Cholesterol Flashcards
1
Q
Cholesterol Structure
A
- Steroid nucleus: four fused rings
- Branched hydrocarbon tail attached to C-17 of ring D
- Presence of -OH group at C-3 of ring A gives rise to term sterol
- Membrane cholesterols have free -OH at this position
- Most plasma cholesterols have a fatty acid esterified at the C-3 -OH producing a cholesteryl
- Adding FA makes molecule more hydrophopic
- Lipoproteins required for transport throughout body
2
Q
Sources of Cholesterol
A
-
Dietary (~30%)
- Animal sources only
- Amount ingested has little effect on plasma levels
- Plant sterols (phytosterols) compete for absorption and can be used to reduced dietary cholesterol absorption
-
De Novo Synthesis (~70%)
- All tissues capable but primarily liver, adrenal cortex, intestine, and reproductive organs.
- Occurs in cytosol and SER
- All C’s from acetyl CoA and fatty acid β-oxidation
- Requires a lot of NADPH & ATP
- Dietary fat is the main determinant of plasma cholesterol (source of Acetyl CoA)
3
Q
Cholesterol
De Novo Synthesis
A
- Condensation of two acetyl-CoA to form acetoacetyl-CoA.
- Addition of a third acetyl-CoA to form HMG-CoA by HMG CoA Synthase
- Cytosolic isozyme makes HMG-CoA that flows into cholesterol
- Mitochondrial isozyme makes HMG-CoA used in generation of ketone bodies
-
HMG-CoA reduced to Mevalonate by HMG-CoA Reductase
- Rate-limiting and regulated step of cholesterol synthesis
- Statin drugs act as competitive inhibitors
-
Mevalonic acid converted in multi-step reaction to produce Isopentenyl pyrophosphate (IPP)
- Requires 3 ATP per IPP made
- IPP [5C] used to build Geranyl pyrophosphate (GPP) [10C] then Farnesyl pyrophosphate (FPP) [15C].
- FPP used to synthesize:
- Dolichol: used in N-linked glycosylation
- Ubiquinone (CoQ): ETC
- Prenylated proteins: used in post-translational processing
- FPP used to synthesize:
-
Two FPP condensed to produce Squalene [30C]
- Requires a total of 18 ATPs
- Squalene converted to Lanosterol through ring-closure by SER-associated Squalene monooxygenase
- Several steps including removal of 3 methyl groups and movement of double bonds to produce Cholesterol.
4
Q
Cholesterol Biosynthesis
Regulation Overview
A
HMG-CoA Reductase is the major control point for regulation of cholesterol biosynthesis.
- Transcriptional Control
- Proteasomal Degradation
- Phosphorylation/Dephosphorylation
- Hormonal Regulation
5
Q
HMG-CoA Reductase
Transcriptional Regulation
A
Controlled by SREBP-2 (sterol regulatory element binding protein-2) binding to SRE (sterol response element)
- SREBP-2 is an integral SER protein normally associated with another SER-membrane protein SCAP (SREBP cleavage activating protein)
- SCAP contains a sterol-sensing domain
-
When [sterol] high:
- SCAP binds to a third ER membrane protein Insig (insulin-induced gene product), causing retention of SREBP-2/SCAP in the ER.
-
When [sterol] low:
- SCAP no longer interacts with Insig and SREBP-2/SCAP complex translocated to the Golgi apparatus.
- In the Golgi, two proteases (S1P and S2P) cleave SREBP-2 to produce a soluble N-terminal domain that enters nucleus and acts as a transcription factor at the HMG-CoA Reductase gene.
- Other genes upregulated by SREBP-2 include:
- HMG-CoA Synthase
- Low-density lipoprotein receptor (LDL-R)
- Proprotein convertase subtilisin kexin 9 (PCSK9)
6
Q
HMG-CoA Reductase
Proteasomal Degradation
A
When [sterol] high:
HMG-CoA Reductase interacts with Insig in the ER membrane leading to ubiquitination and subsequent proteasomal degradation of the reductase.
7
Q
HMG-CoA Reductase
Covalent Regulation
A
HMG-CoA Reductase is controlled by phosphorylation by AMP-activated protein kinase (AMPK).
- Increased [AMP] leads to phosphorylation of HMG-CoA Reductase and inactivation.
- AMPK also regulates Acetyl-CoA Carboxylase from fatty acid synthesis.
- Thus, in conditions of low [ATP], fatty acid and cholesterol synthesis is decreased.
8
Q
HMG-CoA Reductase
Hormonal Regulation
A
Insulin and Glucagon indirectly mediate HMG-CoA reductase activity via cAMP levels.
- Glucagon causes increased [cAMP] leading to inhibition of HMG-CoA reductase phosphatase via PKA
- Maintains the the reductase in its phosphorylated & inactive form.
- Insulin causes decreased [cAMP] leading to increased HMG-CoA reductase phosphatase activity
- Results in increased reductase activity
Thyroxine can up-regulate HMG-CoA Reductase synthesis.
Glucocorticoids can down-regulate HMG-CoA Reductase synthesis.
9
Q
Cholesterol Removal
A
- Ring structure of cholesterol cannot be brown down into CO2 and H2O
- Cholesterol is either:
- Converted to bile salts
- Excreted as intact cholesterol in the bile
10
Q
Bile
A
- Watery heterogenous mix which includes:
-
phosphatidylcholine
- PC solubilizes cholesterol in bile
- bile salts
-
phosphatidylcholine
- Functions as a surfactant to help emulsify dietary fats
- Reduction in PC production or increased cholesterol production can lead to formation of lithogenic bile
11
Q
Bile Salts
Synthesis & Metabolism
A
Two compounds represent the primary bile acids:
Cholic acid
Chenodeoxycholic acid
-
Bile acids produced in the liver from cholesterol through:
- Shortening
- Attachment of a carboxylate group to the hydrocarbon chain
- Attachment of -OH groups to the sterol ring
- Rate-limiting step:
- addition of -OH group at C-7 of the B-ring by cholesterol-7-α-hydroxylase
- Up-regulated by cholesterol
- Down-regulated by cholic acid
- addition of -OH group at C-7 of the B-ring by cholesterol-7-α-hydroxylase
- Modification makes the molecule amphipathic
- Once bile acids formed, they are conjugated with glycine or taurine to produce bile salts.
- Conjugation further increases amphipathic nature
- Bile salts secreted into the intestine where they are metabolized by bacteria.
- Once in the intestines bile salts can either:
- Be excreted (~5%)
- Increased with bile acid sequestrants sometimes used to treat hypercholesterolemia
- Be reabsorbed for reuse through process called enterohepatic recirculation
- Be excreted (~5%)
12
Q
Steroid Hormone Synthesis
A
Cholesterol is the precursor for all classes of steroid hormones including glucocorticoids, mineralocorticoids, and sex hormones:
- Cholesterol converted to pregnenolone by sidechain cleavage enzyme (CYP11A1).
- End product depends on which enzymes are present in a given tissue:
- Adrenal cortex produces cortisol, aldosterone, and androgens
- Ovaries and placenta produce estrogen and progesins
- Testes produce testosterone
- Synthesis of all steroid hormones involve:
- Shortening of the hydrocarbon chain
- Hydroxylation of the steroid nucleus by a series of cytochrome P450 hydroxylases
-
Defects in specific enzymes of a given pathway can results in lack of a specific hormone or shunting to alternate pathways.
- Most common are the congenital adrenal hyperplasias (CAH)
13
Q
Congenital Adrenal Hyperplasias
(CAH)
A
- Caused by a defect in 3-β-Hydroxysteroid Dehydrogenase
- Results absence of all steroid hormones
- Autosomal recessive
14
Q
17-α-Hydroxylase Deficiency
A
- Virtually no sex hormones or cortisol
- Increased production of mineralocorticoids (Aldosterone) causing:
- Sodium and fluid retention
- Hypertension
- Female-like genitalia
15
Q
21-α-Hydroxylase Deficiency
A
- Most common form of CAH
- Partial and complete deficiencies known
- Mineralocorticoids and glucocorticoids virtually absent or deficient
- Overproduction of androgens leading to masculinization of external genitalia in females & early virilization in males
16
Q
11-β1-Hydroxylase Deficiency
A
- Decrease in serum cortisol, aldosterone, and corticosterone
- Increased production of deoxy-corticosterone
- Causes fluid retention through RAAS suppression
- Overproduction of androgens causing masculinization and virilization
17
Q
Plasma Lipoproteins
A
- Transports lipids through plasma in soluble form
- Composed of:
- Neutral lipid core containing triacylglycerol and cholesteryl esters
- Shell of apolipoproteins, phospholipid, and free cholesterol.
- Lipid:protein ratio determines size and density which is the basis for classification
18
Q
Apolipoprotein Classification
A
- Classified into several major classes with one or more members in each class.
- Major classes as A, B, C, and E
- Each type of apolipoprotein can be associated with one or more type of lipoprotein.
20
Q
Chylomicron Lifecycle
A
- Microsomal triacylglycerol transfer protein (MTP) loads Apo B-48 with lipids in the SER of intestinal cells.
- Particle moved to the Golgi, secreted into the lymphatic system, then moved into the blood as a nascent chylomicron.
- Apo E and Apo C-II from HDL added to the nascent CM forming complete CM.
-
Lipoprotein lipase (LPL) activated by Apo C-II and hydrolyzes up to 90% of triacylglycerol from lipoproteins releasing glycerol and free fatty acids
- LPL is a membrane-bound enzyme on endothelial cells of capillaries in most tissues other than liver
- Apo C-II is returned to HDL.
- Remaining particle is a CM remnant which are rapidly taken up by the liver via Apo-E receptor.
- Endosome fuses with lysosomes where apolipoproteins and cholesteryl esters degraded releasing amino acids, free cholesterol, and fatty acids.
24
Q
VLDL Lifecycle
A
- Microsomal triglyceride transfer protein (MTP) loads lipids onto Apo B-100 in hepatocytes
- Defective in abetalipoproteinemia
- Nascent VLDL released into blood where it picks up Apo E and Apo C-II from HDL.
- As mature VLDL travels through the blood, lipoprotein lipase degrades the TAGs increasing the density of the particle to the intermediate density lipoprotein (IDL).
- Some TAGs from VLDL exchanged for cholesteryl esters from HDL by cholesterol ester transfer protein (CETP).
- Some IDLs can be taken up by the liver in an Apo E-mediated process.
- Remainder is digested by LPL to produce low density lipoproteins (LDL).
- Apo C-II and Apo E eventually returned to HDL.
26
Q
LDL Lifecycle
A
-
LDL receptors (LDL-R) binds Apo B-100 on LDLs and Apo E on IDLs.
- LDL-R found on the cell surface of various tissues including liver and muscle localized to clathrin-coated pits.
- Binding triggers endocytosis of LDL (or IDL).
- Proton pump reduces pH causing lipoprotein to dissociate from the receptor.
- Receptor-containing membrane recycled to the surface.
- Lipoprotein degraded in lysosomes to free cholesterol, fatty acids, phospholipids, and amino acids.
- Cholesterol transferred to the SER where it can affect cellular cholesterol metabolism.
29
Q
PCSK-9
(Proprotein Convertase Subtilisin Kexin 9)
A
- PCSK-9 is a serine protease which blocks LDL-R recycling to the cell surface.
- Increased [cholesterol] → decreased PCSK-9 expression via SREBP-2 → increased LDL-R recycling → greater internalization of LDL → reduction in plasma LDL levels.
30
Q
LDL
&
Atherosclerosis
A
- Macrophages have a non-specific scavenger receptor that is capable of internalizing LDL that has been oxidized by exposure to superoxide or other oxidants.
- In response to endothelial cell injury, macrophages accumulate at the site of injury and phagocytize oxidized LDL becoming foam cells.
- Foam cells form plaques in vessels leading to atherosclerosis
- Narrowing of coronary vessels can lead to hypoxic damage and MI.
32
Q
HDL Lifecycle
A
HDL plays an important role in reverse cholesterol transport & removal from periphery.
- Lipid to added to Apo A-1 in the blood to form nascent HDL
- Apo A-1 produced by liver and intestine.
- N-HDL particles consist of phospholipid and Apo A-1, Apo A-II, Apo C-II, and Apo-E.
- Discoid shape
- Cholesterol released from non-hepatic tissues by the ABCA1 transporter (aka cholesterol efflux regulatory protein - CERP)
- Cholesterol taken up by nascent HDL.
- Cholesterol then esterified to plasma enzyme lecithin:cholesterol acyltransferase (LCAT).
-
LCAT activated by Apo A-1 on HDL surface and transfer the fatty acid from C-2 of Phosphatidylcholine to cholesterol to produce lyso-PC and cholesteryl ester.
- As cholesteryl ester accumulate in HDL it becomes spherical.
- Mature spherical HDL carries the cholesteryl esters to the liver and steroidogenic cells where they are taken up via the scavenger receptor cell B type 1 (SR-B1)
- Cholesterol used for bile acid synthesis, disposal via bile, or hormone synthesis.
34
Q
Cholesterol
&
Heart Disease
A
High levels of LDL and low levels of HDL are independent risk factors for cardiovascular disease.
- Total cholesterol and lipoprotein levels measured after 12 hour fast (no CM present)
- Then again after precipitation of Apo B containing lipoproteins (VLDL and LDL) giving HDL-cholesterol
- Seperate assay to determine triglyceride (TG) levels
- Friedewald equation used to obtain an estimate of HDL levels
