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Biochemistry Exam 3 > L27- Steroids and Fat-Soluble Vitamins > Flashcards

L27- Steroids and Fat-Soluble Vitamins Flashcards

1
Q

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

A

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

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2
Q

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

A

Pancreatic cholesterol esterase.

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3
Q

Where in the cell does cholesterol synthesis occur?

A

In both the cytoplasm and the ER.

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4
Q

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

A

3

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5
Q

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

A

HMG-CoA synthase.

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6
Q

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

A

HMG-CoA reductase.

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7
Q

What is the cofactor for HMG-CoA reductase?

A

NADPH.

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8
Q

What is the most important regulatory step in cholesterol synthesis?

A

The HMG-CoA reductase reaction.

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9
Q

How is the enzyme HMG-CoA reductase inhibited?

A

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).

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10
Q

Statins are competitive inhibitors of which enzyme in cholesterol synthesis?

A

HMG-CoA reductase.

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11
Q

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

A

Isopentenyl pyrophosphate (IPP).

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12
Q

How many carbons are in isopentenyl pyrophosphate?

A

5

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13
Q

How many carbons are in geranyl pyrophosphate?

A

10

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14
Q

How many carbons are in farnesyl pyrophosphate?

A

15

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15
Q

Name two isoprenoids for which farnesyl pyrophosphate is a precursor.

A

Dolichol and coenzyme Q (ubiquinone).

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16
Q

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

A

Squalene.

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17
Q

How many carbons are in squalene?

A

30

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18
Q

What is the function of squalene monooxygenase?

A

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

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19
Q

What is the first steroid product in cholesterol biosynthesis?

A

Lanosterol.

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20
Q

How many carbons are in lanosterol?

A

30

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21
Q

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

A

27

22
Q

What is the defect in Wolman’s syndrome?

A

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.

23
Q

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

A

Via conversion to bile salts in the liver.

24
Q

What is the rate limiting step for bile acid synthesis?

A

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

25
Q

Steroids always end in ‘ol.’ Bile salts always end in…

A

‘cholic acid.’

26
Q

In which organ are bile acids stored?

A

The gallbladder.

27
Q

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

A

The enterohepatic circulation.

28
Q

Where in the cell are steroid hormone receptors located?

A

In the cytoplasm or nucleus.

29
Q

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

A

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

30
Q

Which vitamins are fat-soluble?

A

Vitamins A, D, E and K.

31
Q

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

A

Retinol.

32
Q

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

A

Retinoid acid.

33
Q

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

A

Vitamin A.

34
Q

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

A

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

35
Q

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

A

Rhodopsin (opsin with retinal bound to it).

36
Q

True or False. Vitamin D is derived from cholesterol.

A

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

37
Q

What is the precursor of vitamin D?

A

7-dehydrocholesterol.

38
Q

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

A

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

39
Q

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

A

The liver.

40
Q

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

A

The kidneys.

41
Q

What is the most potent form of vitamin D3?

A

1,25-dihydroxycholecalciferol.

42
Q

What is the main function of vitamin D3?

A

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

43
Q

Vitamin D deficiency can lead to which condition in children?

A

Rickets.

44
Q

Vitamin D deficiency can lead to which condition in adults?

A

Osteomalacia.

45
Q

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

A

Vitamin E.

46
Q

What is the main function of vitamin E?

A

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

47
Q

Which fat soluble vitamin is important for blood clotting?

A

Vitamin K.

48
Q

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

A

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

49
Q

Which enzyme is inhibited by the drug warfarin?

A

Vitamin K epoxide reductase.

50
Q

What is the function of vitamin K reductase?

A

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

Biochemistry Exam 3 (6 decks)

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