CBS - Lipids and Membrane Structures

Question 1

What are the functions of lipids?

Answer 1

• Energy Stores – Triacylglycerol
• Precursors for Vitamins, Hormones (Steroids)
• Bile Acids (by-products of digestion) – Cholic acid
• Membrane structure – Phospholipids

Question 2

Describe the characteristics of triacylglycerols.

Answer 2

It is a water-insoluble, hydrophobic molecule.

Characteristics:

• It is an ester of 3 fatty acids and a glycerol
• It it as energy storage
• it is stored in adipose tissue

Question 3

Describe fatty acids.

Answer 3

They are long chain aliphatic carboxylic acids.
CH3(CH2)nCOOH

They are metabolised via the β-oxidation pathway to generate ATP.

Question 4

What are the different types of fatty acids?

Answer 4

FULLY SATURATED (no double bonds):

• myristic acid (14:0);
• palmitic acid (16:0);
• stearic acid (18:0)

UNSATURATED (1 double bond)
- oleic acid (18:1 D9)

POLYUNSATURATED (PUFA) : more than one double bond

• linoleic acid (18:2 D9,12) : essential fatty acid
• arachidonic Acid (20:4 D5,8,11,14)

Question 5

What is the fluid mosaic model?

Answer 5

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character.

Question 6

What are the major groups of lipids in membranes?

Answer 6

The two major groups are phospholipids and glycolipids. They have a similarity to the triacylglycerols (TAGs).

PHOSPHOLIPIDS:
They are made up of a glycerol and 2 fatty acids and a phosphate-containing group.

GLYCOLIPIDS:
They are made up of a glycerol and 2 fatty acids and sugars.

Question 7

Describe the features of phospholipids.

Answer 7

Phospholipids are the major components of all membranes.

They spontaneously form bi-layers in aqueous environment.

They are composed of a “polar head group” (such as serine, choline, ethanolamine or inositol) that is attached to a backbone (glycerol) through a phosphate group.
Two fatty acyl side chains are linked to the glycerol backbone via ester bonds.

They are “amphipathic”: they have both a polar head group and non-polar FA tail.

Question 8

List some common phospholipids.

Answer 8

• Phosphatidylcholine (PC)
• Phosphatidylserine (PS)
• Phosphatidylethanolamine (PE)
• Phosphatidylinositol (PI)
• Sphingomyelin (SPH)

Question 9

List some common head groups found on phospholipids.

Answer 9

• choline
• serine
• ethanolamine
• inositol

Question 10

What groups is phosphatidylcholine (PC) made up of?

Answer 10

• choline
• phosphate
• glycerol
• fatty acyl chain

Question 11

What groups is sphingomyelin (SPH) made up of?

Answer 11

• choline

- ceramide (made up of sphingosine and a fatty acyl chain)

Question 12

Describe cholesterol and its features.

Answer 12

It is a steroid, with a common multi-ring structure. It has similar physical properties of phospholipids, with a hydrophobic and hydrophilic region.

We can derive cholesterol from our diet and through de novo synthesis.

It is the precursor of steroid hormones e.g. testosterone, oestrogen, cortisol, etc.

Question 13

What are some important points about the variation in composition of cellular membranes?

Answer 13

• Only the plasma membrane contains carbohydrates.
• The inner mitochondrial and nuclear membranes have more protein in their membranes.
• There is far more cholesterol in the plasma membrane than other membranes.

Question 14

How are the lipid bilayers asymmetrical?

Answer 14

The two halves of the bilayer have different lipid compositions.

• PC and SPH face the extracellular environment
• PS and PE face the cytosolic side

Question 15

What is membrane fluidity?

Answer 15

Fluidity is the ease with which lipid molecules move about in the plane of the bilayer

It is important in the regulation of membrane function (e.g. movement of proteins, signalling, exocytosis).

Question 16

How is membrane fluidity regulated?

Answer 16

The lipid composition of membrane defines its fluidity.

• an increase in short chain fatty acids reduce the van der Waals interactions between fatty acids and increases fluidity
• kinks in unsaturated fatty acids reduce van der Waals interactions with other lipids and increases fluidity
• high cholesterol content restricts the random movement of polar heads, orders the lipid bilayer and decreases fluidity – it is rich in highly ordered regions of the membrane lipid rafts

Question 17

What are lipid rafts?

Answer 17

They are specialised membrane microdomains of membranes.

They are more ordered – and less fluid. This is due to increased level of cholesterol.

There is also an increased level of sphingomyelin.

It allows for close interactions between receptors and signalling molecules.

Question 18

What are the 3 classes of membrane proteins, and how are the organised?

Answer 18

INTEGRAL (INTRINSIC) PROTEINS – embedded in lipid bilayer, most span the entire bilayer; the transmembrane spanning domains are α-helices or β-sheets

ANCHORED PROTEINS – anchored to membrane by covalent bonds with fatty acids

PERIPHERAL (EXTRINSIC) PROTEINS – attach to membrane surface by ionic interactions with integral proteins or with polar head group of phospholipids

Question 19

Describe an integral membrane protein.

Answer 19

An example of an integral membrane protein is an ion channel.

The residues interacting with the centre of the membrane will be hydrophobic side chains, and vice versa.

There also needs to be a channel through which things can move through which polar substances can move through a hydrophobic environment.

Question 20

Describe anchored proteins.

Answer 20

They are not deeply embedded in the bilayer, but are covalently linked to fatty acid chains or on the cell surface to glycolipids.

Example of a glycolipid-anchored protein: the enzyme alkaline phosphatase

Example of a fatty acyl-anchored protein: RAS a signalling G-protein

Question 21

Describe peripheral proteins.

Answer 21

Peripheral membrane proteins attach to the membrane surface by interactions with other membrane proteins or with the polar head group of phospholipids

Spectrin is an important structural protein on the cytoplasmic surface of erythrocytes that interacts with other proteins such as ankyrin.

Question 22

Describe and give some examples of phospholipases.

Answer 22

Phospholipases are an important class of enzymes that bind to membrane and selectively hydrolyses components of phospholipids.

Examples include:

• Phospholipase A1 (PLA1)
• Phospholipase A2 (PLA2)
• Phospholipase C (PLC)
• Phospholipase D (PLD)

Question 23

What are the different sites of action of phospholipases?

Answer 23

(from external paper)
Phospholipase A1 and A2 act respectively on the ester bonds that link fatty acids to the sn-1 and sn-2 positions of the glycerol backbone of phospholipids. Their action generates free fatty acids and lysophospholipids.

Phospholipase C acts on the phosphodiester bond that links the headgroup of the phospholipid to the glycerol backbone. It yields phosphorylated headgroups and diacylglycerol (DAG).

Phospholipase D acts on the other side of the phosphodiester bond to yield free headgroups and phosphatidic acid (PA).

Question 24

How can different types of proteins be removed from membranes?

Answer 24

Peripheral proteins can be removed by denaturation (through high salt and urea), whilst anchored proteins and integral proteins will remain attached.

Detergents will disrupt membranes in their entirety, so all proteins can be extracted using this method.

Phospholipases can be very effective, but it depends on how that protein is bound. If the protein is linked to the phospholipid, then it can be removed, but if it is imbedded, then the phospholipases will have no impact.

Question 25

How are detergents able to remove proteins from lipid bilayer?

Answer 25

Detergents have a similar overall chemical structure to phospholipids.

These can disrupt the membrane, and can therefore solubise integral proteins due to their hydrophobic and hydrophilic parts.

Thus, the protein is removed through a type of centrifugation.