biochemistry 6 Flashcards
what is the structure and characteristics of cholesterol
- steroid with 4 linked hydrocarbon rings
- hydrocarbon tail and hydroxyl group at opposite ends
- highly hydrophobic and insoluble in water
what is the functions of cholesterol
- modulates membrane fluidity
- some nerve cells have 25% in the plasma membrane
- precursors to steroid hormones and bile salts
where is cholesterol mainly synthesised and what does it require
- the liver
- ATP and NADPH
what are the stages of cholesterol synthesis
1) acetyl CoA converted to mevalonate - cytoplasm
2) mevalonate is used to synthesise isopentenyl pyrophosphate - cytoplasm
3) 6 molecules of isopentenyl pyrophosphate condense to form squalene - ER
4) squalene cylizes to lanosterol
5) lanosterol converted into cholesterol - ER
what is the name of the first step in cholesterol synthesis and what enzyme is used
- commitment step
- HMG-CoA reductase
what is required for the second step of cholesterol synthesis
- 3ATP and one decarboxylation
how many isomers of isopentenyl pyrophosphate are there and are they in equilibrium
- 2 isomers
- in equilibrium
what is added in stage 3 of cholesterol synthesis and how do they combine
- two additional isopentenyl pyrophosphates added to form farnesyl pyrophosphate
- tail-tail coupling of farnesyl pyrophosphates yield squalene
how is squalene activated
- activated with the formation of squalene epoxide
what does squalene epoxide form
- protonated to form carbocation that cylizes to form a tetracyclic stricture
what is HMG CoA reductase activity controlled by
- mRNA controlled by sterol regulatory element binding protein (SREBP)
what happens when HMG CoA reductase becomes inactive
- high cholesterol
- SREBP bound to SREBP cleavage activating protein (SCAP) at ER
what happens when cholesterol levels decrease
- SCAP escorts SREBP to the golgi
- golgi serine and matalloprotease act of SREBP
- DNA-binding domain moves to the nucleus to alter HMG CoA reductase gene expression
what happens when cholesterol levels are high
- mRNA controlled by SREBP
- mRNA negative feedback by metabolites of mevalonate
- HMG CoA reductase degradation with high cholesterol
- AMP-activated kinase phosphorylates and inactives HMG CoA reductase
what are the sources of cholesterol
- meats
- dairy products
- cakes and biscuits
- foods containing coconut oil or palm oil
how is cholesterol absorbed in the body
- intestine by sterol transporters
- cholesterol esterified by Acyl-CoA-cholesterol acyltransferase-2
- incorporated into chylomicrons - lipoprotein
what are lipoproteins
- soluble protein-lipid particles
- proteins component use to target to specific tissue
what is a chylomicron
dietary lipids and cholesterol
what is a low-density lipoprotein (LDL)
- supplies cholesterol to tissues
- receptor mediated endocytosis by LDL receptor binding internalisation (recycled after) and LDL hydrolysis
what is a high-density lipoprotein
- removes free cholesterol from tissue
- transports cholesterol to liver
what does the liver do with cholesterol
- eliminates cholesterol from the body as bile salts or unchanged
what steroid hormones and vitamins does cholesterol make
steroid hormones - progesterone, testosterone, oestrogen, cortisol
vitamins - D, calcium, phosphorus
what are bile salts
- detergents that render dietary lipids more accessible for digestion by lipase
- made from cholesterol
what is dietary lipids function
- fatty acids are main lipid constituent of diet
- primary role in energy metabolism - beta oxidation and energy storage
