14_Biochem Block 3.2 Biosynthesis Of Sterols

Front 1

How is cholesterol stored inside cells? What is the enzyme involved? (p240)

Back 1

It is esterified by acyl-CoA-cholesterol acyl transferase (ACAT).

Front 2

What form of cholesterol is found in membranes and what form is found in storage?

Back 2

The sterol with a hydroxyl group is found in membranes. The sterol with an ester group replacing the hydroxyl group is the stored form (as droplets inside cells)

Front 3

In what form is cholesterol typically found floating around in the serum?

Back 3

In the esterified form.

Front 4

To minimize the risk of cardiovascular events, what levels of LDL and HDL are idea? (p241)

Back 4

Low LDL and high HDL is best.
L=LDL
H=HDL

Front 5

Cholesterol on its own is not very soluble, however in the serum, concentrations are very high. How is this possible?

Back 5

Due to its association with HDL and LDLs.

Front 6

All 27 carbons of cholesterol are derived from what? (p242)

Back 6

Acetate. Specifically, Acetyl-CoA

Front 7

There are 37 steps in the biosynthesis of cholesterol however we're breaking it down into four stages. What are those stages?

Back 7

1. Acetate (3C) is converted to mevalonate (6C).
2. Mevalonate is converted to an activated isoprene (5C with double bond)
3. Activated isoprene is condensed to 30C molecule called Squalene.
4. Squalene is converted to cholesterol

Front 8

Although cholesterol can be synthesized in all cells, where is it most readily produced? Why? What are three others? Why? (p242, my written note in blue)

Back 8

In the liver due to bile synthesis. Other tissues include intestine, adrenal cortex, and reproductive tissues. These tissues manufacture a lot of cholesterol for hormone purposes.

Front 9

Stage 1: Formation of Mevalonate
1. Where does this rxn occur?
2. What three sources can Acetyl-CoA be derived? (p243 top)

Back 9

1. In cytosol
2. B-oxidation of FA, oxidation of ketogenic aa (e.g. leucine, isoleucine), pyruvate dehydrogenase rxn.

Front 10

1. What is the enzyme of the major regulatory point in cholesterol synth?
2. Where is it located?
3. What energy source does it use?
4. What rxn does it catalyze?

Back 10

1. HMG-CoA reductase
2. Integral protein of ER with active site exposed to cytosol.
3. Requires NADPH for E
4. Catalyzes hydrolysis of thioester bond, generating an alcohol group.

Front 11

There are two isoforms of HMG-CoA SYNTHASE - where are they located and what are they involved in? (p243)

Back 11

1. Cytosol isoform is involved in cholesterol synthesis.
2. Mitochondrial isoform is involved in ketone body synthesis.
**these rxns are identical - only in different locations**

Front 12

What is both the committed and rate limiting step in cholesterol synthesis?

Back 12

HMG-CoA REDUCTASE

Front 13

The first two steps in cholesterol synth are identical to:

Back 13

Ketone body synthesis in the mitochondria.

Front 14

2 Acetyl-CoA is condensed to ? by this enzyme:

Back 14

Acetoacetyl-CoA via thiolase

Front 15

Acetoacetyl-CoA combines with an Acetyl-CoA to form ? by this enzyme:

Back 15

beta-Hydroxy-beta-Methylglutaryl-CoA (HMG-CoA) via HMG-CoA synthase

Front 16

HMG-CoA is reduced to ? by this enzyme that requires these energy co-factors:

Back 16

Mevalonate via HMG-CoA reductase; requires 2 NADPH molecules.

Front 17

Mevalonate is converted to ? via this enzyme, requiring what energy?

Back 17

5-Phosphomevalonate via Mevalonate-5-phosphotransferase requiring 1 ATP

Front 18

5-Phosphomevalonate is converted to ? via this enzyme that requires this energy unit?

Back 18

5-pyrophosphomevalonate via Phosphomelavonate Kinase, requiring 1 ATP

Front 19

5-pyrophosphomevalonate is converted to this ? intermediate via this enzyme - that also decarboxylates the intermediate for form what?

Back 19

5-pyrophosphomevalonate is phosphorylated to 3-Phospho-5-Pyrophosphomevalonate [intermediate] via pyrophosphomevalonate decarboxylase - this is decarboxylated (giving off CO2) by the same enzyme to produce activated isoprenes: delta3-Isopentenyl pyrophosphate and Dimethylallyl pyrophsosphate.

Front 20

The only difference between delta 3-isopentenyl pyrophosphate and Dimethylallyl pyrophosphate is:

Back 20

The position of the double bond.

Front 21

Two activated isoprenes are condensed head to tail via this enzyme to make a 10C molecule called? Note that once this happens, another activated isoprene is added using the same enzyme to produce a 15-carbon molecule called?

Back 21

Prenyl Transferase makes a 10C compound called Geranyl Pyrophosphate. The same enzyme adds another activated isoprene to make a 15C compound called Farnesyl Pyrophosphate.

Front 22

Two molecules of Farnesyl Pyrophosphate are combined via this enzyme that requires this energy subunit to produce a 30 Carbon molecule called what?

Back 22

They combine via squalene synthase, requiring NADPH, and produce a molecule called Squalene.

Front 23

What does squalene monooxygenase do? (p246)

Back 23

It converts squalene into an epoxide.

Front 24

Once squalene is made, it is ready to be converted to a four-ring steroid nucleus. 1st Squalene must be converted to a ? with this enzyme that requires this energy unit: (p246)

Back 24

Squalene is converted to an epoxide - Squalene 2,3-epoxide - via Squalene Monoxygenase requiring 1 NADPH and OXYGEN

Front 25

Once the epoxide, Squalene 2,3-epoxide is made, it is cyclized to ? via this enzyme?

Back 25

Converted to Oxidosqualene via Oxidosqualene Cyclase to form Lanosterol.

Front 26

Once Lanosterol is produced, there is a multistep process to finally make cholesterol. What are the three major events during this multistep process?

Back 26

1. Dimethylation
2. Double bond reduction
3. Double bond movement.

Front 27

It was confusing at first how cholesterol pathway disorders could affect development until researchers realized:

Back 27

that cholesterol can be attached to proteins and affect their activity. Example include hedgehog family of transcription factors.

Front 28

What are SREBPs and how do they function?

Back 28

They are Sterol Regulatory Element-Binding Proteins. When cholesterol is high, SREBPs are inactive and retained in the ER. When cholesterol levels decline, SREBP undergoes proteolytic cleavage, releasing a region of the protein that enters the nucleus where it acts as a transcription factor by binding to sterol reg elements (SRE) in the DNA to activate transcription of sterol-regulated genes.

Front 29

See p247 for pathway that results in SREBP cleavage to make TF that turns on sterol synthesis.

Back 29

See p247 for pathway that results in SREBP cleavage to make TF that turns on sterol synthesis.

Front 30

How is HMG-CoA reductase regulated in terms of its phosphorylation?

Back 30

The active form is dephosphorylated (insulin) and the inactive form is phosphorylated (glucagon)

Front 31

After a meal, is HMG-CoA reductase turned on or off?

Back 31

After a meal, HMG-CoA is dephosphorylated by insulin and therefore activ.

Front 32

What are the three major ways in which HMG-CoA reductase can be regulated?

Back 32

1. Transcriptional
2. Phosphorylation
3. Proteolysis/degradation.

Front 33

How do statins work?

Back 33

They act as competitive inhibitors of HMG-CoA reductase.

Front 34

What was the pleiotropic action of statin example she used in class?

Back 34

Where blocking HMG-CoA reductase, you also block modification of geranylgeranyl that increases the expression of eNOS - and therefor improve endothelial function.

Front 35

Bile acids are derivatives of ?, a #-carbon molecule with # hydroxyl groups.

Back 35

Derivative of cholic acid, a 24-carbon molecule with 3 hydroxyl groups.

Front 36

What three important things does bile acids do?

Back 36

1) They act as emulsifying agents to prepare dietary triacyglycerols for hydrolysis by pancreatic lipase. 2) They facilitate the absorption of fat-soluble vitamins, esp Vit D. 3) Since cholesterol is not oxidized to CO2 and H2O, it is excreted as free cholesterol and as bile acids.

Front 37

Name these structures. What is the one on the right useful for and how is it different than the one on the left.?

Back 37

Cholesterol and Cholic Acid. It is useful form of cholesterol that can be excreted. It has additional OH groups, fewer Carbons, and is more water soluble.