15 - Cholesterol Flashcards
Why is cholesterol important?
- Essential component of animal membranes
- REgulates membrane fluidity, curvature, permeability, fission and fusion
- Forms cholesterol rich lateral membrane domains (small and highly dynamic) together with sphingolipids.
- Influences transmembrane protein function
- Precursor of steroid hormones, vitamin D and bile acids
List four hormones that cholesterol is the precursor for
- Androgens
- Estrogen
- Mineralcorticoids
- Glucocorticoids
All carbons in cholesterol are derived from ___?
acetyl-CoA / acetate
Cholesterol is made by condensation of isoprene units
Squalene cyclization yields a sterol
What are the three main sources of acetyl-CoA (used to synthesized cholesterol)?
- Pyruvate (via pyruvate dehydrogenase)
- Fatty acid oxidation
- Catabolism of ketogenic amino acids
All these generate acetyl-CoA in mitochondrial matrix, but cholesterol synthesis takes place in the cytosol.
Give three fates for acetyl CoA
Surplus energy (+ in cytosol)
- Cholesterol
- Fatty acids (needs lots of ATP)
Energy deficit (mitochondria) - TCA cycle, oxidative phosphorylation and eventually ATP
NADPH is used in fatty acid and cholesterol synthesis, list one important step required to provide this in the cytosol.
(hint): this step also involves the transport of cholesterol main building block!
The tricarboxylate transport system
- Uses acetyl CoA in the mitochondrial matrix and generates acetyl CoA in the cytosol
- Citrate is transported into the cytosol, pyruvate is transported back into mitochondria
- Necessary transport for all synthetic use of acetyl-CoA
- Generation of cytosolic acetyl-CoA from citrate requires ATP (citrate lyase)
- The tricarboxylate transport system provides cytosolic NADPH for lipid biosynthesis
Recall the steps of cholesterol biosynthesis (list enzymes as well)
What is the RLS?
Hints: what is the first commited step of mevalonate? Cholesterol?
What is lanosterol?
- Acetyl-CoA reacts with Acetyl-CoA to form acetoacetyl-CoA (thiolase)
- Acetyl CoA reacts with acetoacetyl CoA to form HMG-CoA (HMG-CoA synthase)
- HMG-CoA reacts to mevalonate using NADP (HMG-CoA reductase) and NADPH - RATE LIMITING STEP and first committed step of mevalonate synthesis
- Mevalonate is phosphorylated and decarboxylated to isopentenyl pyrophosphate (energy costly activation by pyrophosphorylation, part of product is isoprene unit). This product is a precursor to LOTS of isoprene containing molecules (eg. retinol and ubiquinone and prenylated proteins like heterotrimeric G proteins, tether to membrane)
- Isoprene units are condensated, isopentenyl pyrophosphate condensated to dimethylallyl pyrophosphate (activated isoprene units)
- 6 Isoprene units are condensed head to tail to form squalene (first committed step of cholesterol biosynthesis), geranyl pyrophosphate to farnesyl pyrophosphate (prenyltransferase) to squalene (Squalene synthase)
- Oxidosqualene cyclase catalyzes the formation of the sterol ring system to make lanosterol (regulatory intermediate)
- 19 more reactions to covert lanosterol to cholesterol (15 need NADPH, 10 need oxygen)
Defects in cholesterol synthesis before and after lanosterol are ____
Before lanosterol: fetal fatal
After lanosterol: Neurological defects
What is the main regulatory point in cholesterol synthesis?
The converstion of HMG-CoA to mevalonate (HMG-CoA reductase)
List the intermediates between acetyl CoA and cholesterol
- Acetyl CoA
- HMG CoA
- Mevalonate
- Mevalonate pyrophosphate
- Isopentyl pyrophosphate
- Geranyl pyrophosphate
- Farnesyl pyrophosphate
- Cholesterol
What is dimethylallyl pyrophosphate?
isopentyl pyrophosphate can be converted to dimethylallyl pyrophosphate, which can be converted to geranyl pyrophosphate to make cholesterol or other farnesylated and geranylgeranylated proteins (eg. Heme A, some Ras proteins, retinoids etc.)
OR it can become isopentenyl adenosine (tRNA)!
What is HMG CoA reductase? What are the two classes?
- Catalyzes the RLS of cholesterol biosynthesis
- Class I (eukaryotic) and class II (prokaryotic)
- Transmembrane protein in ER
- C terminal half is catalytic domain tetramer (on cytoplasmic side of ER)
Describe HMG CoA reductase as a drug target (name them as a class of drug)
- Competitive HMG-COA reductase inhibitors (statins) lower low density lipoprotein (LDL) cholesterol in blood
List the four levels of regulation of HMG-CoA reductase
- Transcription (sterol regulatory element is bound by transcription factor when sterol levels are low)
- Translation
- Phosphorylation
- Degradation
Describe what happens at sterol regulatory binding element when there is low and high cholesterol
Low: Sterol regulatory element binding protein (SREBP) becomes freed from ER by translocation to golgi and two cleavages, allowing it to translocate to the nucleus and promote cholesterol synthesis
High: SREBP is retained in the ER by INSIG and SCAP
