8 - Cholesterol Flashcards
What kind of molecule is cholesterol?
What is its role?
steroid
it increases and decreases membrane stiffens
it also changes interaction with the cytoskeleton
Describe the synthesis of cholesterol up to isopentyl pyrophosphate
1) condensation of 2 acetyl CoA molecules to form acetoacetyl CoA
2) condensation of another molecule of acetyl CoA to form HMG-CoA
3) HMG-CoA is rescued to generate Mevalonate
4) Mevalonate undergoes sequential phosphorylation, followed by decarboxylation
to form 3-isopentyl pyrophosphate
How is the enzyme HMG CoA reductase (step 3) regulated ?
Under negative feedback control forms he end product cholesterol, the intermediate mevalonate and bile salts
Describe the synthesis of cholesterol from isopentyl pyrophosphate to squalene
1) isopentenyl pyrophosphate is isomerised to dimethyallyl pyrophosphate
2) it condenses twice with a molecule of isopentenyl pyrophosphate to form 15C farnesyl pyrophosphate
3) 2 farnesyl pyrophosphate molecules condense to form 30 carbon squalene and 2 molecules of pyrophosphate
Describe the synthesis of cholesterol from squalene
3 steps
1) squalene is reduced to form squalene epoxide
2) formation of lanosterol (formation of four rings). Catalysed by squalene epoxide lanosterol-cyclase.
3) 19 step conversion of lanosterol to cholesterol
What molecules can cholesterol produce?
steroids
vitamin D
bile salts
How is the problem of insolubility of lipids in aqueous solution saved to allow transport of lipids?
packaged with lipoproteins
What is the structure of lipoproteins?
- phospholipid monolayer containing cholesterol and apoproteins
- packed in the core is cholesterol ester and triacylglycerols
Give an overview of the process of receptor mediated endocytosis of LDLs
- LDLRs receive the LDLs and send them into an early endosome
- at the early endosome, the LDLRs are recycled back to the plasma membrane
- the LDLs are transferred to lysosomes where it is degraded to free cholesterol
