Mne Lecture 13 - Cholesterol Synthesis

Front 1

What does cholesterol look like?

Back 1

Has 4 rings: 
3 - 6 membered rings
1 – 5 membered ring

REMEMBER: 

3 position: -OH group
*Cholesterol is hydrophobic. Soluble in membranes. Only one oxygen. Way you deal with it in body depends on hydrophobicity.

Double bond between 5 and 6: delta 5 double bond

Side chain at C 17

Has 27 carbons
--> All of these carbons come from acetyl coA. The red dots are the carboxy carbons from acetyl coA. 

Cholesterol is precursor of steroids

Front 2

Where does cholesterol come from?

Back 2

Center have Acetyl-coA
For this lecture, all he’s really concerned about is that it is essential for cholesterol biosynthesis.
Everything comes from acetyl-coA.
There is no other way into cholesterol.
If you want to make cholesterol, it’s very easy to make acetyl-coA.
All of these pathways feed through to acetyl Co-A, one of those things being cholesterol. 

**one thing he can ask: where does cholesterol come from? Acetyl-coA

Front 3

How are isoprenes made?
What is thought to be derived from isoprenes?

Back 3

Making Isoprenes
How?
-so the first step is phosphorylation to mevalonic acid then we get another phosphate added, pyrophosphate formed.
-Then another phosphate is added to the hydroxy group on mevalonic acid which brings about instability.
-Once that phosphate is added the compound is unstable so the carboxy group is released and we get isopentenyl pyrophosphate and dimethylallyl pryophosphate. 
*27 carbons in cholesterol, 2 in acetate. Have to lose a carbon somewhere (otherwise would have 26 or 28)


Isoprenes make up many compounds:
Vitamin A 
Cholesterol
Steroid hormones 
Bile Acids
Vitamin D
other compounds on right

Front 4

What enzymatic reaction do statins inhibit and why do they inhibit?

Back 4

1. 3 acetyl co-a’s --> HMG coA
2. NOW to mevalonic acid via HMG-CoA reductase!
*This is the Main controlling step to cholesterol biosynthesis and the main step that is easily controlled by drugs.

Statins are competitive inhibitors of HMG-CoA reductase 

-competitive inhibitor does not need to look like substrate
-note structure of inhibitor (statin) and melavonic acid (product) and how they're similar
-look at R groups, most of these statins differ by their R groups, each company has their own specific form of statins that are simply modifications of the same product.

Front 5

Cholesterol is Stored and Transported as

Back 5

ESTERS

LCAT is involved in the TRANSPORT (outside cells -- in HDL)
ACAT is involved in STORAGE (inside cells).

Now we have cholesterol and want to use it in cell membranes or make steroids from it
If you don’t have cholesterol you diemake it and transport it to tissues that need it
Have to transport it through the blood stream, but you can’t simply pump it into the blood stream. It’s not water soluble.
To transport it, have to make it hydrophilic, put it in a form that can be transported. 
So what we’re going to be doing is to modify cholesterol. Often going to be forming an ester.
This is cholesterol ester here. Often this is going to be part of another complex, this isn’t what is really solubilizing cholesterol.
These esters are also used to store cholesterol inside of cells. 
There are two ways of making this cholesterol. The end product is exactly the same, the enzyme is different.
Two dif pw, two dif fxns.. Different parts of the cell different parts of the body.

Within cells, take cholesterol and add fatty acyl coA to get the cholesterol ester. This is the STORAGE FORM. The enzyme that does this is ACAT acyl-coA cholesterol acyl transferase *** KNOW IT : Know what it is, where it’s located, what it does. 

In the circulation, in these molecules that are transporting cholesterol, specifically HDL, there is another enzyme which does exactly the same thing, makes a cholesterol ester. BUT this one take cholesterol and takes a fatty acid but it takes the fatty acid from phosphatidylcholine 
So the substrate of this enzyme is different but the end product is the same. It’s in HDL, HDL is the transport form of cholesterol.

Front 6

T/F: If you eat a big meal with no cholesterol in it you won't see an increase of cholesterol in your body.

Back 6

FALSE.

Have HMG co-A reductase going from b-hydroxy-b-methyl-glutaryl-CoA to mevalonic acid. Going to be making cholesterol. Feedback mechanism: cholesterol feeds back on HMG co-A reductase. This is a long-term effect, brings about proteolysis of HMG-coA-reductase. Often when we think of product level feedback as simply turning down the enzyme. This actually destroys the enzyme. Insulin INCREASES the activity, GLUCAGON decreases. Think: Big breakfast, had NO cholesterol. This tells you that you’ve made a mistake. That big meal contains no cholesterol, BUT you’re going to release insulin, which INCREASES CHOLESTEROL LEVELS. Continually eating, giving yourself big insulin boosts, actually increase cholesterol levels.

Front 7

How is cholesterol transported around the body?

Back 7

Take in big fatty meal containing cholesterol
take this fatty food and solublize in intestine and get it through these two sets of cell membranes into blood supply
these things come through as chylomicrons (1st fat and protein complexes that are coming into blood stream) go through capillaries and acted upon by lipoprotein lipases.
Have proteins, fats, lipoprotein lipases break down fats.
They don’t digest these particles completely, so you have these smaller particles which are called chylomicron remnants that go to the liver and get reprocessed.
Liver reprocesses all the things coming in .. What liver puts out is VLDL (very low density lipoprotein).

HDL: more protein less fat 
LDL: fats tend to float, low density, higher fat content lower protein level VDL comes back from the capillaries, brings out LDL.
Thought of as bad cholesterol because it is not destined back to liver but ALL tissues. Receptor on it (APOB-100 ** REMEMBER THIS ** directs LDL and VLDL to tissues) so that it’s targeted directly to tissues. Have cholesterol in these tissues outside the liver. If we have too much, need to recirculate back to liver. These tissues, the cells themselves, can release LDL and that goes back to liver. That’s why HDL is considered good (away from tissues and back to liver rather than to tissues).

Front 8

General comparison of composition of chylomicrons, VLDL, HDL, LDL

Back 8

-Cholesterol is hydrophobic and requires protein association for solubility

-A number of different lipoprotein complexes are found in the circulation and are characterized by unique sets of apoproteins and different ratios of lipid to protein resulting in particles of varying density
**apolipoproteins signal where the complex goes to!
**remember that APO B 100 targets LDL and VLDLs to LDL receptor on cells. This is how these particles are targeted to cells. HDL does NOT have this protein!

-Each form of lipoprotein complex plays a particular role in cholesterol transport and control

__

ratio is different
high density lipoprotein.. If you go through here find that it has the most protein. High density, most protein
Chylomicrons have lowest amount of protein, lowest density, at least as low as VLDL. 
*not just the amount of protein, also the kinds of protein present.

Front 9

Uptake into cells is mediated by?

Back 9

LDL and VLDL uptake into cells is primarily via LDL-receptor (recognizes protein - apoB/apoE)

**APO B 100 !!!!

Front 10

2 Primary Bile acids?
2 Secondary Bile acids?

Where are each made?

Back 10

2 primary bile acids made in gall bladder (7 a-hydroxylase) and secreted into intestine (*most are conjugated to either glycine or taurine, but some are not going to be conjugated):
-chenodeoxycholic acid
-cholic acid

2 secondary bile acids converted from 1' bile acids by bacteria in the intestine:
-7-deoxycholate
-lithocholate

Both 1' and 2' are in the intestine, both conjugated and not, and they form these lipid complexes and all go back to the liver. In the liver, they are put back into gall bladder and stored there to be used again.

**95% of what goes into intestine is brought back into body --> emulsifying fat, so expect it to be taken back up again
**5% excreted --> 90% of excreted cholesterol is done via this process. Other 10% via steroids (men --> androgens).

Front 11

7-a-hydroxylase

Why is it important? How is it controlled?

Back 11

Cholesterol --> 7-hydroxycholesterol --> bile acids

7-hydroxycholestrol : enzyme that makes it is 7-a-hydroxylase = BILE ACID SYNTHESIS

CONTROLLING STEP OF BILE ACID SYNTHESIS (besides availability of cholesterol)


How is this reaction controlled?
*in the liver
Important points of regulation: 
1. Bile acids inhibit AND 2’ bile acids inhibit.
2. They do so by multiple mechanisms. That shows the importance of this reaction. 
3. Acts through two different hormone systems, nuclear receptors and cell membrane receptors (2nd membrane systems)

Front 12

How much cholesterol do we eat / day? How does this compare to how much we make / day?

Back 12

De novo synthesis is 1 g/day

most people only take in 0.3 g/day

Plasma levels of 150 -200 mg/dl are maintained mainly by de novo synthesis

very hard to regulate cholesterol levels by diet alone
*need another way of controlling

Front 13

Four Main Mechanisms of Cholesterol Regulation

Back 13

HOW DO WE CONTROL CHOLESTEROL BIOSYNTHESIS

1. HMG coA reductase 
--> statins inhibit main control step of cholesterol biosynthesis
-->other than drugs?
Feedback from cholesterol (causes phosphorylation) 
-->Have HMG co-A reductase going from b-hydroxy-b-methyl-glutaryl-CoA to mevalonic acid. Going to be making cholesterol. Feedback mechanism: cholesterol feeds back on HMG co-A reductase. This is a long-term effect, brings about proteolysis of HMG-coA-reductase. Often when we think of product level feedback as simply turning down the enzyme. This actually destroys the enzyme. Insulin INCREASES the activity, GLUCAGON decreases. 
Think: Big breakfast, had NO cholesterol. This tells you that you’ve made a mistake. That big meal contains no cholesterol, BUT you’re going to release insulin, which INCREASES CHOLESTEROL LEVELS. Continually eating, giving yourself big insulin boosts, actually increase cholesterol levels.

2. ACAT
-->sequester cholesterol inside cells by forming ester.. Controls free/available cholesterol

3. Can control the amounts that are circulating, the amounts going to or from tissues
--> transport molecules LDL/HDL
-->APOB-100 targets VLDL and LDL to tissues!

4. Can control excretion
-->Cholesterol cannot excrete as cholesterol, can secrete as steroids and bile acids (Two main ways of secretion, main one being bile acids)
-->Quaker Oats -- absorbs bile acids when you excrete your Quaker oats take bile acids with it, perhaps can reduce cholesterol