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Metabolism > Cholesterol > Flashcards

Cholesterol Flashcards

1
Q

what is the starting point for cholesterol synthesis (source of all the carbons)

A

acetyl CoA

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2
Q

where does most cholestrol come from

A

it is made in vivo

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3
Q

why is cholestrol so hard to regulate

What might be the best way to regulate it

A

it is mostly made in vivo, with little contributions from the diet.

best way to regulate might be to eat less = less acetyl CoA

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4
Q

what is a main synthetic product of cholesterol

A

steroid hormones

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5
Q

how is most cholesterol excreted

A

through bile acids via the intestines

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6
Q

cholesteryl esters are used to what

A

store and transport cholesterol

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7
Q

what is the activated precursor of cholesterol

A

isopentenyl pyrophosphate

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8
Q

what is the main regulatory step of cholesterol synthesis

A

HMG CoA reductase rxn

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9
Q

what generates acetyl CoA

A

food is converted to Acetyl CoA which can be made into choelesterol

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10
Q

how many carbons are in cholesterol? where do they come from

A

27, all from acetyl CoA

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11
Q

how many six-membered rings and how many five membered rings are in cholesterol?

A

3 six membered rings and 1 five membered rings

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12
Q

how many carbons does the hydrocarbon side chain have? what carbon is it attached to?

A

8 carbons: attached to carbon 17

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13
Q

what do red dots in diagram indicate?

A

red dots in diagram show where COO- from acetate go to

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14
Q

what is removed to make steroid hormones

A

hydrocarbon sidechain

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15
Q

what is cholesterol made from

A

isoprenes; 5 carbon units

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16
Q

how many hydroxyls are in cholesterol and at what position? what is the resulting property of the molecule

A

only one at carbon 3: (3-hydroxy)

it is very hydrophobic

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17
Q

where is the only double bond

A

between C5 and C6: called delta-5

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18
Q

how many double bonds do isoprenes have

A

2; one at each end

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19
Q

cholesterol can be used to make what vitamin

A

vitamin D

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20
Q

isopentenyl pyrophosphate has many products it can form

A

vitamin A, vitamin E, vitamin K, rubber, carotenoids, plant hormones, abscisic acid and gibberellic acid, rubber, phytol chain or chlorophyll, dolichols, quinone electron carriers: ubiquinone, plastoquinone, isoprene

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21
Q

On the first diagram what products can become acetyl CoA

A

-Starch glycogen and sucrose can all converge to become glucose
– Alanine glucose and serine can all converge to become pyruvate
-Pyruvate can become a seal coat way

-Phospholipids and triacylglycerol’s can be broken down to fatty acids which can then be converted to a acetyl CoA

Phenylalanine and isoleucine are two amino acids that can be converted to acetyl CoA

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22
Q

Starting from acetyl CoA, name what molecules it must be converted to an sequential order to become cholesterol

A

Acetyl CoA is converted to acetoacetyl-CoA which is then converted to mevalonate and that is converted to isopentenyl pyrophosphate which is converted to cholesterol

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23
Q

According to the diagram what are the possible fates for cholesterol

A

It may become steroid hormone which is the main synthetic product

-It may be converted to bile acids which is the main way in which it is excreted.

– It may become cholesteryl esters which is a form that is used for storage and transport

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24
Q

What is the main regulatory step of Cholesterol synthesis

A

The main regulatory step is the reduction catalyzed by HMG CoA way reductase from acetoacetyl-CoA to mevalonate

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25
Q

Describe the structure of isoprene

A
  • Four carbon chain with a fifth carbon or methyl group sticking out of the second carbon in the chain
  • there is a double bond on either end of the molecule and there are no other Atoms other than carbon and hydrogens
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26
Q

What is another name for isopentenyl pyrophosphate?

A

Activated isoprene

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27
Q

How many carbons does squalene have?

How many are removed when it is converted to cholesterol

A

30 Carbons.

Three carbons are removed when squalene is converted to cholesterol

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28
Q

What is the first reaction of cholesterol synthesis?

A

Two molecules of acetyl CoA are reacted to make acetoacetyl-CoA.

– Enzyme: thiolase

– CoA – SH is released

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29
Q

In cholesterol synthesis what is acetoacetyl-CoA reacted with? what product is made?

What enzyme is used?

What is released?

A

Acetoacetyl-CoA is reacted with acetyl CoA to make beta – hydroxy – Beta – methylglutaryl CoA. (HMG – CoA)

Enzyme: HMG – CoA synthase

Released: CoA – SH

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30
Q

How many acetyl – Coa form acetoacetyl-CoA

A

2

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31
Q

What is a third Acetyl CoA reacted with to make HMG – CoA

A

Acetylacetyl CoA via the enzyme HMG-CoA synthase

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32
Q

What is HMG – CoA reacted with to make what product?

What enzyme is used?

What is released?

A

It is reacted with two NADPH molecules and two protons to make Mevalonate

Enzyme: HMG – CoA reductase

Released: NADP+ and CoA– SH

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33
Q

What type of reaction happens in the rate limiting step of cholesterol synthesis

A

The carboxy carbon is reduced: electrons from two NADPH molecules are added and CoA-SH is released

34
Q

What is production of mevalonate inhibited by?

What kind of inhibition occurs?

What differences are there between the Statin drugs?

A

It is competitively inhibited by Statins

Statins have different our groups in two different places labeled R1 and R2

35
Q

Name the four Statin drugs

A

Compactin

Simvastatin (zocor)

Pravastatin (Pravachol)

Lovastatin (Mevavor)

36
Q

How is mevalonate converted to Delta three – Isopentenyl pyrophosphate?

What enzyme is used?

A

-Mevalonate is phosphorylated sequentially on C5 to form pyrophosphate

– Then phosphorylation at C3 occurs and is followed by immediate decarboxylation at C-1 and subsequent removal of the phosphate on C3 follows that.

37
Q

What are the two activated isoprenes?

A

Delta three – isopentenyl pyrophosphate and dimethylallyl pyrophosphate

38
Q

How many carbons do each of the precursors of cholesterol have?

A

Acetyl – CoA has two carbons and 1 Carbonyl carbon

Acetoacetyl – CoA has four carbons and 2 carbonyl carbon’s

H MG – CoA has a five carbon chain with a six carbon on the third carbon in the chain; it also has a hydroxyl group on the third carbon; Hey carboxyl group on one end and a thio carboxyl group on the other (with CoA )

Mevalonate has a five carbon chain with a methyl group on the third carbon, hydroxyl group on the third carbon, and another hydroxyl group on the fifth carbon; there is no coA in this molecule

IsoPentenyl pyrophosphate has a four carbon chain with a methyl group on the second carbon and a double bond between C1 and C2; the diphosphate group is attached to the fourth Carbon.

Dimethylallyl pyrophosphate has a four carbon chain with a methyl group on the C2 and a double bond between C2 and C3; the pyrophosphate is attached to the fourth carbon

39
Q

How is squalene made from activated isoprenes?

How many steps?

A

There are three steps

  1. Geranyl pyrophosphate is made by fusing two activated isoprenes
  2. A third isoprene is added to Geranyl pyrophosphate to make Farnesyl pyrophosphate
  3. Two Farnesyl pyrophosphates fuse to form squalene Using NADPH
40
Q

What is squalene reacted with to make its product?

What is this product?

What enzyme is used?

A

Squalene is reacted with NADPH and a proton as well as with O2 to make squalene – 2, 3 – epoxid

Enzyme: squalene monooxygenase

Released: H2O and NADP+

41
Q

In animals, what is squalene– 2, 3 – epoxide made into?

What enzyme is use?

A

It is made into Lanosterol

Enzyme: cyclase

42
Q

What three outcomes are possible from squalene – 2, 3 – epoxide?

A

-in plants it can be converted to stigmasterol the a multistep reaction

– And fungi it can be converted to ergosterol

-in animals it is converted to cholesterol eventually by way of Lanosterol

43
Q

What enzyme is used to make cholesrol into Cholesteryl ester within the cell

What is cholesterol reacted with?

What is released?

A

Acyl-CoA-Cholesterol Acyl Transferase (ACAT)

it is reacted with FA-CoA

released: CoA-SH

44
Q

What is cholesterol reacted with to make cholesteryl ester in HDL?

what enzyme is used?

what is the other product?

A

it is reacted with Phosphatidylcholine (lecithin) to make cholesteryl ester and Lysolecithin

enzyme: Lecithin-Cholesterol Acyl Transferase (LCAT)

45
Q

How much cholesterol is made de Novo per day

A

1 g

46
Q

How much cholesterol do we consume per day?

A

0.3 g

47
Q

What are the plasma levels of cholesterol maintained mainly by de novo synthesis

A

150 to 200 mg/dL

48
Q

What are the four main mechanisms of cholesterol regulation

A
  1. HMG – CoA reductase:
    – Main biosynthetic control
    – Phosphorylation decreases activity
    – Level of enzyme is also regulated
  2. -Acyl-CoA :cholesterol acyltransferase, ACAT
  3. LDL/HDL ratios
  4. Excretion of bile acids
49
Q

What are the regulators of HMG – coenzyme a reductase?

A

Activators: insulin

Inhibitors:

  • glucagon
  • intracellular cholesterol (Stimulates proteolysis of HMG – CoA reductase )
50
Q

Name Regulators for ACAT

A

Activator: Intracellular cholesterol

51
Q

What is ACAT

A

Acyl- CoA cholesterol acyltransferase

52
Q

What regulates receptor mediated endocytosis of cholesterol (LDL Cholesterol)

A

Intracellular cholesterol via proteolysis of HMG – CoA reductase

53
Q

How is LDL-cholesterol Taken up by the cell?

A

By receptor mediated Endocytosis

54
Q

What path does Cholesterol take in the body?

A

Deliver releases cholesterol through bio acids in the intestine and as LDL through the blood stream.

– Cholymicrons enter capillaries after being absorbed.
– Lipoprotein lipase breaks up cholymicrons and VLDL to Produce free fatty acids (This happens in capillaries of mammary, muscle, or adipose tissue)

  • VLDL remnants are sent to the liver or are converted to LDL to be sent either to the liver or extrahepatic tissue.
  • Cholymicron remnants are sent to the liver
  • extrahepatic tissue makes HDL from precursors coming from the liver and intestine. Produced HDL’s are sent to the liver.
55
Q

Why does Cholesterol need to be associated with proteins in order to be transported?

A

Because it is hydrophobic

56
Q

What differentiates different lipoprotein complexes

A

Their unique sets of Apoproteins and different ratios of lipid to protein resulting in particles of varying density

57
Q

What is important about each form of lipoprotein complex?

A

Each plays a particular role in cholesterol transport and control

58
Q

Which lipoprotein carries the most cholesterol by percent wt.

A

LDL’s carry 8 percent free cholesterol and 37% cholesteryl esters

VLDL’s have 7% free cholesterol and 12% cholesteryl esters

HDL have to percent free cholesterol and 15% cholesteryl esters

Chylomicrons have 1% free cholesterol and 3% cholesteryl esters

59
Q

What percent of lipoproteins is made out of protein

A

HDL has the highest percentage with 55%

LDL has the second highest with 23%

VLDL has 10%

Chylomicrons have 2%

60
Q

What percent of triacylglycerol it’s do lipoproteins have

A

Chylomicrons have 85% triacylglycerol’s

VLDL has 50% chylomicron

LDL has 10% chylomicron

HDL has 4% chylomicrons

61
Q

What are the important Apolipoproteins

A
  • ApoB 100: has a molecular weight of 513,000 and associates with BL DL, LDL and binds to LDL receptor.
  • ApoA-1 associate with HDL and date activate LCA T; interacts with ABC transporter

MW: 28,331

ApoA 2 : associates with HDL MW 17,380, respectively

62
Q

What Apoproteins bind associate with HDL?

A

ApoD and ApoA 1 and 2 Are exclusively associated with HDL

ApoA 3 associates with HDL but is also in chylomicrons

ApoC 2 and 3 Associated with HDL but also with chylomicrons and VLDL

ApoC 1 Associates with HDL and VLDL

ApoE associates with HDL, VL DL, chylomicron

63
Q

Described LDL particle receptor mediated endocytosis

A

LDL particle with cholesteryl esters inside associates with an LDL receptor through a POB – 100

LDL particles Is taken in, receptor molecules disassociate and the lysosome degrades the endosome with the LDL particle inside

64
Q

What Apoproteins are recognized by the LDL receptor?

A

LDL particles are primarily taking in through the LDL receptor which recognizes protein apoprotein B apo protein E

65
Q

what 2 bile acids are primarily synthesized int the liver

A

chenodeoxycholic acid & cholic acid

66
Q

how much of bile acids are conjugated?

via what?

A

Most bile acids (98%) are conjugated via an amide bond to either glycine or taurine

67
Q

Are only conjugated Bile acids secreted into the gall bladder and intestine?

A

Both free and conjugated bile acids are secreted into the gall bladder and intestine

68
Q

what happens to Biles acids after they are excreted?

A

Conjugated bile acids are converted into the free form in the intestine

69
Q

what do bacteria do to bile acids?

name some of the products

A

Bacteria convert the primary bile acids to dozens of secondary bile acids including

7-deoxycholate and lithocholate

70
Q

What do bile acids do in the intestine?

in what form do they do this?

how much is reabsorbed?

A

In the gut primary and secondary bile acids emulsify fat, 95% are reabsorbed and return to the liver.

71
Q

What happens to Bile acids in the liver?

A

In the liver, the primary and secondary bile acids are reconjugated to glycine or taurine and resecreted.

72
Q

what percentage of bile acids are excreted?

what percentage of all excreted cholesterol does this account for?

10% is excreted as what?

A

5% of primary and secondary bile acids are excreted.

  • Main excretory pathway for cholesterol (90%)
  • 10% is excreted as steroid hormones and their derivatives
73
Q

What happens to bile acid in each compartment?

A

Cholesterol is converted to primary BA CoA

Primary bile acid CoA is converted to either bile acid or conjugated bile acid

It is excreted into the gallbladder and remains in that form

When it enters the intestine conjugated bile acid , it becomes bile acid
-then it is reacted to become bowel acid lipid complexes

  • 5% is excreted and then 95% return to the liver
74
Q

What is the first reaction in bile acid synthesis?

A

Cholesterol is reacted with NADP H and one proton as well as Oxygen to make 7-hydroxy cholesterol

Enzyme: 7 alpha – hydroxylase

75
Q

What is 7 – hydroxy cholesterol converted to?

A

It can either be converted to Cholyl– CoA or to chenodeoxycholyl-CoA

There are several steps. (Don’t worry what they are)

Both reactions use NADPH and a proton as well as oxygen and two molecules of CoA -SH are used to make Cholyl – CoA

released: propionoyl-CoA

76
Q

What are the differences between the conjugated bile acids

What are the names of the bile acids?

A

Glycocholic acid and taurocholic acid

  • NH-CH2-COOH
  • NH-CH2-CH2-SO3H
77
Q

Why are bile acids toxic

A

Because they are acidic

78
Q

How much control in bile acid synthesis

A

Bile acid synthesis is under excruciating control

-7-hydroxycjolesterol is major control point

79
Q

What are androgens and glucocorticoids made from

What else are cholesterol derivatives involved in?

A

Secondary cholesterol

Cholesterol derivatives are used in the messenger system

80
Q

what are the major control points of bile acid synthesis

A

LXR-alpha (Liver X Receptor) :

  • activator: Cholesterol
  • inhibitor: 7-Hydroxylase by
  1. increase BA production which activates LCA (Lithocholic acid), JNK (Jun N-terminal kinase) and FXR (Farnesoid X Receptor) activity
  2. LCA (Lithocholic acid) increases PXR (Pregnane X Receptor) activity which inhibits 7-hydroxylase
  3. JNK (Jun N-terminal kinase) inhibits 7-hydroxylase
    • FXR (Farnesoid X Receptor) activate SHP which inhibits LXR-alpha (Liver X Receptor) which is activator of 7- hydroxylase

LRH: Liver Receptor Homolog

81
Q

What in the intestines promotes bile acid excretion?

A

Some foods bind to bile acids

Oatmeal (soluble fiber) lowers cholesterol

Chia seeds also reduce cholestol by increasing bile acid secretion

82
Q

Give the bile acid control regulators name and abreviations

A 
FXR:	Farnesoid X Receptor
LXR:	Liver X Receptor
PXR:	Pregnane X Receptor
JNK:	Jun N-terminal kinase
LRH:	Liver Receptor Homolog
SHP (name not on slide)

LCA: (Lithocholic acid), in intestines
BA in intestines

Metabolism (8 decks)

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