Cholesterol Metabolism Lecture Sep 11 Flashcards
What are the 4 functions of cholesterol?
- in membranes
- bile salts for digestion of fat
- steroid hormones
- vitamin D synthesis
What is the one precursor of cholesterol?
What are the sources of this precursor?
Acetyl CoA
can be from pyruvate dehydrogenase
beta oxidation of fatty acids
oxidation of ketogenic amino acids like leucine and lysin
In general terms, what are the 4 stages of cholesterol synthesis?
- 3 acetyl CoAs make mevalonate (6C)
- Mevalonate converted to isoprenes (5C)
- Six isoprenes condense to form squalene (30C)
- Squalene is cyclized and converted to cholesterol (27C)
What is the rate limiting step and key regulatory step in cholesterol synthesis?
HMG CoA reductase
combining the 3 acetyl CoAs to form mevalonate
Where does cholesterol synthesis take place in the cell?
In the cytosol
At what step in cholesterol synthesis do statin drugs work?
Statins inhibit HMG-CoA reductase which is the rate limiting/regulated step of the synthesis. It’s the conversion of HMG-CoA to Mevalonate.
In what three general ways is HMG CoA reductase regulated?
transcription
degradation
phosphorylation
Describe the transcriptional regulation of HMG CoA reductase.
When cholesterol is abundant, the TF sterol response element binding protein (SREBP) is sequestered in intracellular membranes in complex with SREBP cleavage activing protein (SCAP)
WHen the cholesterol levels drop within the membrane, the membrane becomes more fluid and this alters the comformation of SREBP and SCAP in such a way that SCAP cleaves the DNA binding domain of SREBP.
This allows SREBP to translocate to the nuclease where it binds to a promoter element near the HMG CoA reductase gene, recruits RNA polymerase II, and increases the transcription of the protein, HMG CoA reductase
This is a sensitive regulatory mechanism because the free DNA binding domain of SREBP has a short half life.
What do high sterol levels trigger in regards to HMG CoA reductase?
proteolysis, degradation
WHere is HMG CoA reductase located in the cell?
It is an integral membrane protein of the ER membrane.
How is HMG CoA reductase regulated thorugh phosphorylation?
What will turn it off? What will turn it on?
Phosphorylation turns of HMG CoA reductase OFF.
It is phosphorylated by AMP-K.
This makes sense because during fasted/low energy conditions, you don’t want to be using your precious energy to synthesize sterols.
At times of low energy, AMP is high, which allostericaly activates AMP-K. Sterols will also allosterically activate AMP-K.
So AMP-K phosphorylates and turns it OFF.
During the fed state, you can use energy to make sterols, so HMG CoA reductase is dephosphorylated by an insulin sensitive phosphatase, turning it ON.
After mevalonate is synthesized from acetyl CoA, what happens to it?
It’s phosphorylated 3 times, then decraboxylated and dephosphorylated to form the 2 isoprenes: isopentenyl pryophosphate and dimethylallyl pyrophosphate
Both isoprenes are then used in cholesterol synthesis
Besides cholesterol syntehsis, what are two biosynthetic reactions can isoprenes be used in?
synthesis of CoQ and dolichol phosphate
What happens to the isoprenes after they’re synthesized?
3 isoprenes condense to form farnesyl pyrophosphate.
2 farnesyl pyrophosphate join together to form squalene (a 30C unsaturated, branched chain)
What happens to squalene in cholesterol synthesis?
It’s hydroxylated then cyclized to lanosterol.
lanosterol undergoes reactions to remove methyl groups and eventually forms the 27C cholesterol.
What organ is the main source of cholesterol synthesis?
How does it export the cholesterol?
The liver!
It can export cholesterol in the form of cholesterol esters or bile salts.
Bile acids are used in digestion to emulsify dietary fat. TO be effective detergents, what must be done to cholesterol?
It has to be amphipathic with a polar end and a nonpolar end.
What is the rate limiting step for bile acid synthesis?
How is it regulated?
The 7alpha hydroxylase step which is the hydroxylation at the 7 carbon of cholesterol.
Bile acids will inhibit this enzyme (feedback inhibition)
After bile acids have been made, what else can be done to make them even better detergents?
Make them into bile acid conjugates by binding their side chains to amino acids (either taurine or glycine).
How are bile salts recycled?
THey are degraded by gut bacteria to remove AAs conjugates and the 7C hydroxyl to make secondary bile salts.
Most of the biles salts and secondary bile salts are taken up by the blood and brought to the liver where they are recycled. Those that are recycle go to the gallbladder for storage.
From the gallbladder they can be reintroduced to the GI tract through the common bile duct (in the event of a meal).
5% of the bile salts are not recycled and lost through feces. This is pretty much the only way we lose cholesterol from the body (since we don’t break it down for fuel), and why we need to be able to synthesize it in our body.
More specifically, what do bile salts do in the GI tract?
They act as detergents and make dietary fats soluble and accesible to lipase.
the lipase will cleave break the TG in the bile salts down to fatty acids and 2-monoacylglycerol.
THe bile salts emulsify the FA and 2-MG into a micelle which can be taken up by gut epithelial cells, where they will be put back together into a triacylglycerol that can leave the gut epithelial cell trhough a nascent chylomicron.
Once the dietary fat and cholesterol is taken up by epithelial cells, what is it packaged in?
THey are packaged as nascent chylomicrons.
- THey are brought into the SER
- In the golgi they are packaged with ApoB-48 (a chylomicron tag)
- The nascent chylomicrons then leave the cell and go into the lymph.
- THe lymph travels up to the neck and then dumps the chylomicrons out into the blood
How do nascent chylomicrons become mature chylomicrons?
WHen the nascent chylomicrons reach the blood, HDL particles (the good cholesterol) transfer the proteins ApoCII and ApoE, convertin it to a mature chylomicron.
WHat particle is ApoB48 associated with?
What particle are ApoCII and ApoE associated with?
B48 is with nascent chylomicrons
CII and E are with mature chylomicrons
What does ApoCII activate?
It activates lipoprotein lipase, which is expressed on the capillary beds of cells that need fatty acids.
This means that cells can only take fatty acids off of mature chylomicrons.
Besides donating proteins to chylomicrons, what role does HDL play?
It monitors cholesterol concentration in the membranes of other particles.
If it’s low, HDL will donate cholesterol.
If it’s high, HDL will receive cholesterol.
It will transfer cholesterol esters, proteins, triglycerides, and phospholipids among the lipoproteins and cells.
WHat is the relative composition of chylomicrons?
Mostly traicylglycerol
Some protein (ApoB-48, ApoCII, ApoE etc)
cholesterol
cholesterol ester
phospholipid
What is the difference between chylomicrons and VLDL?
Chylomicrons transport dietary fats and cholesterols, whereas VLDL carries synthesized fats.
WHat happen to both chylomicrons and VLDL particles as they travel through the capillaries?
They get smaller and smaller as the LPL along the capillary beds removed fatty acids.
When a chylomicron gets too small to be used by the tissues, it becomes a remnant and is sent to the liver to be recycled.
As VLDL gets smaller and smaller, it’s reduced to an intermediate density lipid (IDL) and then to a low density lipid (LDL), which should be taken up by the liver for reuse.
What Apo protein classified VLDL particles?
ApoB-100
To become mature it also receives ApoCII and ApoE from HDL particles, just like chylomicrons.
How are cholesterol esters synthesized?
Why are they synthesized?
What are they used for?
Cholesterol is exterified to fatty acyl CoA to form cholesterol ester. THe reaction is catalyzed by acyl-CoA-cholesterol actyltransferase (ACAT)
This makes it even less soluble. Most cholesterol exported from the liver is esterified to the unsaturated fatty acid linoleate.
Cholesterol esters are used to make membranes or packaged in VLDL for export to other tissues
What does the body do with LDL?
It removes the LDL from the blood in two ways.
- ENdocytosis into cells (especially liver cells) through an LDL receptor. THis is the preferred way.
- If the receptor is mutated or overwhelmed, LDL will build up in the blood. WHen this happens, the LDL will be expsed to oxidative stress and become oxidized. When this happens, the macrophages will phagocytose the LDL.
What causes atherosclerotic placque buildup?
When LDL concentration is too high in the blood for the LDL receptor to bring it into cells, the LCL becomes oxidized and phsgocytosed by the macrophage.
The LDL then builds up in macrophages, forming foam cells. The foam cells can stick to a blood vessel wall and form a placque.
What receptors do macrophages use to take up oxidized LDL?
scavenger receptors: SR-A1 and SR-A2.
What does the ligand binding domain of the LDL receptor actually bind to?
The ApoB100 protein on the LDL
What is the LDL receptor’s cytoplasmic domain linked to in the cell?
clathrin
WHat results from mutations in the LDL receptor?
familial hypercholesterolemia:
hyperlipidemia, premature CVD, xanthomas
WHat hormones are derived from cholesterol?
WHere are they synthesized?
Steroid hormones derive from cholesterol.
They are synthesized in the adrenal cortex and the gonads.
What are the 5 steroid hormones we discussed?
glucocorticoids
mineralocorticoids
androgens
estrogens
progestins
Give an example of a glucocorticoid. WHat does it do?
cortisol
it mobilizes stored fuel in the fasted state or times of stress
GIVe an example of a mineralocorticoid. WHat does it do?
Aldosterone
It regulates salt loss and fluid volume (via increased fluid reuptake in the kidney)
GIve an example of an androgen. WHat does it do?
testosterone
male development
give an example of an estrogen. what does it do?
estradiol
female development
Give an example of a progestin. WHat does it do?
Progesterone
maintains pregnancy and female development
WHat is the first step of all steroid hormone synthesis?
WHere in the cell does it happen?
WHat enzyme?
Removal of most of the side chain to form pregnenolone
This happens in the mitochondria
Cyp proteins do all the steps.
What causes congenital adrenal hyperplasia?
mutation in the Cyp21 enzyme, which causes an issue with steroid hormone biosynthesis.
(any block below progesterone will cause this because you’ll get a buildup of DHEA and the the androgens, but NOT the estradiol)
In these patients, cirulating aldosterone and cortisol are decreased, but androgens are increased.
THe phenotype can vary between mild (masculinization alone) and severe (masculinization plus fuel metabolism issues and water reuptake/salt loss issues due to lack of aldosterone and cortisone).
In congenital adrenal hyperplasia, what steroids build up and what are low?
What are the symptoms?
Aldosterone and cortisol are low.
Androgens build up. (Estrogens do NOT)
Symptoms include maculanization, lipid metabolism issues, and salt loss and water reuptake issues
In steroid hormone synthesis, what two reactions occur in the mitochondria? Where do the rest occur?
In the mitochondria:
- Synthesis of pregnenolone
- the demolase and hydrosylase reactions to create cortisol
The rest of the rxns are in the cytosol
How do steroid hormones act?
Steroid hormones all act through transcriptional regulation.
THey diffuse through cell membranes and bind ligand-activated transcription factors.
The steroid receptor has three domains: the transactivation domain, the DNA-binding domain, and the ligand-binding domain.
THe hormone binds to the ligand binding domain. THis causes dimerization of the receptor. The DNA binding domain then binds to a discrete element in the promoter of the steroid hormone regulated gene. THen the transactivation domain makes contact with other transcription factors to recruit RNA polymerase II to initiate transcription of the gene. Thi sis true for ALL steroid receptors.
WHat does cortisol transcriptionally upregulate?
PEP-CK, which is an important gluconeogenic enzyme.
What is vitamin D important for in the body?
calcium homeostasis and uptake.
What kind of receptor does Vitamine D use?
It uses a ligand activated transcription factor similar to the steorid hormone receptors
In what three tissues do reactions need to occur in order to form the most potent form of vitamine D?
What is the most potent form of vitamin D?
The skin, the liver, and the kidney
1,25-dihydroxycholecalciferol (calcitriol)
What is the biggest structural difference between vitamin D and the steroid hormones?
Steroid hormones preserve the 4 ring structure of cholesterol
Vitamin D breaks open one of the rings through UV light.
What is the purpose of IDL?
WHat is the purpose of LDL?
IDL return lipids to the liver
LDL delivers cholesterol to cells
