M3: Phospholipids L18

Question 1

Under which metabolic circumstances should TG synthesis be favoured?Select ALL that apply.

A) After a fatty meal
B) When cells are dividing
C) When fasting for > 12h
D) To maintain membrane rafts balance
E) In case of stress, to provide signalling molecules

Answer 1

A) After a fatty meal

Question 2

What types of lipids are found in membranes?

Answer 2

Membrane lipids include:

• phospholipids
• glycolipids
• sterols

Question 3

Under which metabolic circumstances should PL synthesis be favoured?Select ALL that apply.

A) After a fatty meal
B) When cells are dividing
C) When fasting for > 12h
D) To maintain membrane rafts balance
E) In case of stress, to provide signalling molecules

Answer 3

B) When cells are dividing (for membrane synthesis)
D) To maintain membrane rafts balance (Phospholipids are involved in the maintenance of the rafts and non-rafts structures.)
E) In case of stress, to provide signalling molecules (PIP3, DAG, PIP2 are derived from phospholipids)

Question 4

What is the cellular location for the synthesis of phospholipids?

Answer 4

The endoplasmic reticulum

Question 5

How are the glycerophospholipids PI (Phosphatidylinositol), PG (Phosphatidylglycerol) and Cardiolipin synthesized?

Answer 5

Step 1 = Activation of phosphatidic acid

• Phosphatidic acid was previously formed from glycerol-3-phosphate
• CTP is the source of energy. It combines with Phosphatidic acid and make CDP-diacylglycerol and PPi via the enzyme CDP-diacylglycerol synthase (∆G˚ = 0).
• CDP-diacylglycerol is a high energy intermediate. It can be formed bc it is driven by the hydrolysis of PPi to 2Pi which has a -∆G. Favourable irreversible reaction.

Step 2 = Adding inositol or glycerol 3-phosphate
- High energy intermediate (CDP-diacylglycerol) can interact with inositol to generate Phosphatidylinositol via PI synthase.
OR
- High energy intermediate (CDP-diacylglycerol) can interact with glycerol-3-phosphate to generate phosphatidylglycerol phosphate via PG-3-P synthase enzyme.
- Phosphatidylglycerol phosphate loses a phosphate via pG-3-P phosphatase which makes PG.
-2 PG’s are associated via cardiolipin synthase to make cardiolipin. (a glycerol is lost in the process)

Question 6

What are the two pathways that can be used to make PE (phosphatidylethanolamine) and PC (phosphatidylcholine)?

Answer 6

1. Kennedy pathway (makes PE & PC)

2. Methylation via PEMT in the liver (the 1 carbon pathway) => Makes PC from PE

Question 7

What are the steps in the kennedy pathway?

Answer 7

Step 1 = priming the head group with ATP by adding a phosphate to ethanolamine or choline.
Step 2 = Activating the head group (phosphoethanolamine/phosphocholine) with CTP which releases PPi.
Step 3 = Add diacylglycerol to CDP-ethanolamine or CDP-choline (releases CMP) to form PE or PC.

Question 8

Why do we need to activate the polar head group (phosphoethanolamine/phosphocholine) with CTP in the kennedy pathway?

Answer 8

To raise the internal energy of the system to generate a molecule of higher order. Anabolic reactions require energy.

Question 9

What are the steps of the PEMT (Phosphatidylethanolamine methyltransferase) pathway in the liver? (the 1 carbon pathway)

Answer 9

Step 1 =Adding 1 methyl group from Ado-Met to PE via PEMT.
Ado-Met is made in the 1 carbon pathway where an adenine group is added to methionine from ATP. This forms Ado-Met and a pyrophosphate is released. Ado -met has a sulfonium moiety, which makes the methyl group unstable and favourable to be detached.
Step 2 = Repeat
Step 3 = Repeat => end product: Phosphatidylcholine (PC)

Question 10

Which function is highly dependent on abundant PC production in liver?

Answer 10

The production of lipoproteins and bile. PC is involved in lipoprotein production, they will coat the membranes of the lipoproteins bc they are amphipathic.
Producing bile: This is crucial. PC is in very high proportion in bile. The composition of bile is exquisitely regulated. It has to contain the proper amount of cholesterol, phosphatidylcholine, and bile salts. If you have the proper ratios of the 3 constituents you have normal bile and it will emulsify lipids coming from diet. If you don’t have enough of any of the constituents you will get gall stones in the bladder due to the bile salts crystallizing with cholesterol. So to maintain the amount of PC in the bile you need the PE to PC pathway (PEMT) on top of the Kennedy pathway.

Question 11

Identify a “high energy intermediate” in the PEMT-mediated conversion of PE to PC.

Answer 11

Ado-Met (s-adenosyl-L-methionine)

Question 12

What phenotype is observed in case of a PEMT deficiency? Can you propose a mechanism?

Answer 12

Gall stones, fatty liver.
You would get a fatty liver in PEMT deficient mice because there would be an accumulation of PE since PE can’t be converted to PC. The fatty acids will accumulate in the form of triglycerides in the liver. This is called non-alcoholic fatty liver.

Question 13

How do you make PS (Phosphatidylserine)?

Answer 13

Serine is added to PE (phosphatidylethanolamine) via PE serine transferase. This does not require an input of energy and it is an irreversible reaction. PS is formed.

Question 14

How do you make PE (Phosphatidylethanolamine)?

Answer 14

PS (Phosphatidylserine) loses a CO2 by PS decarboxylase to make PE. This does not require an input of energy and it is an irreversible reaction.

Question 15

Name the glycerophospholipids.

Answer 15

1. PI (phosphatidylinositol)
2. PG (phosphatidylglycerol)
3. Cardiolipin
4. PE (phosphatidylethanolamine)
5. PC (Phosphatidylcholine)
6. PS (Phosphatidylserine)

Question 16

How do you form sphingolipids?

Answer 16

Step 1 = adding an activated acyl (palmitoyl-CoA) to the serine backbone. The CoA is hydrolized which releases the energy required to drive the rxn fwd.
-Another fatty acyl-CoA is used to generate a ceramide (2 fatty acids on a serine group).

Step 2a = transfering a phosphatidylcholine headgroup to ceramide to make sphingomyelin.

Step 2b = adding a glucose headgroup to ceramide using UDP-Glucose to make cerebroside (a glycosphingolipid)

Question 17

Where are phospholipases found and what do they do?

Answer 17

Phospholipases are found in the plasma membrane. Phospholipids are remodeled (not really broken down) at the plasma membrane via phospholipases.

Question 18

Why do phospholipases cleave phospholipids?

Answer 18

1. To generate signaling molecules
2. To convert phospholipids
3. To modulate the shape of the plasma membrane

Question 19

Look at lecture 18 slide 33 to see where each phospholipase cleaves a phospholipid to generate different molecules

Answer 19

lecture 18 slide 33

Question 20

What are the different phospholipases?

Answer 20

Phospholipase A1, A2, C, and D.

Question 21

What disease does a deficiency in sphingomyelinase (phospholipase) cause?

Answer 21

Nieman-Pick Type A/B disease.

These patients will have problems with endocytosis of lipoproteins because of the immaturation of the endosomes.

Question 22

How do phospholipases generate signalling molecules?

Answer 22

Phospholipase C hydrolizes PIP2 to IP3 and DAG in the phosphoinositide GPCR pathway when it is stimulated by G-alpha-q due to an external signal.

Question 23

How do phospholipases convert phospholipids?

Answer 23

via deacylation/reacylation = the “Lands’ Cycle”
Through the use of phospholipase A2s on phospholipids, you’re releasing one fatty acid and generating lysophospholipids such as phosphaditic acid. These lysophospholipids are the substrates for regenerating other phospholipids. This is the Land’s cycle where you change the head group or the fatty acid associated with the glycerol backbone.

Question 24

How do phospholipases modulate the shape of the plasma membrane?

Answer 24

The shape of the phospholipids induces modifications in the structure of the plasma membrane. Ex: if you need to endocytose, the flat membrane is going to invaginate. You need to convert the lipids that are in the cylinder shape to cone shape in order to invaginate.