L27- Steroids and Fat-Soluble Vitamins

Question 1

What is the function of cholesterol with regards to cell membranes?

Answer 1

It intercalates itself between phospholipids to reduce the fluidity of the membrane.

Question 2

Which pancreatic enzyme hydrolyzes dietary cholesterol esters into cholesterol and free fatty acids?

Answer 2

Pancreatic cholesterol esterase.

Question 3

Where in the cell does cholesterol synthesis occur?

Answer 3

In both the cytoplasm and the ER.

Question 4

How many molecules of acetyl-CoA are needed to form HMG-CoA?

Answer 4

3

Question 5

Which enzyme condenses acetyl-CoA with acetoacetyl-CoA to form HMG-CoA?

Answer 5

HMG-CoA synthase.

Question 6

The conversation of HMG-CoA to mevalonic acid is catalyzed by which enzyme?

Answer 6

HMG-CoA reductase.

Question 7

What is the cofactor for HMG-CoA reductase?

Answer 7

NADPH.

Question 8

What is the most important regulatory step in cholesterol synthesis?

Answer 8

The HMG-CoA reductase reaction.

Question 9

How is the enzyme HMG-CoA reductase inhibited?

Answer 9

By feedback inhibition from high levels of cholesterol, via phosphorylation by a cAMP-dependent kinase, and by the rate of synthesis of the enzyme, which has a short half life (3 hours).

Question 10

Statins are competitive inhibitors of which enzyme in cholesterol synthesis?

Answer 10

HMG-CoA reductase.

Question 11

In cholesterol synthesis, mevalonic acid can undergo a series of reactions involving decarboxylation and phosphorylation to give rise to which isoprene unit?

Answer 11

Isopentenyl pyrophosphate (IPP).

Question 12

How many carbons are in isopentenyl pyrophosphate?

Answer 12

5

Question 13

How many carbons are in geranyl pyrophosphate?

Answer 13

10

Question 14

How many carbons are in farnesyl pyrophosphate?

Answer 14

15

Question 15

Name two isoprenoids for which farnesyl pyrophosphate is a precursor.

Answer 15

Dolichol and coenzyme Q (ubiquinone).

Question 16

Condensation followed by NADPH-dependent reduction of two molecules of farnesyl pyrophosphate yields what product?

Answer 16

Squalene.

Question 17

How many carbons are in squalene?

Answer 17

30

Question 18

What is the function of squalene monooxygenase?

Answer 18

It uses both NADPH and oxygen to oxidize squalene, generating a squalene epoxide, which then undergoes cyclization.

Question 19

What is the first steroid product in cholesterol biosynthesis?

Answer 19

Lanosterol.

Question 20

How many carbons are in lanosterol?

Answer 20

30

Question 21

Lanosterol undergoes a series of demethylations and rearrangements before it is converted to cholesterol. How many carbons are in cholesterol?

Answer 21

27

Question 22

What is the defect in Wolman’s syndrome?

Answer 22

Wolman’s syndrome is a cholesterol ester storage disease whereby a defect in a lysosomal esterase prevents the hydrolysis of cholesterol esters. While LDL uptake is normal, there is nevertheless an accumulation of cholesterol esters in cells.

Question 23

What is the major route of cholesterol removal in the human body?

Answer 23

Via conversion to bile salts in the liver.

Question 24

What is the rate limiting step for bile acid synthesis?

Answer 24

Conversion of cholesterol to 7-alpha-hydroxycholesterol by 7-alpha-hydroxylase.

Question 25

Steroids always end in 'ol.' Bile salts always end in...

Answer 25

'cholic acid.'

Question 26

In which organ are bile acids stored?

Answer 26

The gallbladder.

Question 27

Bile acids are secreted into the intestines, where they are reabsorbed and shuttled back to the gallbladder. What is the name of this circulation?

Answer 27

The enterohepatic circulation.

Question 28

Where in the cell are steroid hormone receptors located?

Answer 28

In the cytoplasm or nucleus.

Question 29

What happens to steroid hormone receptors when they become bound by a hormone?

Answer 29

They increase the transcription of specific target genes (i.e. they act as sequence-specific DNA-binding transcription factors).

Question 30

Which vitamins are fat-soluble?

Answer 30

Vitamins A, D, E and K.

Question 31

Which form of vitamin A plays an important role in vision?

Answer 31

Retinol.

Question 32

Which form of vitamin A is an important intercellular signaling molecule involved in growth and differentiation?

Answer 32

Retinoid acid.

Question 33

Deficiency of which fat soluble vitamin can lead to night blindness?

Answer 33

Vitamin A.

Question 34

True or False. Trans-retinal absorbs lights and subsequently isomerizes to 11-cis-retinal.

Answer 34

False. It is the other way around: 11-cis-retinal isomerizes to all-trans-retinal.

Question 35

What is the active form of the light-receptor protein opsin?

Answer 35

Rhodopsin (opsin with retinal bound to it).

Question 36

True or False. Vitamin D is derived from cholesterol.

Answer 36

False. It is derived from an intermediate of cholesterol synthesis (7-dehydrocholesterol).

Question 37

What is the precursor of vitamin D?

Answer 37

7-dehydrocholesterol.

Question 38

7-alpha-hydroxycholesterol and 7-hydrocholesterol are intermediates of two different metabolic pathways. To which pathway do they each belong?

Answer 38

7-alpha-hydroxycholesterol is a precursor of bile salts while 7-hydrocholesterol is a precursor of vitamin D.

Question 39

Where in the body does vitamin D3 get hydroxylated at C25?

Answer 39

The liver.

Question 40

Where in the body does vitamin D3 get hydroxylated at C1?

Answer 40

The kidneys.

Question 41

What is the most potent form of vitamin D3?

Answer 41

1,25-dihydroxycholecalciferol.

Question 42

What is the main function of vitamin D3?

Answer 42

It increases the intestinal absorption of calcium to promote bone formation.

Question 43

Vitamin D deficiency can lead to which condition in children?

Answer 43

Rickets.

Question 44

Vitamin D deficiency can lead to which condition in adults?

Answer 44

Osteomalacia.

Question 45

Alpha-tocopherol is another name for which fat-soluble vitamin?

Answer 45

Vitamin E.

Question 46

What is the main function of vitamin E?

Answer 46

It terminates the peroxidation of polyunsaturated fatty acids (it acts as an antioxidant).

Question 47

Which fat soluble vitamin is important for blood clotting?

Answer 47

Vitamin K.

Question 48

For which reaction in the synthesis of prothrombin is vitamin K a cofactor?

Answer 48

The formation of gamma-carboxyl glutamate residues from glutamate residues by vitamin K-dependent carboxylase.

Question 49

Which enzyme is inhibited by the drug warfarin?

Answer 49

Vitamin K epoxide reductase.

Question 50

What is the function of vitamin K reductase?

Answer 50

It regenerates vitamin K to its active form so it may be reused by vitamin K-dependent carboxylase.