Lecture 6 - Lipids, Membranes And Signaling

Question 1

What is the basic structure of fatty acids?

Which are most common in people?

Answer 1

Carboxylic acid with long hydrocarbon side chains

Fatty acids with 16 or 18 carbons

Question 2

How does the omega nomenclature system work for fatty acids?

Answer 2

Specifies placement of final double bond, identifies the position of the last double bond relative to the last carbon (the omega carbon)

Question 3

Which configuration of double bonds is most common in biology?

Answer 3

Cis

Question 4

How does saturation vs unsaturation affect lipid packing?

Answer 4

Saturated FAs are linear and pack tightly with their atoms along the length in van der Waals contact with neighboring molecules.

Unsaturated FAs have at least 1 double bond that are in cis conformation and reduce the packing van der Waals forces between FAs

Question 5

How does unsaturation affect fluidity?

Answer 5

Increased degree of unsaturation increases fluidity of substance (hence unsaturated FAs are liquid at room temp with lower melting point compared to saturated FAs and it has to do with reduced non covalent interactions between individual FAs)

Question 6

What effect does partial hydrogenation have on vegetable oils?

Answer 6

Converts cis double bonds to single bonds and trans double bonds, preserves shelf life and stability of product

Question 7

What impact do trans double bonds have on health?

Answer 7

Increased risk of heart disease

Increased blood TAGs and LDL cholesterol

Increased inflammatory response

Question 8

What is the impact of trans double bonds on lipid packing?

Answer 8

Increased packing ability, increased van der Waals forces between FAs, decreases fluidity

Question 9

Describe the structure of TAGs

What is the function of TAGs

Answer 9

3 fatty acid chains linked to glycerol backbone with ester linkages

Energy storage (more effective than carbohydrates for energy storage because they are less oxidized (aka more reduced) and carbohydrates pull water with them for storage but TAGs do not)

Question 10

Structure of glycerophospholipids

Answer 10

Glycerol backbone with 2 fatty acids and polar head made of phosphate group and alcohol group

These lipids are named after their polar heads

Question 11

Describe the structure of sphingolipids

Answer 11

Sphingosine backbone with fatty acid attached via amide linkage to backbone, different polar heads attached to carbon 1

Question 12

Sphingomyelin is the ___ _____ sphingolipid.

It is composed of a ____ with a ____ head group.

Answer 12

Most common

Ceramide, phosphocholine

Question 13

Cerebrosides have a ___ with a ___ head group

Gangliosides have a ___ with a _____ head group

Answer 13

Ceramide, single carbohydrate

Ceramide, polysaccharide

Question 14

What is a sterol?

What is the general structure?

Answer 14

Structural lipids present in membrane

3 six membered rings and 1 five membered ring all fused together

Question 15

Cholesterol is a component of _____ and is a precursor to ___ and ___.

Answer 15

Membranes

Bile acids and steroids

Question 16

Lipid bilayers are ___ for biological systems

They form ___ due to the ___ ___

Answer 16

A barrier

Spontaneously, hydrophobic effect

Question 17

Is the thickness of the phospholipid bilayer constant?

Answer 17

No - depends on length of FA chains and how they interact with one another.

Question 18

What does the melting point of a membrane (transition from crystalline state to fluid state) depend on?

Answer 18

Length of FA chains (longer = more van der Waals forces) and degree of saturation (more saturated = more solid)

Question 19

How does cholesterol affect membrane fluidity?

Answer 19

Ring structure of cholesterol restricts movement of nearby FA chains and decreases fluidity

Insertion of cholesterol can prevent packing and increase fluidity

Dual action helps maintain membrane fluidity over broad range of temperatures

Question 20

Is the lipid bilayer symmetric?

Answer 20

No - certain FAs are more common on one side than the other.

Question 21

How can lipids move in the membrane?

Answer 21

Laterally (very fast and has low thermodynamic barrier)

Lipid can flip from outer layer to inner layer (has to be catalyzed by ATP and enzymes such as flippase (out to in) or floppase (in to out) or scramblase (moves in out or out in))

Question 22

How do peripheral membrane proteins associate with membrane?

Answer 22

Through hydrogen bonds with polar head groups

Question 23

What is a lipid raft?

Answer 23

A region of the membrane that is tightly packed with glycosphingolipids and cholesterol. They are thicker, more ordered and associate with specific proteins. The components of the raft move as a unit and are important for signaling.

Question 24

Why are lipid rafts important to signaling?

Answer 24

They cluster membrane components of signaling cascades together and create a localized signaling unit which enhances interactions and makes signaling more efficient.

Question 25

What is a distinguishing feature of carriers?

Answer 25

The molecule that they transport binds to them, causes a conformational change and then that change allows passage of the transporter molecule.

NOTE: a ligand binding to a membrane protein and changing the shape of the protein is NOT considered a carrier because the ligand is not being transported through the membrane.

Question 26

Receptor

Answer 26

Protein in the membrane which binds a signaling molecule / ligand and transfers information from the environment to the cell’s interior

Question 27

Second messenger

Answer 27

Small molecules that are generated intracellularly when a signaling molecule activates a receptor

Question 28

What are the 4 main steps of signal transduction?

Answer 28

1) specificity
2) amplification
3) desensitization / adaptation
4) integration

Question 29

Describe the process of G protein coupled receptors

Answer 29

1) Epinephrine binds to transmembrane region
2) GDP on G alpha subunit swapped for GTP
3) G alpha subunit moves to adenylate Cyclades and either stimulates it (Gs) or inhibits it (Gi)
4) Adenylate Cyclades affects cAMP production
5) cAMP activates Protein kinase A
6) PKA phosphorylates cellular proteins and causes signal transduction

Question 30

How are signal cascades turned off?

Answer 30

1) intrinsic GTPase activity of G proteins
2) short half life of second messengers
3) reversal of phosphorylation by removal of phosphate group by phosphatase
4) desensitization of receptor to a signal

Question 31

Single fatty acids form a ____ lipid barrier.

Phospholipids form a ___ lipid barrier

Triacylglycerols form a ___ lipid barrier

Answer 31

Micelle (triangular shape)

Bilayer (rectangular shape)

Liposome (bilayer that forms a circular shape, inside contains hydrophilic pocket)

Question 32

How does the composition of the cell membrane change during apoptosis?

Answer 32

During apoptosis, scramblase flips phosphatidylserine (PS) from the inner to the outer leaflet. Under normal circumstances this is energetically unfavorable because the polar head group has to travel through the hydrophobic center of the bilayer for this to happen