T09 - Cholesterol Biosynthesis

Question 1

Which tissues of the body contains the most cholesterol, on an mg/g basis? List the top four.

Answer 1

brain > adrenal > lung > kidney

Question 2

What are the three ways by which tissues obtain cholesterol?

Answer 2

de novo synthesis from acetate

uptake of cholesterol-rich lipoproteins

diet

Question 3

How many carbons does cholesterol have?

Answer 3

27

Question 4

How many steps does cholesterol synthesis take?

Answer 4

18

Question 5

How does progressing along the cholesterol synthesis pathway affect solubility?

Answer 5

solubility in water of successive intermediates decreases as cholesterol is assembled

Question 6

What is the rate-limiting enzyme in cholesterol synthesis?

Answer 6

HMG-CoA reductase

Question 7

What regulates HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis?

Answer 7

SREBP family of proteins which are transcription factors

Question 8

SLOS syndrome is caused by

Answer 8

a defect in Δ⁷-sterol reductase, the last enzyme in the cholesterol synthesis pathway

Question 9

The brain, out of all other tissues, contains the most cholesterol in the body. What is this cholesterol used for?

Answer 9

cholesterol is located in myelin sheaths

Question 10

How is the brain unique in its cholesterol utilization?

Answer 10

it can only synthesize cholesterol de novo because of the blood/brain barrier

Question 11

How do cells choose between receptor-mediated endocytosis of cholesterol and de novo synthesis of cholesterol?

Answer 11

cells prefer LDLR over de novo synthesis (exception: muscle cells, which have low LDL receptors, tend to make their own cholesterol)

Question 12

What are the two pathways by which cholesterol is metabolized in mammalian cells?

Answer 12

cholesterol → bile acids (95% of the time)

cholesterol → steroid hormones (5% of the time)

Question 13

Describe the mammal’s preference for using acetate to produce fatty acids vs. to produce cholesterol.

Answer 13

10x more fatty acid synthesis than cholesterol synthesis

Question 14

Where is HMG-CoA reductase located?

Answer 14

located in the membrane of ER

Question 15

Write out the first six reactions of the cholesterol synthesis pathway. Identify where HMG-CoA reductase acts.

Answer 15

acetyl-CoA → acetoacetyl-CoA → HMG-CoA → [HMG-CoA reductase] → mevalonate →→→ farnesyl pyrophosphate → squalene →→→

Question 16

Differentiate between the Bloch and Kandutsch-Russel pathways of cholesterol synthesis.

Answer 16

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

Question 17

(T/F) The steps of cholesterol synthesis differ based on tissue.

Answer 17

True

Question 18

Describe the characteristics of the intermediates of cholesterol synthesis.

Answer 18

7 water soluble intermediates

4 isoprenoid intermediates

7 sterol intermediates

Question 19

What is triparanol and how does it act?

Answer 19

drug that inhibits the 2nd last to step of cholesterol synthesis → accumulation of hydrophobic intermediates → toxic

Question 20

Which is more harmful — mutations to enzymes earlier in the cholesterol synthesis pathway, or mutations to enzymes later in the pathway?

Answer 20

mutations to enzymes later in the pathway are more harmful

Question 21

Where are enzymes associated with cholesterol synthesis located in the cell?

Answer 21

located in either cytosol or ER

Question 22

What are the two major pathways of cholesterol synthesis?

Answer 22

Bloch pathway (liver)

Kandutsch-Russel pathway (skin)

Question 23

What is an alternative fate of isopentenyl pyrophosphate, a cholesterol synthesis intermediate?

Answer 23

aromatic compound

Question 24

What is an alternative fate of dolichols, a cholesterol synthesis intermediate?

Answer 24

protein glycosylation

Question 25

What is an alternative fate of quinone, a cholesterol synthesis intermediate?

Answer 25

electron transfer

Question 26

Isopentenyl pyrophosphate, a cholesterol synthesis intermediate, can be converted to which compounds/vitamins?

Answer 26

vitamins A/E/K dolichols quinones

Question 27

What is the precursor to vitamin D3?

Answer 27

7-dehydrocholesterol

Question 28

The conversion of 7-dehydrocholesterol to vitamin D3 is a multi-step process. Where does the first step take place?

Answer 28

skin

Question 29

Describe what happens in the first step of vitamin D3 formation.

Answer 29

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

Question 30

Is vitamin D3 active after it is formed from 7-dehydrocholesterol?

Answer 30

No, it is inactive and has to be hydroxylated twice to become active

Question 31

Where does the first hydroxylation of inactive/unmodified vitamin D3 take place?

Answer 31

liver

Question 32

What enzymes mediate the first hydroxylation of inactive/unmodified vitamin D3?

Answer 32

CYP2R1 -or- CYP27A1

Question 33

Where does the second hydroxylation of inactive/unmodified vitamin D3 take place?

Answer 33

**kidney** mono-hydroxylated vitamin D3 transported by vitamin D binding protein from liver → kidney

Question 34

What enzymes mediate the second hydroxylation of inactive/unmodified vitamin D3?

Answer 34

only CYP27B1

Question 35

What is the product of the second hydroxylation of inactive/unmodified vitamin D3? What does this compound do?

Answer 35

1α,25-dihydroxyvitamin D3 binds to activates vitamin D receptor, which regulates transcription of vitamin D3 target genes that control calcium metabolism

Question 36

Describe the half-life of 1α,25-dihydroxyvitamin D3.

Answer 36

1α,25-dihydroxyvitamin D3 is quickly inactivated by enzymes in vitamin D target tissues, so its blood levels are very low

Question 37

Which form of vitamin D3 is clinically measured?

Answer 37

25-hydroxyvitamin D3 (the immediate precursor to 1α,25-dihydroxyvitamin D3)

Question 38

What increases plasma levels of 25-hydroxyvitamin D3?

Answer 38

increased exposure to sunlight increased dietary intake of vitamin D

Question 39

Write out the steps of the vitamin D3 synthesis pathway. (6)

Answer 39

7-dehydrocholesterol → [UV] → previtamin D → [36 hrs] → vitamin D3 → [CYP27A1/CYP2R1] → 25-hydroxyvitamin D3 → [CYP27B1] → 1α,25-dihydroxyvitamin D3

Question 40

Mutations in the genes that produce vitamin D3 cause what disease?

Answer 40

rickets

Question 41

When cells were sated for cholesterol, but were fed excess cholesterol, what were the three possible outcomes?

Answer 41

reduction in levels of HMG-CoA reductase stimulated ACAT (cholsterol → cholesteryl esters) reduced LDLRs

Question 42

What are the two forms of post-translational regulation of HMG-CoA reductase?

Answer 42

geranylgeranyl pyrophosphate 24,25-dihydrolanosterol (both intermediates in the cholesterol synthesis pathway)

Question 43

What is the one form of transcriptional regulation of HMG CoA reductase?

Answer 43

cholesterol inhibits SREBPs, and SREBPs are needed to activate cholesterol synthesis

Question 44

Write out the pathway of events that occur when 24,25-dihydrolanosterol builds up. (6)

Answer 44

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

Question 45

Where are SREBPs made?

Answer 45

made as precursor proteins in ER

Question 46

Write out the pathway of events involving SCAP and SREBP that occur when a cell is starved for cholesterol. (5)

Answer 46

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

Question 47

Write out the pathway of events involving SCAP and SREBP that occur when a cell has excess cholesterol.

Answer 47

cholesterol binds to SCAP → SCAP binds to Insig → SCAP/SREBP complex can't move to Golgi → SREBP can't be activated

Question 48

Describe the half life of HMG CoA reductase.

Answer 48

short half-life

Question 49

Statins inhibit

Answer 49

HMG-CoA reductase

Question 50

Why are there so many different statins? (2)

Answer 50

(1) different statins have different potency in reducing levels of LDL-cholesterol (2) different patients respond differently to different statins

Question 51

What are two conditions that lead to a Class I statin recommendation?

Answer 51

LDL-C \> 190 mg/dL → start on high-intensity statin or DM + age 40-75 → start on moderate-intensity statin

Question 52

In ASCVD prevention and risk assessment, how do you treat patients age 0-19? (2)

Answer 52

counsel on lifestyle to prevent ASCVD if diagnosed with familial hypercholesterolemia → start on statin

Question 53

In ASCVD prevention and risk assessment, how do you treat patients age 20-39?

Answer 53

counsel on lifestyle to prevent ASCVD if family hx of premature ASCVD AND LDL-C \> 160 mg/dL → start on statin

Question 54

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?

Answer 54

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%

Question 55

What is the clinical presentation of Smith-Lemli-Opitz Syndrome? (4)

Answer 55

feeding problems mental retardation microcephaly polydactyly

Question 56

What is the inheritance pattern of Smith-Lemli-Opitz Syndrome?

Answer 56

autosomal recessive

Question 57

On a cellular level, why is Smith-Lemli-Opitz Syndrome harmful?

Answer 57

leads to accumulation of 7-dehydrocholesterol, which means not enough cholesterol is produced

Question 58

How is Smith-Lemli-Opitz Syndrome diagnosed? (2)

Answer 58

gene sequencing measurement of 7-dehydrocholesterol in blood

Question 59

What is the treatment of Smith-Lemli-Opitz Syndrome?

Answer 59

cholesterol supplementation

Question 60

Describe the effects of a mutation in the mevalonate kinase gene.

Answer 60

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

Question 61

Under what conditions would you be able to reduce the risk of a heart attack to 0%?

Answer 61

if you were able to reduce blood [LDL] to under 55 mg/dL

Question 62

Draw out the pathway of the alternate fates of cholesterol biosynthetic intermediates.

Answer 62

Question 63

What are the top three cholesterol-producing tissues in the body? Keep in mind that this is _separate_ from cholesterol content.

Answer 63

liver \> adrenal \> small intestine