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Biochemistry I > Chapter 29: Cholesterol Metabolism > Flashcards

Chapter 29: Cholesterol Metabolism Flashcards

1
Q

Circulating levels of cholesterol are a balance between

A
  • What is consumed in diet

- What is made in the body

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2
Q

Consumption of less cholesterol causes the body to

A
  • Produce more

- Makes it difficult to reduce cholesterol by dietary means alone

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3
Q

Cholesterol is ubiquitous in

A
  • Animal cells
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4
Q

Cholesterol solubility

A
  • Considered amphipathic

- Weakly soluble in water

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5
Q

Cholesterol is linked with

A
  • Cardiovascular disease
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6
Q

Cholesterol is often found stored in cell or

A
  • Transported around the body
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7
Q

Cholesterol esters

A
  • Hydropobic
  • Much less polar fatty acids
  • Transport cholesterol
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8
Q

The more hydrophobic cholesterol esters are transported in

A
  • The core of a lipoprotein

- FA is attached

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9
Q

Cholesterol is transported in

A
  • The outer phospholipid coat of lipoproteins

- (Phospholipid monolayer on the outside)

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10
Q

All carbon atoms of the cholesterol molecule are derived from

A
  • Acetate
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11
Q

Rings of the cholesterol molecule are attached to a

A
  • Branched hydrocarbon chain
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12
Q

Carbons of the cholesterol molecule come from acetate via

A
  • Acetyl-SCoA
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13
Q

Cholesterol molecule structure is characterized as

A
  • Weakly amphipathic

- Hydrophobic

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14
Q

Cholesterol esters are very

A
  • Hydrophobic
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15
Q

Cholesterol has the following functions

A
  • Precursors of bile acids
  • Membrane stabilization
  • Precursor of steroid hormones
  • Cholecalciferol (vitamin D3)
  • NOT an energy substrate
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16
Q

Cholesterol is not broken down, but

A
  • Converted to bile acids and secreted from the body

- NOT an energy substrate (no ATP production from breakdown)

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17
Q

Cholesterol derivatives are precursors to active

A
  • Vitamin D

- Means we can make vitamin D in the body

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18
Q

We can make vitamin D in the body by

A
  • Exposing the skin to sunlight
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19
Q

Enzymes for cholesterol biopsyntehsis are located in

A
  • Cytosol

- ER

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20
Q

Enzymes for cholesterol biosyntehsis

A
  • Produced in most tissues

- NADPH-dependent (NADPH from the PPP)

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21
Q

PAtheay of cholesterol biosyntehsis overall

A
  1. Acetoacetyl-SCoA formation in cytoplasm
  2. HMG-SCoA production (also found in ketogenesis)
  3. Production of mevalonic acid (committed step)
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22
Q

Acetoacetyl-SCoA formation in cytoplasm

A
  • Acetyl-SCoA must exit mitochondris
  • Two acetyl-SCoA molecules condensue to form acetoacetyl-SCoA
  • Acetyl-SCoA transferase
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23
Q

HMG-SCoA production (also found in ketogenesis) is catalyzed by

A
  • Cytosolic HMG-SCoA synthase
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24
Q

Production of mevalonic acid (committed step) is catalyzed by

A
  • HMG-SCoA reductase
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25
Q

First intermediate that is unique to the pathway of cholesterol biosynthesis

A
  • Mevalonic acid
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26
Q

Condensation of a third molecules of acetyl-SCoA with acetoaetyl-SCoA to form

A

-

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27
Q

The rate limiting step of cholesterol biosynthesis is catalyzed by

A
  • HMG-CoA reductase
  • Irreversible
  • Far from equilibrium
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28
Q

HMG-SCoA is reduced to

A
  • Mevalonic acid in an irreversible reaction
  • NADPH requirement
  • Key step in cholesterol biosynthesis
29
Q

Insulin _____ HMG-SCoA activity

A
  • Insulin promotes its activity
30
Q

HMG-SCoA is _____ when active

A
  • Dephosphoprylated

- Connected with biosynthesis in the presence of insulin

31
Q

Mevalonic acid is phosphorylated and decarboxylated to

A
  • Isopentenyl pyrophosphate
32
Q

Isopentenyl pyrophosphate is formed by

A
  • Phosphorylation and decarboxylation of mevalonic acid
33
Q

Isopentenyl pyrophopciate is a

A
  • 5C isporene unit
34
Q

Two 5C isoprene units condense to form

A
  • 10C geranyl pyrophosphate
35
Q

A third 5c isoprene is added, forming

A
  • 15C unit

- Farnesyl pyrophosphate

36
Q

Finally, 2 farnesyl groups condense to form

A
  • 30C linear compound
  • Squalene
  • Reaction is catalyzed by squalene synthase
37
Q

Squalene is converted to

A
  • Cholesterol

- Squalene > Lanosterol > cholesterol

38
Q

Squalene is transformed by

A
  • Series of ring closure steps
  • Oxidized to the sterol (Lanosterol)
  • Which is converted to cholesterol in 19 steps
39
Q

End product inhibition of HMG CoA reductase by

A
  • Endogenously produced and by dietary cholesterol

- End product feeds back and inhibits this enzyme

40
Q

Proteasome degradation of the enzyme is stimulated by

A
  • Cholesterol

- (Rate of HMG CoA reductase degradation)

41
Q

Reversible covalent phosphorylation of HMG CoA reductase

A
  • Inhibition by phosphorylation by AMP-activated protein kinase
  • Favored by glucagon and epinephrine
  • Active in the dephosphorylated state
42
Q

Regulation of cholesterol biosynthesis (HMG-SCoA reductase) involves

A
  • Dietary cholesterol
  • Bile acids
  • Hormones
  • Drugs
43
Q

Regulation of cholesterol biosynthesis by HMG-SCoA reductase is inhibited/activated by

A
  • Inhibited by phosphorylation

- Activated by dephosphorylation

44
Q

Dietary cholesterol returning to the liver reduces the activity of

A
  • HMG-SCoA reductase enzyme
45
Q

HMG-SCoA reductase is a massive target for

A
  • Statins
46
Q

The statins

A
  • Resemble HMG-CoA

- Competitive inhibitors of HMG-CoA reductase

47
Q

In the hepatocyte, cholesterol is converted to

A
  • Bile acids
48
Q

Bile acids are polar derivatives that represent

A
  • The sole end product of cholesterol metabolism
49
Q

Two phases of the synthesis of bile acids

A
  • Hydroxylation

- Conjugation of cholesterol with amino acids

50
Q

Overall goal of bile acid synthesis

A
  • To increase the solubility of bile acids so they can be secreted out of the body
  • Done by adding hydroxyl groups and attaching amino acids (making bile acid conjugates)
51
Q

Bile acids are _____ water-soluble than cholesterol

A
  • More water-soluble

- Something becomes more polar, it becomes more water-soluble

52
Q

Attaching amino acids during bile acid synthesis makes

A
  • Bile acid conjugates

- Conjugates are much more water soluble than original biel acid

53
Q

Choelsterol exretion requires the formation of

A
  • Primary bile acids

- Ch

54
Q

Conjugation occurs in

A
  • The liver

- Prior to secretion into the biliary canaliculus

55
Q

Taurine conjugate

A
  • Taurocholic acid
56
Q

Glycine conjugate

A

-

57
Q

Bacterial metabolism

A
  • Process by which secondary bile acids are formed

- Formed from primary bile acids

58
Q

Essentially, bacterial metabolism is considered

A
  • Dehydroxylation
59
Q

Bile acids are the only significant

A
  • Excretion product of cholesterol

- Aid cholesterol solubility

60
Q

Bile acid sequestrants

A
  • Colestipol (Colestid)

- Cholestyramine (Questran)

61
Q

Bile acid sequestrants are used in the treatment of

A
  • Hypercholesterolemia
62
Q

Anionic resin-bile acid conjugates

A
  • Excreted in the feces

- Serum cholesterol levels are reduced since existing cholesterol is diverted into the replacement of these bile acids

63
Q

Bile acid sequestrants cause liver cells to express

A
  • More LDL receptor proteins

- Further reduces circulating LDL-cholesterol levels

64
Q

Generally, bile acid sequestrant drugs are extremely

A

-

65
Q

Substances secreted into the bile (enterohepatic circulation of bile acids) are first stored in

A
  • Gall bladder

- Then released into the intestine

66
Q

After being released into the intestine, the substances secreted into the bile are then

A
  • Reabsorbed into the intestine

- Then returned to the liver via the portal circulation

67
Q

Colestipol is a bile acid sequestrant effective in

A
  • Reducing circulating cholesterol levels
68
Q

Bile acid sequestrants have a complimentary charge that allows

A
  • Binding to bile acids

- Prevention of recycling of bile acid(avoiding return to the liver)

69
Q

With bile acid sequestrants, the liver directs cholesterol that is not recycled to

A
  • Bile acid formation

- Much better option for cardiovascular risk

Biochemistry I (34 decks)

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