regulates it own biosynthesis
- Concentration of LDL receptors determined by need for cholesterol
- Regulates HMG-CoA reductase synthesis/degradation
More cholesterol in circulation —> _____________ _____ receptor synthesis
Increase LDL receptor synthesis
Usually ppl who have trouble regulating cholesterol have an issue with (2)
- LDL receptor
And/or
- regulation of HMG-CoA reductase
LDL receptor structure
- Single transmembrane protein
- Majority faces outside
- LDL binding domain is on N-terminus
- Has a cytosolic domain
Homozygous individuals =
Result?
No LDL receptors
—> high levels of LDL
—> familial hypercholesterolemia
Heterozygous individuals =
1/2 levels of LDL receptors
What happens when there are mutations in the LDL receptor?
Cholesterol is not getting into cells and there is no feedback to stop its biosynthesis —> liver thinks there is not enough cholesterol so it makes it…
How does LDL receptor work?
Receptor-mediated endocytosis
- feedback control *
Process of receptor mediated endocytosis
Receptors cluster in region called coated pits that have high conc of protein clathrin —> LDL binds to receptor —> membrane invaginates to form a vesicle —> vesicle fused with endosomal compartment with a slightly lower pH —> drop in pH separates the LDL receptor and LDL particle —> cholesterol unloaded
When LDL is brought into liver cell:
- Decrease cholesterol synthesis
- HMG-CoA reductase activity and synthesis decreases
- Increase degradation of HMG-CoA reductase
