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
Squalene monoxygenase
Catalyzes reaction from squalene to squalene 2,3 epoxide
Where does most cholesterol synthesis happen?
The liver is the main source of cholesterol synthesis. The liver can export cholesterol in the form of cholesterol esters or bile acids
Sythesis of Bile acids?
Starts from cholesterol to cholic acid and chenocholic acid
What are Bile acids used for?
Bile acids are used in digestion to emulsify dietary fat. To be effective detergente, bile acids must be amphipathic
What is key rate limiting step in bile acid synthesis?
The rate limiting step in bile acid synthesis is hydroxylation at the #7 carbon of cholesterol by 7a-hydroxylase; bile acids inhibit this enzyme.
What are the AAs bile acid side chains can bind to? What doe they create?
Taurine and Glycine Bile acid conjugates. Conjugates are better detergents than unconjugated bile salts
What is path through body of bile acids?
Add pIc. Bile salts are efficiently recycled. They can be degraded by gut bacteria to remove amino acid conjugates and the hydroxyl at carbon 7; these are secondary bile salts. Eventually they are excreted in feces The 5% loss of bile salts is the major source of lost cholesterol.. the only way we lose cholesterol and why we need to either get it through diet or synthesize it de novo
Secondary bile salts
Can look for bile salt profile in pts.. can see profile.. some pathologies associated.
Chylomicrons
Dietary fat and cholesterol are packaged as nascent chylomicrons by intestinal epithelial cells. The chylomicrons enter the lymphatic circulation first, and then are transported to the blood. This is to bypass the liver on first pass* - not like normal circulation that goes to the liver first to remove toxins*
ApoB-48
Puts a tag on cells to identify …
NP1L1 in brush boarder
Important for asetamime in pts with high cholesterol..
How does nascent chylomicron convert to a mature chylomicron?
Add pic. In the blood, HDL particles transfer ApoCII to nascent chylomicrons to make mature chylomicrons. ApoCII activates lipoprotein lipase (LPL)
HDL function (HDL is the good cholesterol)
Maintain homeostasis for other proteins to take immature VLDLs to mature VLDLs adn same for chylomicrons.. Cholesterol homeostasis - they can take excess cholesterol to membranes and can also donate cholesterol to tehm
What is distibution of triacylglycerol, Cholesterol, cholesterol ester, proteins, and phopholipids in chylomicrons?
Add PIc. TG’s by far most
What hapepns to chylomicrons
Chylomicrons get smaller and smaller as it moves throughout body.. becomes a chilomicron reminant (not used anymore but is taken up and recycled by liver) Have similar role as VLDLs in body BUT VLDLs take synthesized fat from liver Chylomicrons carry DIETARY fat VLDL to intermediate density protein to LDL.. then it gets small enough that it gets taken up to liver.
LDL accumulation
Very important in cardiac health.. if not taken up by liver can cause arthrosclerotic plaques.
How is a cholesterol ester made? Why?
Makes cholesterol and fatty acids more dense and more transportable.. Used to make membranes or packaged in VLDL for export to other tissues Esterification to a fatty acid makes cholesterol even less soluble. Cholesterol exported by the liver is often esterified to the unsaturated fatty acid linoleate
What enzyme is used in esterification of cholesterol?
ACAT - Acyl-CoA-cholesterol acyltransferase
What does VLDL look like? What is the Apoprotein?
Apoprotein B-100 vs the one for VLDL, Apo B48 is for chylomicrons.. ..
mono layer not bilayer…
Peripheral apoprotein for signaling..
HDL - cholesterol transport..
VLDL secreted from the liver is nascent VLDL. To become mature VLDL, it receives ApoCII and ApoE from circulating high density lipoprotein (HDL). HDL functions to transfer cholesterol esters, proteins, triglycerides and phospholipids among lipoproteins and cells
produced by liver to donate lipoproteins and ensure other tissues.
ApoCII, ApoA, ApoE..
Donating cholesterols and taking cholesterols.. also making cholesterol esters (CE) using LCAT (enzyme) also donates CE to vldls adn takes tg and pl from VLDLs… ADD PIC>>> VLDL turned into IDL to LDL then taken up into liver.. packaged again to VLDL, or used in bile salt synths, or used as fuel..
Pathophysiology of LDL (the bad cholesterol)
Add pic.. LDL can be taken up through endocytocis (usually by liver where it is recycled) the remaining FAs cand be used elsewhere.. IF LDL receptor is overwhelmed from excess cholesterol in diet or if there is a mutation in the LDL receptor.. get a build up of LDL = bad. Then they are oxidized (oxidized LDLs). They are taken up by macrophages.. if too much LDL to process.. teh macrophages start to fill up and expand to foam cells.. The foam cells then stick to vessel walls and cause plaques.. Then clots can form and you know what happens..
LDL receptor
Lipoprotein particles can be endocytosed in target cells that express appropriate receptors. Macrophages use scavenger receptors SR-A1 and AR-A2. Most other cells use the LDL receptor. The LDL receptor’s ligand binding domain binds ApoB100.
Ligand binding domain (where LDL binds) if far from the membrane.. to grab big particles. THe binding domain binds to ApoB100 (only particles with this!) Diffeent domains of the protein. Mutations cause hypercholeseolemia. Cannot take up LDLs which causes faom cells and plaque.
What is familial hypercholesterolemia? What is mutated? What are the symptoms?
Mutations in the LDL receptor are a cause of familial hypercholesterolemia.
Symptoms: hyperlipidemia premature CVD xanthomas
What is does normal LDL receptor do/process?
Add pic. marks 34.18
What is a way that atherosclerotic plaques are formed?
LDLs building up in body…
When they build up they are oxidized and taken up by macrophages
When an overload of LDL the macrophages swell up and turn into a foam cell they are so full..
The foam cells attached to the inside of blood vessel.. this causes the arteriosclerosis… leads to clot formation and bad stuss.
