M3: TG Lipid Functions L17

Question 1

What is the difference between endogenous and exogenous energy production from triglycerides?

Answer 1

Endogenous triglycerides are synthesized and exogenous triglycerides are ingested. Then, both endogenous and exogenous triglycerides are transported, stored, and mobilized to generate energy.

Question 2

Summarize triglycerides’ journey from ingestion to energy prouction.

Answer 2

1. Intake lipids
2. Lipids are lipolysed (digested) in intestine using pancreatic lipase.
3. In the enterocytes, triglycerides are reassembled and stored into chylomicrons to be secreted into blood.
4. When they reach their site of delivery (ex: adipose tissue or muscle) the triglycerides will be hydrolyzed (lypolized) and inserted into their storage site.
5. Fatty acids are Re-esterified again to be stored under the form of lipid droplets.
6. When we exercise and need extra energy, we need to mobilize the FAs for extra energy, triacylglicerides need to be broken down by lypolisis.
7. The free fatty acids will be transported to other organs for energy through the blood.

Question 3

What are the three ways Triglycerides undergo lipolysis (breakdown)? Where do the reactions occur?

Answer 3

1. Pancreatic Lipase (in the GI tract)
2. Lipoprotein lipase (blood vessels)
3. Lipid droplets (adipose and liver)

Question 4

What are the ways triglycerides undergo lipogenesis (synthesis)? Where do the reactions occur?

Answer 4

1. In enterocytes and muscle/adipose tissue

2. ‘De novo’ (in adipose and liver)

Question 5

Describe how lipolysis happens in the GI tract. When does this happen? How is it regulated?

Answer 5

Lipolysis in the GI tract occurs during and after a meal (in the fed state).
The reaction takes place using one enzyme (pancreatic lipase).
In the small intestine, pancreatic lipase is being secreted by the pancreas and meets with the micelles (from diet after being solubilized by the bile salts). The pancreatic lipase breaks down triglycerides into monoglyceride and 2 free fatty acids. These can be taken up by the cell.
This reaction is regulated by substrate availability.

Question 6

Describe how lipolysis happens in blood vessels on the surface of adipose tissue and muscle. When does this happen? How is it regulated?

Answer 6

Lipolysis in the blood vessels occurs during and after a meal (in the fed state).
The reaction takes place using one enzyme (lipoprotein lipase - LPL).
1. Chylomicrons are travelling through the blood until they get by the muscle cell or adipose tissue.
2. At the surface of the endothelial cells that coat the tissues, there are lipoprotein lipases. The chylomicron will meet the lipoprotein lipase at the surface of the tissues and it will break down the triglycerols in the same way that pancreatic lipase does.
3. The cells can take up the free fatty acids and reassemble them to regenerate triglycerides. They are stored as lipid droplets (stable).

This reaction is regulated by substrate availability.

Question 7

At what time do you start breaking down triglycerides during exercise?

Answer 7

During aerobic lipolysis. This is the breakdown of triglycerides stored in muscle and adipose tissue which happens the most after hours (3-4) of exercise. This happens AFTER you’ve exhausted the available glucose and glycogen in your body.
Order: free ATP->creatine phosphate->anaerobic glycolysis (no ox-phos)->aerobic glycolysis->aerobic lipolysis

Question 8

What is the starting point of lipolysis when fasting or during exercise?

Answer 8

Triglycerides are stored in lipid droplets that have different structures depending on the tissue that they’re in. Lipolysis is used to mobilize the fatty acids stored as triglycerides in the lipid droplets so they can be used as energy.

Question 9

What is the difference between lipid droplets stored in adipose tissue and lipid droplets stored in the liver?

Answer 9

1. Adipose cell = tissue specialized in storing neutrolipids (triglycerides & cholesterylester) into huge lipid droplets that constitute most of the volume of the cell. The nucleus and mitochondria are squeezed by the size of the lipid droplet.
2. Main function of the liver is not to store lipids. The lipid droplets in the liver don’t occupy the whole volume of the cell. They are in small vesicles in the cell of the hepatocytes.

Question 10

Describe how lipolysis happens in lipid droplets. When does this happen? How is it regulated?

Answer 10

Lipolysis in lipid droplets occurs when fasting or during exercise.
The reaction takes place using 3 enzymes (ATGL, HSL, and MGL).
1. In the lipid droplet, the triacylglycerol is lipolysed to diacylglycerol via adipose triglyceride lipase (ATGL).
2. Diacylglycerol is lipolysed to monoacylglycerol via hormone-sensitive lipase (HSL).
3. Monoacyl glycerol is lipolysed to glycerol via Monoacyl glycerol lipase.
Free fatty acids are released at each of these steps (one at each step) which are absorbed into the blood and carried by albumin. Albumin is amphipathic, has a hydrophobic pocket, so it can carry free fatty acids. This will then be delivered to the muscle (for example to generate ATP).
4. The glycerol backbone that is left is a soluble sugar. It can travel through the blood all the way to whatever tissue needs energy (generates energy via fructose metabolism).

This reaction is regulated by hormonal regulation: PKA phosphorylates perilipin and HSL.

Question 11

Describe the regulation of lipolysis in lipid droplets.

Answer 11

Protein kinase A phosphorylates perilipin and HSL:

1. adrenaline binds to the beta-adrenergic receptor (GPCR).
2. GTP bound Gαs associates with AC and makes cAMP out of ATP.
3. CAMP activates PKA which phosphorylates many things including HSL and perilipin.
4. The phosphorylated HSL needs to access the pool of triglycerides in the lipid droplets. So PKA phosphorylates perilipin as well. Perilipin will change structure which will allow HSL to interact with the core of the lipid droplets and hydrolise the DAG.

Question 12

What is perilipin?

Answer 12

Amphipathic protein with hydrophobic residues that can easily interact with the core of the lipid droplets. Perilipin coats the lipids. Protein that maintains the lipid droplets protecting them from being hydrolyzed

Question 13

What is different about lipolysis of lipid droplets in the liver vs in the adipose tissue?

Answer 13

In the liver, HSL is able to do the same job as ATGL. It can hydrolize the triacylglycerol to diacylglycerol AND hydrolize the diacylglyerol to monoacylglycerol.

Question 14

Where does the (re)synthesis of triacylglycerols occur in enterocytes?

Answer 14

The only place you can resynthesize neutrolipids inside the cell is in between two membranes. -> hydrophobic environment. Therefore it occurs in the ER between 2 membranes. Since it’s occuring in between 2 membranes, as you re-form more triglycerides it enriches the “core” and starts to form a vesicle that can eventually budd off and form a chylomicron.

Question 15

Describe the (re)synthesis (lipogenesis) of triacylglycerols in the enterocyte.

Answer 15

1. Activation by CoA via Acyl-CoA Synthetase:
   - Fatty acid binds one phosphate group (along with the adenosine) from ATP using Acyl-CoA synthetase. The remaining Ppi is hydrolized by inorganic pyrophosphatase to make 2pi which provides the energy needed to form the intermediate.
   - The intermediate binds CoA via Acyl-CoA synthetase to form Acyl-CoA and AMP.
2. the 2-monoacylglycerol pathway:
   - 2-Monoacylglycerol from the diet uses the acyl-CoA formed in the first reaction and the MGAT (2-monoacylglycerol acyltransferase) enzyme to transfer an acyl chain onto it to form diacylglycerol.
   - The same thing happens to diacylglycerol via the enzyme DGAT (diacylglycerol acyltransferase) to form Triacylglycerol. This is what accumulates in the core and generates a lipoprotein (chilomicron).

Question 16

Why do we need to add a CoA to the fatty acid when re-synthesizing triacylglycerol in the enterocyte?

Answer 16

1. To make a high energy intermediate (activate it) with a thioester bond which will make the 2-monoacylglycerol pathway more favourable.
2. To trap fatty acids inside the cell. (Like adding P to Glucose to make G6P)

Question 17

Describe the Triacylglycerol ‘de novo’ synthesis in the liver and adipose. What is the name of the pathway?

Answer 17

The glycerol 3 phosphate pathway:
1. generating Glycerol-3-Phosphate: There are two ways
(a) DHAP synthesis via gluconeogenesis (pyruvate->oxalo-> PEP->->DHAP). DHAP to glycerol-3-phosphate via Glycerol-3-phosphate dehydrogenase (reverse reaction from fructose metabolism in the liver).
(b) Phosphorylation of Glycerol (glycerol to Glycerol-3-P using glycerol kinase from the fructose metabolism pathway in the liver)
Adipocytes lack glycerol kinase so reaction B doesn’t happen in adipocytes.

2. Adding acyl-CoA’s:
   - We use Glycerol 3 phosphate and 2 activated acyl-CoAs and you generate phosphaditic acid.
   - Once phosphaditic acid is generated there is a crossroads. It can be used to generate triacylglycerol OR it can be used to generate phospholipids.
3. Dephosphorylation via Lipin enzyme of
   Phosphatidic acid to DAG via Lipin.
   Lipin is the key regulatory enzyme to decide whether you generate phospholipids or triacylglycerol. The enzyme is regulated at the translational level. When Lipin is highly abundant, the triglyceride pathway is favoured.
4. Adding acyl-CoA to DAG to form triacylglycerol via DGAT.

Question 18

How is lipogenesis regulated by insulin? What state are you in?

Answer 18

Occurs in the fed state:

1. Intestinal cells transfer food derived glucose to blood.
2. [Glucose] in the blood raises and pancreas releases insulin.
3. When insulin binds to its receptor it generates a cascade of events that leads to the synthesis of all the enzymes required for the synthesis of fatty acids into TAG. So if you have more insulin, the cells are producing more of the enzymes involved in lipogenesis and they will be better at storing fatty acids under the form of triglycerides.
   * *insulin stimulates lipogenesis in adipose tissue and liver

Question 19

Go see the summary of triglycerides in the fed and fasting state.

Answer 19

Module 3: L17 Slide 35.

Question 20

Where are membrane lipids found?

Answer 20

In the plasma membrane.

Question 21

What are the two general types of phospholipids?

Answer 21

1. Phospholipid with a glycerol backbone (glycerophospholipids)
2. Phospholipid with a sphingosine backbone (sphingolipids)

Question 22

How is lipid distribution in membranes maintained?

Answer 22

Lipids can either do transverse diffusion or lateral diffusion.
Transverse diffusion: There are proteins in the membrane called flippase, floppase, and scramblase that transfer phospholipid molecules from one bilayer leaflet to the other. The structure of the membrane is defined by the balance of what phospholipids are on the outside and inside of the plasma membrane.
Lateral diffusion: pairwise exchange of neighbouring phospholipid molecules in the same bilayer leaflet.

Question 23

What lipids make up microdomains in membranes?

Answer 23

All of these make up RAFTS or NON-RAFTS

1. Phospholipids
2. Sphingomyelin
3. Cholesterol
4. Lipid anchors: anchor proteins at the cell surface. RAFTS and Non-RAFTS use different lipid anchors.

Question 24

What are Caveolae? What are their roles?

Answer 24

Rafts can assemble and form invaginations (i.e. caveolae)
Caveolin = coating protein
Roles:
- assemble signaling complexes
- provide elastic capacity to certain membranes

Question 25

What happens when you modulate the composition of RAFTS or Calveolae?

Answer 25

Caveolae (or RAFTS) can be involved in signalling. If you modulate the composition of RAFTs (ex:+ or - cholesterol, sphyngomyelins), it will modulate the concentration of solution proteins in the RAFTS. Therefore, if you deplete the membranes of cholesterol or sphyngomyelins you disrupt the RAFTs and you have a structure that does not permit signalling.

Question 26

What is an example of a function of Calveolae?

Answer 26

• Caveolae structure can adapt depending on plasma membrane ‘tensions’
• They are abundant in mechanically stressed cells (muscle, fibroblasts, endothelial, adipocytes)

Therefore, they can serve as a buffer for the plasma membrane.
Ex: muscle cells can be stretched and extended or un-stretched. The plasma membrane copes with these constraints by having a reservoir of plasma membrane in the form of flattened and unflattened calveolae.