Lecture 52 Flashcards
Cholesterol Metabolism
1
Q
cholesterol metabolism overview
A
- most cells can synthesize cholesterol
- liver, intestine, and steroidogenic tissues (adrenal cortex, testes, ovaries) contribute most significantly to the overall cholesterol pool
- liver is the central organ that controls the overall cholesterol homeostasis (synthesis, distribution, elimination)
pg 1351
2
Q
sources of liver cholesterol
A
- diet (from chylomicron remnants)
- de novo synthesis (from acetyl CoA)
- delivered via HDLs (reverse cholesterol transport) -> collect excess cholesterol from peripheral tissues
pg 1351
3
Q
routes for cholesterol clearance from the liver
A
- secretion into VLDL/LDL (sent back to peripheral tissues for membrane structure/function)
- secretion as free cholesterol into the bile (emulsification of fats)
- conversion to bile salts and acids (only way we can clear from body)
pg 1351
4
Q
3 protein transporters for cholesterol
A
- Niemann-Pick type C1 (NPC1): integral membrane protein in late endosomes/lysosomes that transfer LDL-cholesterol from lysosomes to other cellular compartments (ER, PM, etc)
- Niemann-Pick type C2 (NPC2): ubiquitously expressed soluble protein in the lysosomal lumen that transfer cholesterol to NPC1
- Niemann-Pick like type C1 (NPCL1): a protein responsible for dietary cholesterol and biliary cholesterol absorption expressed in small intestines as well as in hepatocytes
pg 1352
5
Q
ezetimibe
A
blocks NPCL1 and inhibits the absorption of dietary cholesterol and decreases the amount of cholesterol available to liver cells, used as cholesterol lowering drug
pg 1352
6
Q
liver cholesterol synthesis de novo
A
- multiple steps divided into 6 stages
- stage 1: synthesis of HMG CoA
- stage 2: synthesis of mevalonate
- stages 3-6: synthesis of cholesterol
- enzyme localization: cytosol, smooth ER membranes, peroxisomes
pg 1353
7
Q
liver synthesis stages 1 and 2
A
- stage 1: 2 acetyl CoA -> acetoacetyl CoA (using thiolase) -> HMG-CoA (using HMG CoA synthase)
- stage 2: HMG-CoA -> mevalonate (uses NADPH, HMG CoA reductase)
- rate limiting committed step: HMG-CoA reductase -> key regulated irreversible step, integral membrane protein of ER facing the cytosol
- produces mevalonate
pg 1354
8
Q
regulation of HMG-CoA reductase activity
A
- regulation at the level of enzyme protein product (sterol-dependent) -> enzyme gene expression, enzyme degradation (transcriptional regulation, how much enzyme will be produced)
- regulation via covalent modifications (sterol-independent) -> phosphorylation/dephosphorylation
pg 1355
9
Q
transcription factor SREBP-2
A
- integral protein of the ER membrane
- associated with a 2nd ER membrane protein, SCAP (SREBP cleavage-activating protein), which has a sterol-binding domain (INSIG)
- once released, it translocates to the nucleus and binds DNA at the cis-acting sterol regulatory element (SRE) upstream of the HMG CoA reductase gene -> activates gene expression
pg 1356
10
Q
regulation -> high cholesterol
A
- high sterol levels -> NO new sterols needed
- SCAP binds sterols and retains the SCAP-SREBP complex on the ER in an inactive state
- indirectly cause HMG CoA redutase to be ubiquinated, thus targeted for degradation by proteasomes
pg 1357
11
Q
regulation -> low cholesterol
A
- low sterol levels -> new sterols NEEDED
- SCAP protein is not bound to sterols (they are low)
- SCAP-SREBP-2 complex is transported to the golgi
- in golgi, SCAP is cleaved to release SREBP-2
- free SREBP-2 moves to the nucleus where it binds an SRE and initiates transcription of HMG CoA reductase
pg 1358
12
Q
regulation via covalent modifications
A
activity is influenced by the energy state of the cell:
- elevated AMP (low energy): HMG CoA reductase is phosphorylated, which is its INACTIVE form (high glucagon: fasting or glucocorticoids: stress)
- elevate ATP (high energy): HMG CoA reductase is dephosphorylated, which is its ACTIVE form (high insulin and thyroxine, well-fed state)
pg 1359
13
Q
statin drugs
A
- contain a side group that resembles HMG CoA
- compete for the active site of the HMG CoA reductase enzyme (competitive inhibition)
- helpful in lower cholesterol in bloodstream by blocking rate-limiting step
pg 1360
14
Q
cholesterol synthesis: stage 3
A
- mevalonate to isoprenyl pyrophosphates (IPP)
- loss of CO2, 3 ATP
- requires ATP
- works to keep molecules water soluble
pg 1361
15
Q
cholesterol synthesis: stages 4-6
A
- 4: isoprenyl pyrophosphates -> squalene
- 5: squalene -> lanosterol
- 6: lanosterol -> cholesterol (inborn error of metabolism)
pg 1361
16
Q
cholesterol synthesis: stages 3-6
A
- phosphorylation of mevalonate and presence of pyrophosphate in subsequent structures help keep these water-insoluble compounds in solution
- pyrophosphate is released in each of 4 condensation steps, making the reactions irreversible
- beginning with squalene, intermediates in cholesterol biosynthesis are nonphosphorylated and are so hydrophobic that they require an intracellular sterol carrier protein to keep them soluble
pg 1361
17
Q
FPP is a chemotherapeutic target
A
- farnesyl pyrophosphate (FPP)
- responsible for farnesylation -> so chemotherapy works to block this
- FPP links Ras (small GTP-binding protein) to the membrane
- Ras mutations are seen in 1/3 of human cancers
pg 1362
18
Q
Smith-Lemli-Opitz syndrome
A
- defect in the conversion of lanosterol to cholesterol (step [8]: 7-dehydrocholesterol reductase)
- autosomal recessive
- symptoms: intellectual delay, ambiguous genitalia, hypotonia, microcephaly, syndactyly, limb abnormalities/deformities, polydactyly
pg 1362
19
Q
required substrates for cholesterol synthesis
A
- acetyl CoA - C skeleton
- NADPH - reducing equivalents
- ATP - energy needed
pg 1363
20
Q
under what conditions will de novo synthesis of cholesterol be active?
A
- in well-fed state (when ALL substrates are available)
- when cholesterol supply is low
pg 1363
21
Q
cholesterol degradation and clearance
A
- humans canNOT degrade the sterol nucleus to CO2 and H2O
- excreted into the bile as cholesterol or converted to bile acids and salts and excreted
- very little loss (5% daily in feces) due to the efficiency of enterohepatic circulation of bile salts
pg 1364
22
Q
bile salts and bile acids
A
- during production of bile acids from cholesterol, a number of oxygen atoms have been added, which increases the solubility of the compound
- in the final transformation from a bile acid to a bile salt, one of two amino acids (glycine or taurine) are added to the bile acid to form bile salt, which even further increases the solubility
- the final compounds are amphipathic in nature, and so acts as an emulsifying agent on lipids (including cholesterol)
NEED TO SEE NEW SLIDE IN BB
pg 1365
23
Q
enterohepatic circulation of bile
A
- bile acids (from cholesterol) converted to bile salts by addition of AA in liver
- bile salts travel through bile duct to duodenum
- about 5% of bile salts lost through fecal excretion per day, the rest go back to the liver via the portal vein
pg 1366
24
Q
bile sequestrants and LDL cholesterol
A
- Cholestyramine is a bile acid sequestrant, which binds bile in the GI tract to prevent its reabsorption and promote excretion
- removal of bile form the liver relieves inhibition on bile acid synthesis and promotes diversion of cholesterol to bile salts
- dietary fiber may also act as a kind of natural bile acid sequestrant
pg 1367
25
Q
cholelithiasis (gallstones)
A
- derived from cholesterol and result from too little bile salts and/or too much cholesterol (the bile salts are required to solubilize free cholesterol)
- possible causes…
- inefficient enterohepatic cycling of bile salts
- liver dysfunction resulting in bile salts deficiency
- other idiopathic reason for decreased bile salts
pg 1368
