What is the first step in the synthesis of mevalonate?
Reactants: 2x Acetyl-CoA
Products: Acetoacetyl-CoA + CoA-SH
What is the 2nd step in the synthesis of mevalonate?
A second condensation...
Reactants: Acetoacetyl-CoA + Acetyl-CoA
Enzyme: HMG-CoA Synthase
Products: HMG-CoA + CoA-SH
What is the 3rd and final step in the synthesis of mevalonate?
Reactants: HMG-CoA + 2 NADPH + 2 H+
Enzyme: HMG-CoA Reductase
Products: Mevalonate + 2 NADP+ + CoA-SH
After formation of mevalonate...
what is the next (2nd) step of cholesterol synthesis?
(generally, no specific enzymes etc.)
formation of isoprenoid units from mevalonate by loss of CO2
After formation of isoprenoid units...
what is the next (3rd) step in cholesterol synthesis?
(generally, no specific enzymes)
condensation of six isoprenoid units to form squalene
After squalene formation...
what is the next (4th) step in cholesterol synthesis?
(generally, no specific enzymes)
cyclization of squalene to form lanosterol
(lanosterol goes on to form cholesterol)
What is the main step for regulation of cholesterol synthesis?
the HMG-CoA Reductase step creating mevalonate near the beginning of synthesis
What endogenous substances directly inhibit HMG-CoA Reductase?
And what drugs inhibit it?
- Bile Acid
(Direct + downstream products of HMG-CoA Reductase activity)
- Statin drugs
How do cholesterol + its metabolites repress transcription of HMG-CoA Reductase?
by repressing activation of SREBP (steroid regulatory element-binding protein)
- cholesterol blocks SREBP from moving to the golgi where its HLH domain can be cleaved and sent to the nucleus to upregulate cholesterol synthesis genes
What protein keeps SREBP from moving from to the golgi for cleavage of its HLH domain?
And how else does this protein decrease cholesterol synthesis?
Insig (Insulin induced gene)
- binds to sterol-sensing domain of SCAP and keeps SCAP/SREBP in the ER
- also directly increases degradation of HMG-CoA Reductase
What protein promotes activation of SREBP and thus its presence in the nucleus as an upregulator of HMG-CoA reductase?
SREBP Cleavage-Activating Protein
What two enzymes cleave SREBP and allow it to travel to the nucleus?
S1P - cleaves the HLH domain off of the rest of the protein
S2P - remove the HLH domain from the golgi membrane for transport to the nucleus
What are the 3 steps of cholesterol absorption from the intestine?
Uptake - of hydrolyzed cholesterol esters as free cholesterol via NPC1L1 transporter into enterocyte
Resynthesis - of cholesterol esters
Secretion - of cholesterol esters into chylomicrons in the lymph
What protein is important in absorption of free cholesterol from the intestine?
What drug blocks it?
Niemann-Pick C1-Like Protein 1
- blocked by ezetimibe
What enzyme is responsible for cholesterol esterification?
Two isoforms... which one is where (types of cells)?
ACAT (Acetyl-CoA Acetyltransferase)
- ACAT1 - in macrophages and Kupfer cells of the liver
- ACAT2 - in hepatocytes and enterocytes
What enzyme is responsible for the conversion of cholesterol esters into free cholesterol?
CEH (Cholesteryl Ester Hydrolase)
AKA Hormone-Sensitive Lipase
- in all the cells where ACAT can catalyze the opposite rxn ( FC ---> cholesterol ester)
What enzymes are responsible for the synthesis of TAGs that contribute to lipid droplets in lipoproteins?
DGAT (Diglyceride Acyltransferase)
- forms TAGs from diacylglycerol and acyl-CoA
How does lanosterol effect HMG-CoA Reductase activity?
it inhibits it via ubiquitination which marks HMG-CoA Reductase for degradation in proteasomes
Describe post-translational regulation of HMG-CoA Reductase
What hormones affect it and how?
Insulin - activates protein phosphatases which dephoshporylate/deactivate AMPK, keeping HMG-CoA Reductase dephosphorylated and thus active
Glucagon - deactivates the same protein phosphatases
- essentially, phosphorylation via AMPK deactivates HMG-CoA Reductase
What key enzyme plays a role in regulating LDL levels and how?
(Proprotein Convertase Subtilisin/Kexin Type 9)
- binds to LDL Receptors and induces their degradation
- this leads to a decrease in LDL metabolism + increase in circulating LDL
What is the receptor with a dual role in HDL metabolism?
What are its two roles in different tissue types?
(Class B Scavenger Receptor B1)
- Liver/Steroidogenic Tissues - binds HDL via Apo A-I and delivers cholesteryl esters to cells
- Other Tissues - accepts cholesterol efflux from cell to HDL, then HDL goes to liver and cholesterol is excreted in bile
What is IDOL?
Inducible Degrader of LDL Receptor (IDOL)
- ligates ubiquitin to the LDLR in response to high intracellular cholesterol
- this leads to LDLR degradation
What is reverse cholesterol transport?
What receptor plays a major role + how?
What 2 transporters also play a major role?
(the 'how' for the transporters is in another card)
transport of cholesterol from peripheral tissues back to the liver via plasma
- mediates efflux of cholesterol to HDL from peripheral tissues AND uptake of cholesterol from HDL (via Apo A-I binding) into the liver
Transporters: ATP-binding cassette transporters A1 + G1 (ABCA1 + ABCG1)
How are ABCA1 and ABCG1 involved in reverse cholesterol transport?
ABCA1 - promotes cholesterol efflux from tissues specifically to poorly-lipidated particles (ie preβ-HDL + Apo-A1) to convert them to HDL3
ABCG1 - mediates transport of cholesterol from cells to (already lipidated?) HDL
What is the enzyme responsible for conversion of discoidal HDL to HDL3 ?
Where is it?
LCAT (Lecithin:Cholesterol Acyltransferase)
(in the plasma)
- binds to discoidal particles with Apo A-I and converts their cholesterol to cholesteryl esters
- non-polar cholesteryl esters move into the interior of the bilayer, forming a nonpolar core surrounded by a now spherical phospholipid layer
How does HDL3 become HDL2 ?
- by accepting cholesterol from tissues via SR-B1 and esterifying it via LCAT
- this increases particle size and forms the less dense HDL2
How does HDL2 convert back to HDL3?
And what is this whole HDL2/3 interconversion mechanism called?
Delivery of cholesterol ester to liver via SR-B1
Hydrolysis of its phospholipids + TAGs by hepatic + endothelial lipase
Known as the HDL cycle