Biochemistry-Cholesterol

Question 1

What cellular membrane has the smallest amount of cholesterol?

Answer 1

ER. This is the site of cholesterol synthesis control (SREBP)

Question 2

Structure of cholesterol

Answer 2

4 fused rings form the rigid backbone, large hydrophobic carbon chain, OH polar face…makes it amphipathic.

Question 3

What is cholesterol’s role in the membrane?

Answer 3

It interferes with lipid packing and keeps the membrane fluid.

Question 4

What determines the solubility of cholesterol in the bile?

Answer 4

Phosphatidylcholine and bile acids solubilize cholesterol in the bile.

Question 5

What are many cholesterol synthesis intermediates used for?

Answer 5

Tethering proteins to membranes. For example prenyl groups stick G proteins on the membranes.

Question 6

What intermediates are the building blocks of cholesterol synthesis? What else are they used for?

Answer 6

Isopentenes. These are also used for synthesis of Vitamin A, CoQ, Dolichol and Heme A.

Question 7

What contributes to a healthy serum cholesterol?

Answer 7

*

Question 8

Where does cholesterol synthesis first begin?

Answer 8

Cholesterol, phosphatidylcholine and bile salts are synthesized in the liver and stored in the gallbladder.

Question 9

What is the role of cholesterol in helping digest a meal you just ate?

Answer 9

Neurohormonal signaling causes bile secretion into the duodenum, which contains cholesterol and cholesterol esters. These help to emulsify fat so pancreatic enzymes can digest the fat. Cholesteryl esterase is also release by the pancreas.

Question 10

What does the intestine do with the cholesterol secreted in the bile?

Answer 10

ABC pumps bring it back into the enterocyte, they also secrete a small amount into the feces. In the enterocyte, the cholesterol either remains in its free form, or is esterified by ACAT. It is then packaged with ApoB-48 in the enterocyte and secreted as a chylomicron headed back to the liver.

Question 11

How can you treat high cholesterol by decreasing reabsorption?

Answer 11

Inhibition of the ATP binding site on the Niemann-Pick C1-like-1 protein (Ezetimibe)

Question 12

Deficiency in what enterocyte cholesterol pump can result in sitosterolemia and coronary heart disease?

Answer 12

ABC-G5 and G8 proteins, these are responsible for secretion of cholesterol and plant sterols

Question 13

Where does cholesterol synthesis take place and where do the carbons come from?

Answer 13

Takes place in the cytosol near the RER. All carbons come from acetyl CoA.

Question 14

What drives the reactions in cholesterol synthesis?

Answer 14

ATP and Acetyl CoA hydrolysis

Question 15

Where do the reducing agents used in cholesterol synthesis come from?

Answer 15

Oxidative decarboxylation of Gluc-6-P

Question 16

What is the committed rate limiting step in cholesterol synthesis?

Answer 16

HMG-CoA -> Mavalonate by HMG-CoA reductase + NADPH.

Question 17

1st step in cholesterol synthesis

Answer 17

3 Acetyl CoA molecules combine to form HMG-CoA. 1) Thiolase adds 2 Acetyl CoAs together to form acetoacetyl CoA. 2) HMG-CoA synthase + another acetyl CoA form HMG CoA

Question 18

2nd step in cholesterol synthesis

Answer 18

HMG-CoA is reduced to Mevalonate by HMG-CoA reductase + 2 NADPH

Question 19

What enzyme is targeted by statins?

Answer 19

Their beta-hydroxy acid competitively binds HMG-CoA reductase and prevents HMG-CoA binding to the enzyme. Note that these drugs can be overcome by higher concentrations of HMG-CoA. Ultimately this results in a decrease in cholesterol synthesis, which causes increased expression of LDLR, which takes more cholesterol out of the blood.

Question 20

3rd step in cholesterol synthesis

Answer 20

3 phosphates are added to 6 carbon mevalonate by mevalonate kinase, it is then decarboxylated to form a 5 carbon isoprene

Question 21

4th step in cholesterol synthesis

Answer 21

3 five-carbon isoprenes combine to form 15 carbon farnesyl pyrophosphate. Those then combine to form 30 carbon units.

Question 22

5th step in cholesterol synthesis

Answer 22

30 carbon unit loses 3 carbons.

Question 23

How is HMG-CoA reductase regulated at the level of transcription?

Answer 23

SREBP is released from ER membrane by its chaperone SCAP -> delivered to golgi -> SREBP’s DNA-binding domain is proteolytically cleaved and released to nucleus -> DNA binding domain activates SRE gene in nucleus -> Increased synthesis of HMG-CoA reductase and increased expression of LDLR -> Cholesterol levels increase -> Negative feedback by sterols inhibits SREBP release by SCAP

Question 24

How is HMG-CoA reductase regulated allosterically?

Answer 24

HMG CoA reductase is associated with the ER membrane. High levels of sterols promote proteolytic degradation of HMG-CoA reductase.

Question 25

How is HMG-CoA reductase regulated by phosphorylation?

Answer 25

When it is not phosphorylated it is active. When its serine is phosphorylated it is inactive. Insulin works to activate phosphatases and turn on cholesterol synthesis. Glucagon works to activate AMP-activated protein kinases that phosphorylate HMG-CoA reductase and shut off cholesterol synthesis.

Question 26

What is the major mechanism by which cholesterol is cleared from the blood?

Answer 26

LDLR

Question 27

How do bile salt sequestrants help to lower cholesterol levels?

Answer 27

Sequestrants interfere with reabsorption of bile salts, which contain cholesterol. The liver up regulates bile salt product and thus uses more cholesterol which will now be excreted as bile. Finally, since cholesterol levels are declining, LDLR expression increases and more cholesterol is taken out of the blood.

Question 28

2 exocrine functions of the liver

Answer 28

VLDL to carry triglycerides and bile.

Question 29

Enterohepatic circulation

Answer 29

Bile salts produced in liver -\> Stored in gallbladder -\> Released after a meal to emulsify fats via ABC -\> 95% of salts are reabsorbed in the terminal ileum -\> Other 5% allows for excretion of cholesterol

Question 30

How do bile salts emulsify fat?

Answer 30

They form micelles and break the fat into small pieces.

Question 31

How are bile acids formed?

Answer 31

In the liver, cholesterol -\> 7alpha-hydroxycholesterol (RDS) -\> Cleaved -\> Bile acid thioesterified w/CoA -\> Thioester conjugated w/taurine or glycine -\> Bile salts

Question 32

What is the purpose of conjugating thioesters with taurine or glycine when synthesizing bile salts?

Answer 32

The conjugates have a lower pKa than the thioesters, which makes them better detergents because they are more polar in the intestine, considering that the intestinal pH is around 6 and the conjugates are 2 and 4.

Question 33

What are the negative feedback functions in bile acid synthesis?

Answer 33

Chenocholic acid and cholic acid feedback to inhibit 7a-hydroxylase

Question 34

How does our normal flora help keep our cholesterol levels in check?

Answer 34

There are normal flora that deconjugate and dehydroxylate bile salts to secondary bile salts. By doing this, bile salts are less soluble and do not return to the liver. This is why we excrete 5% of our bile salts.

Question 35

How are bile salts taken back up in the terminal ileum?

Answer 35

Primary bile salts are returned by Na cotransport. Secondary bile salts are reabsorbed passively.