T09 - Cholesterol Biosynthesis Flashcards
Which tissues of the body contains the most cholesterol, on an mg/g basis? List the top four.
brain > adrenal > lung > kidney
What are the three ways by which tissues obtain cholesterol?
de novo synthesis from acetate
uptake of cholesterol-rich lipoproteins
diet
How many carbons does cholesterol have?
27
How many steps does cholesterol synthesis take?
18
How does progressing along the cholesterol synthesis pathway affect solubility?
solubility in water of successive intermediates decreases as cholesterol is assembled
What is the rate-limiting enzyme in cholesterol synthesis?
HMG-CoA reductase
What regulates HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis?
SREBP family of proteins which are transcription factors
SLOS syndrome is caused by
a defect in Δ7-sterol reductase, the last enzyme in the cholesterol synthesis pathway
The brain, out of all other tissues, contains the most cholesterol in the body. What is this cholesterol used for?
cholesterol is located in myelin sheaths
How is the brain unique in its cholesterol utilization?
it can only synthesize cholesterol de novo because of the blood/brain barrier
How do cells choose between receptor-mediated endocytosis of cholesterol and de novo synthesis of cholesterol?
cells prefer LDLR over de novo synthesis (exception: muscle cells, which have low LDL receptors, tend to make their own cholesterol)
What are the two pathways by which cholesterol is metabolized in mammalian cells?
cholesterol → bile acids (95% of the time)
cholesterol → steroid hormones (5% of the time)
Describe the mammal’s preference for using acetate to produce fatty acids vs. to produce cholesterol.
10x more fatty acid synthesis than cholesterol synthesis
Where is HMG-CoA reductase located?
located in the membrane of ER
Write out the first six reactions of the cholesterol synthesis pathway. Identify where HMG-CoA reductase acts.
acetyl-CoA → acetoacetyl-CoA → HMG-CoA → [HMG-CoA reductase] → mevalonate →→→ farnesyl pyrophosphate → squalene →→→
Differentiate between the Bloch and Kandutsch-Russel pathways of cholesterol synthesis.
Bloch = liver = higher amounts of lanosterol 14-demethylase
Kandutsch-Russel = skin = higher amounts of Δ24-sterol, dictated by need to synthesize 7-dehydrocholesterol, a precursor of vitamin D3
(T/F) The steps of cholesterol synthesis differ based on tissue.
True
Describe the characteristics of the intermediates of cholesterol synthesis.
7 water soluble intermediates
4 isoprenoid intermediates
7 sterol intermediates
What is triparanol and how does it act?
drug that inhibits the 2nd last to step of cholesterol synthesis → accumulation of hydrophobic intermediates → toxic
Which is more harmful — mutations to enzymes earlier in the cholesterol synthesis pathway, or mutations to enzymes later in the pathway?
mutations to enzymes later in the pathway are more harmful
Where are enzymes associated with cholesterol synthesis located in the cell?
located in either cytosol or ER
What are the two major pathways of cholesterol synthesis?
Bloch pathway (liver)
Kandutsch-Russel pathway (skin)
What is an alternative fate of isopentenyl pyrophosphate, a cholesterol synthesis intermediate?
aromatic compound
What is an alternative fate of dolichols, a cholesterol synthesis intermediate?
protein glycosylation
What is an alternative fate of quinone, a cholesterol synthesis intermediate?
electron transfer
Isopentenyl pyrophosphate, a cholesterol synthesis intermediate, can be converted to which compounds/vitamins?
vitamins A/E/K
dolichols
quinones
What is the precursor to vitamin D3?
7-dehydrocholesterol
The conversion of 7-dehydrocholesterol to vitamin D3 is a multi-step process. Where does the first step take place?
skin
Describe what happens in the first step of vitamin D3 formation.
non-enzymatic: exposure to UV light causes scission of B-ring of 7-dehydrocholesterol to form previtamin D → undergoes isomerization over 36 hours → produces vitamin D3
Is vitamin D3 active after it is formed from 7-dehydrocholesterol?
No, it is inactive and has to be hydroxylated twice to become active
Where does the first hydroxylation of inactive/unmodified vitamin D3 take place?
liver
What enzymes mediate the first hydroxylation of inactive/unmodified vitamin D3?
CYP2R1 -or- CYP27A1
Where does the second hydroxylation of inactive/unmodified vitamin D3 take place?
kidney
mono-hydroxylated vitamin D3 transported by vitamin D binding protein from liver → kidney
What enzymes mediate the second hydroxylation of inactive/unmodified vitamin D3?
only CYP27B1
What is the product of the second hydroxylation of inactive/unmodified vitamin D3? What does this compound do?
1α,25-dihydroxyvitamin D3
binds to activates vitamin D receptor, which regulates transcription of vitamin D3 target genes that control calcium metabolism
Describe the half-life of 1α,25-dihydroxyvitamin D3.
1α,25-dihydroxyvitamin D3 is quickly inactivated by enzymes in vitamin D target tissues, so its blood levels are very low
Which form of vitamin D3 is clinically measured?
25-hydroxyvitamin D3 (the immediate precursor to 1α,25-dihydroxyvitamin D3)
What increases plasma levels of 25-hydroxyvitamin D3?
increased exposure to sunlight
increased dietary intake of vitamin D
Write out the steps of the vitamin D3 synthesis pathway. (6)
7-dehydrocholesterol → [UV] → previtamin D → [36 hrs] → vitamin D3 → [CYP27A1/CYP2R1] → 25-hydroxyvitamin D3 → [CYP27B1] → 1α,25-dihydroxyvitamin D3
Mutations in the genes that produce vitamin D3 cause what disease?
rickets
When cells were sated for cholesterol, but were fed excess cholesterol, what were the three possible outcomes?
reduction in levels of HMG-CoA reductase
stimulated ACAT (cholsterol → cholesteryl esters)
reduced LDLRs
What are the two forms of post-translational regulation of HMG-CoA reductase?
geranylgeranyl pyrophosphate
24,25-dihydrolanosterol
(both intermediates in the cholesterol synthesis pathway)
What is the one form of transcriptional regulation of HMG CoA reductase?
cholesterol inhibits SREBPs, and SREBPs are needed to activate cholesterol synthesis
Write out the pathway of events that occur when 24,25-dihydrolanosterol builds up. (6)
24,25-dihydrolanosterol builds up → activate Insig → binds to HMG-CoA reductase in ER membrane → HMG-CoA ubiquitinated → geranylgeranyl pyrophosphate extracts ubiquitinated HMG-CoA → ubiquitinated HMG-CoA degraded by proteasome
Where are SREBPs made?
made as precursor proteins in ER
Write out the pathway of events involving SCAP and SREBP that occur when a cell is starved for cholesterol. (5)
SCAP dissociates from Insig → SCAP escorts SREBP to Golgi → SREBP cut in half → amino-terminal half containing DNA-binding domain migrates to nucleus → activates enzymes for cholesterol synthesis
Write out the pathway of events involving SCAP and SREBP that occur when a cell has excess cholesterol.
cholesterol binds to SCAP → SCAP binds to Insig → SCAP/SREBP complex can’t move to Golgi → SREBP can’t be activated
Describe the half life of HMG CoA reductase.
short half-life
Statins inhibit
HMG-CoA reductase
Why are there so many different statins? (2)
(1) different statins have different potency in reducing levels of LDL-cholesterol
(2) different patients respond differently to different statins
What are two conditions that lead to a Class I statin recommendation?
LDL-C > 190 mg/dL → start on high-intensity statin
or
DM + age 40-75 → start on moderate-intensity statin
In ASCVD prevention and risk assessment, how do you treat patients age 0-19? (2)
counsel on lifestyle to prevent ASCVD
if diagnosed with familial hypercholesterolemia → start on statin
In ASCVD prevention and risk assessment, how do you treat patients age 20-39?
counsel on lifestyle to prevent ASCVD
if family hx of premature ASCVD AND LDL-C > 160 mg/dL → start on statin
In 40-75 year old patients who have LDL-C between 70 and 190 mg/dL, but no hx of DM, what are the possible statin treatment options?
low risk - discuss risks and counsel on lifestyle
borderline risk - discuss risks and consider starting moderate-intensity statin
intermediate risk - start moderate-intensity statin to reduce LDL-C by 30%
high risk - start statin to reduce LDL-C by 50%
What is the clinical presentation of Smith-Lemli-Opitz Syndrome? (4)
feeding problems
mental retardation
microcephaly
polydactyly
What is the inheritance pattern of Smith-Lemli-Opitz Syndrome?
autosomal recessive
On a cellular level, why is Smith-Lemli-Opitz Syndrome harmful?
leads to accumulation of 7-dehydrocholesterol, which means not enough cholesterol is produced
How is Smith-Lemli-Opitz Syndrome diagnosed? (2)
gene sequencing
measurement of 7-dehydrocholesterol in blood
What is the treatment of Smith-Lemli-Opitz Syndrome?
cholesterol supplementation
Describe the effects of a mutation in the mevalonate kinase gene.
though mevalonate kinase is pretty far along the pathway, a defect doesn’t cause as severe of problems because the accumulating intermediate (mevalonate) is water-soluble
Under what conditions would you be able to reduce the risk of a heart attack to 0%?
if you were able to reduce blood [LDL] to under 55 mg/dL
Draw out the pathway of the alternate fates of cholesterol biosynthetic intermediates.
What are the top three cholesterol-producing tissues in the body? Keep in mind that this is separate from cholesterol content.
liver > adrenal > small intestine
