Week 3 - Cholesterol Metabolism Flashcards
Functions of Cholesterol
Membranes (primary) - fluidity and flexibility of membranes Bile acids - detergents that solubilize dietary fatst to use as fuel Steroid hormones - estrogen, testosterone, aldosterone Vitamin D - can get from diet or synthesize it de Novo in body
How is cholesterol moved throughout the body? Where does it come from?
Can be free or esterified to a fatty acid (attached to OH - called a cholesterol ester) Cholesterol can be synthesized de novo or from the diet
What is the precursor for cholesterol?
Acetyl CoA
What are the sources of Acetyl CoA?
Pyruvate dehydrogenase Beta-oxidation of fatty acids Oxidation of ketogenic amino acids (ex leucine, lysine)
What are the 4 stages of cholesterol synthesis?
Three acetyl CoAs make mevalonate (six carbons) (KEY STEP, RATE Limiting) Mevalonate converted to isoprenes (5 carbons) Six isoprenes condense to form squalene (30 carbons) Squalene is cyclized and converted to cholesterol (27 carbons)

What is the key regulatory enzyme in cholesterol synthesis?
b-hydroxy-b-methylglutaryl CoA reductase (HMG CoA reductase)
Where does cholesterol synthesis occur? What energy sources are used?
Cytosol Takes a lot of energy (ATP and NADPH) - but body doesn’t get energy from cholesterol degradation - not an important fuel source at all
Key step in cholesterol synthesis
Add pic. Acetyl CoA to mevalonate Three Acetyl CoAs condense to form HMG-CoA. HMG CoA reductase removes coenzyme A and reduces the aldehyde to an alcohol to form mevalonate
What are HMG Coa reductase regulated by?
HMG CoA reductase is a key regulatory step in cholesterol synthesis. It is regulated by transcription degradation (the more cholesterol there is the more unstable the enzyme is and it is degraded) phosphorylation
What is an important drug target for high cholesterol? What is the drug class?
HMG CoA Statins are the drug class…
Transcriptional regulation of HMG CoA reductase
Add Pic. When cholesterol is abundant, the transcription factor sterol response element binding protein (SREBP) is sequestered in intracellular membranes in complex with SREBP cleavage activating protein (SCAP). When cholesterol levels drop, SCAP cleaves the DNA binding domain of SREBP, which then translocates to the nucleus and regulates transcription of HMG CoA reductase
What is SREBP?
Embedded in membrane near phopholipids and cholesterol When have lack of cholesterol, the conformation of SREBP changes and releases DNA binding domain. When there lac of cholesterol near SREBP, SCAP cleaves the dna binding and then promotes a DNA-binding domain to be released from SREBP
What is SCAP?
SREBP cleavage activating protein
What happens to HMG CoA reductase during high levels of sterol?
Proteolysis, degredation
HMG CoA reductase is normally associated with intracellular membranes.
High sterol levels promote proteolysis of HMG CoA reductase.

What happens to HMG CoA reductase during fasted and fed states?
In fasted / low energy conditions, HMG CoA reductase is phosphorylated by AMP-K.
AMP-K is activated by phosphorylation and allosterically activated by AMP and sterols.
Insulin promotes cholesterol synthesis by activating phosphatases that dephosphosphorylate HMG CoA reductase.

What does AMP-K do? what is it activated by?
Dont want an energy expensive process going on when energy sources are low( ie dont want to make cholesterol during fasted state).
IN FED stae
activate cholesterol AND fatty acid synthesis… cholesterol plays a big role in transporting fatty acids around the body.
AMP activated protein kinase
AMP phosphorylates AMP-K which phosphorylates and inactivates HMG CoA reductase
AMP activated protein kinase kinase
Metformin acts on this in TypeII diabetes
Second steps of cholesterol synthesis.. Mevalonate to isoprene
Add pic Three phosphates are added to mevalonate to form 3-phospho 5-pyrophosphomevalonate, which is dephosphorylated and decarboxylated to form the isoprene D3-isopentenyl pyrophosphate.
D3-isopentenyl pyrophosphate can isomerize to dimetylallyl pyrophosphate.
Both isoprenes are used in subsequent reactions in cholesterol synthesis.

What are isoprenes used in?
Biosynthesis of CoQ and dolichol phosphate and cholesterol synthesis
What happens after the isoprenes are synthesized in cholestrol synthesis?
Add PIc. Three isoprenes condense to form farnesyl pyrophosphate. Two farnesyl pyrophosphates join together to form the thirty carbon squalene. (Geranylation and farnesylation are post-translational modificaitons to many membrane anchored proteins.)

What are the steps from squalene to cholesterol? What are the enzymes?
Add Pic The thirty carbon squalene is hydroxylated, then cyclized to lanosterol. Subsequent reactions reduce the acyl chain and remove methyl groups to form the 27 carbon cholesterol

Cyclase
catalyzes reaction from Squalene 2,3 epoxide to lanosterol






