11 - Intro to Lipid Signalling

Question 1

Structure of phospholipids in the phospholipid bilayer

Answer 1

The phospholipids that make up the membrane have head groups which are charged and face towards the aqueous environment
- half of the monolayer with the heads facing towards the water in the cytoplasm
- the second half is facing out with the polar headgroups facing outside of the cell

Question 2

Features of the fatty acid tails

Answer 2

Fatty acid tails face inwards- can be saturated or unsaturated
- The tail tends to be straight if there are no double bonds- saturated
- Unsaturated the tail tends to be kinked due to the double bond

Question 3

Why does the lipid bilayer form?

Answer 3

The lipid bilayer forms spontaneously in aqueous solution because the fatty acid tails are hydrophobic and tend to gather together to force out water whereas the polar headgroups are hydrophilic

Question 4

What is the structure of vesicles?

Answer 4

In the cell the vesicles that contain insulin etc. are formed of lipids forming a low energy spherical structure whereby the tails form a hydrophobic barrier, and the head groups point out to the cytoplasm
- Liquid filled space in the centre of the vesicle

Question 5

What does the nature of the cell membrane lipids depend on?

Answer 5

• The nature of the cell membrane lipids depends on their amphipathic nature, the degree of packing is dependent on the degree of saturation
• The more unsaturated they are, the more liquid like the bilayer

Question 6

Structure of phosphatidylcholine

Answer 6

The polar head group consists of a alcohol (choline) which is charged, the alcohol is linked to e three carbon glycerol backbone by a phosphate group which is negatively charged
- Phosphoester bond between the glycerol and phosphate, and a phosphoester bond between the phosphate and alcohol
- Glycerol- phosphate on one carbon and on the other two is a fatty acid
- So theres the hydrophilic head, glycerol backbone and two non-polar hydrophobic fatty acid chains attached by ester bonds

Question 7

Give examples of phospholipids

Answer 7

Phosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl choline, phosphatidyl inositol, and sphingomyelin

Question 8

What is the main difference in phospholipids?

Answer 8

The nature of the alcohol head group

Question 9

Differences in phosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl choline, phosphatidyl inositol, and sphingomyelin

Answer 9

1. PHOSPHATIDYL ETHANOLAMINE - ethanol group is positively charged
2. PHOSPHATIDYL SERINE - serine is charged
3. PHOSPHATIDYL CHOLINE - choline is charged
4. PHOSPHATIDYL INOSITOL- important signalling molecule with a ring head group with many HO groups meaning it is water immiscible (the HO groups can be targeted by phosphates meaning a range of signalling molecules can be generated)
5. SPHINGOMYELIN differs from the others in that it doesn’t have a glycerol backbone, has an aliphatic chain instead, called a ceramide, ceramide is a single long allopathic chain with three carbons at the end creating a head group
   * similarly there is a phosphate linking the backbone to the alcohol- choline
   * has an additional fatty acid, but doesn’t have an ester group, instead has a nitrogen

Question 10

What is the structure of phospholipids?

Answer 10

All of the phospholipids have a phosphate linked to a glycerol backbone, phosphoglycerides

Question 11

How does the nature of the fatty acid change vary of a large degree?

Answer 11

Each of the core ones can have a range of fatty acid tails and different chain lengths and different degrees of saturation and unsaturation

Question 12

Function of the lipid-like substance cholesterol

Answer 12

• can be converted into steroid hormones and vitamin D
• plays a structural component of plasma membrane
• easy to forget the important of cholesteol in normal bodily function
  -essential in vast number of hormones in the body

Question 13

Structure of cholesterol

Answer 13

o planar ring structure -hydrophobic
o non-polar hydrocarbon tail which is hydrophobic
o at the top there is a HO group which lends a polar charge to the molecule
o it is lipid-like due to its tail and it polar head and rigid hydrophobic core
o it fits inbetween the glycerolphospholipids in the membrane
o cholesterol lends to the stiffness of the cell membrane

Question 14

Why are cell membranes asymmetric?

Answer 14

o Asymmetric because the functions of the lipids are different
o The two faces of biological membranes always differ from each other

Question 15

Degree of asymmetry of the five major phospholipids

Answer 15

1. Phosphatidylcholine tends towards the outer leaflet
2. the phosphatidylethanolamine is more in the inner leaflet
3. sphingomyelin tends towards the outer leaflet due to its protective nature
4. phosphatidylserine is completely on the inner leaflet (acts as a single during cell death- flips to the outer leaflet and serves as a molecular flag to macrophages to indicate that the cell is undergoing apoptosis)
5. phosphatidylinositol is also completely on the inner leaflet of the cell membrane as it is an important signalling molecules which is acted on by enzymes in the inner leaflet

Question 16

What is a phospholipase and what do they do?

Answer 16

• a phospholipase is an enzyme that converts phospholipids into fatty acids and other lipophilic substances by hydrolysis
• so effectively they use water in the cells to attack ester bonds that hold the phospholipids together to break the fatty acid chains

Question 17

What happens when membrane lipids are hydrolysed?

Answer 17

They are removed from the membrane and therefore the structure and integrity and fluidity of the membrane may be altered

Question 18

Names of 2 enzymes that modify membrane lipids?

Answer 18

• Phospholipase A2
• Phospholipase C

Question 19

Give an example of enzymes that modify phosphatidyl inositol

Answer 19

o Embedded in the inner leaflet of the cell membrane
o One specific chain is called arachidonic acid which is a long chian woth four double bonds- is a famous progenitor of signalling molecules
o Arachidonic acid is attached by a regular ester bond which can be attacked by phospholipase A2
o Phospholipase C is regulated by G-coupled protein receptors and once activatd it cleaves the phosphoester bonds which generates two singalling moleucles
- Releases signalling molecules