Membrane Structure (Module 2 Lecture 4)

Question 1

Name the two types of proteins in a plasma membrane as well as the modified carbohydrate motifs on the extracellular surface.

Answer 1

1. Integral proteins (embedded in the membrane; usually a transmembrane) and peripheral proteins (which are loosely bound).
2. Glycoproteins and glycolipids on extracellular surface

Question 2

Describe two amphipathic lipids in a cellular membrane.

Answer 2

1. Phospholycerides with varied polar head groups

2. Sphingolipids (including sphingomyelin and glycolipids)

Question 3

Describe the asymmetry of plasma membranes.

Answer 3

1. Outer leaflet contains more phosphatidylcholine and sphingomyelin as well as glycolipids.
2. The inner leaflet contains more phosphatidylserine (charged) and phosphatidylethanolamine.

Question 4

How do fatty acids module membrane packing?

Answer 4

1. Cis-configuration unsaturated tails kink and cannot pack as closely as fully saturated tails.
2. The more double bonds, the more fluid the membrane.

Question 5

Describe the movement of phospholipids in the lipid bilayer.

Answer 5

1. They can diffuse laterally in the membrane and can flex and rotate.
2. They can move spontaneously from one side of the membrane to another but it is rare (flip-flopping).
3. Flippases contribute to the synthesis of asymmetrical membranes.

Question 6

Describe the role of cholesterol in plasma membranes.

Answer 6

It stiffens the region of the membrane adjacent to the steroid ring, strengthening the membrane while allowing the middle section of the bilayer to remain relatively fluid.

Question 7

How are integral proteins removed from the plasma membrane?

Answer 7

They can only be removed using detergents because they are usually anchored from membrane leaflet to the other side and have hydrophilic portions on both sides.

Question 8

How are peripheral proteins removed from the plasma membrane?

Answer 8

They are solubilized from the membrane by aqueous solvents like high salt buffers due to the fact that they are not fully embed in the membrane.

Question 9

List four functions of integral proteins in plasma membranes.

Answer 9

1. Transporters
2. Anchors
3. Receptors
4. Enzymes

Question 10

Describe the structure of a typical integral transmembrane protein.

Answer 10

1. Contains alpha-helical structure made of non-polar amino acids.
2. Contains oligosaccharide chains and disulfide bonds, all on the non-cytosolic surface of the membrane.

Question 11

Can some transmembrane proteins pass through the membrane more than once?

Answer 11

Yes, and can have hydrophobic portions that are alpha-helices of beta-pleated sheets.

Question 12

Describe the hormone receptors in plasma membranes linked to G proteins.

Answer 12

They have several spanning domains, like many other hormone receptors and include receptors for epinephrine and glucagon.

Question 13

What is epinephrine?

Answer 13

Adrenaline (a hormone and neurotransmitter)

Question 14

What is a G protein?

Answer 14

Guanosine-nucleotide binding proteins usually serve as switches in the cell membrane that can activate a cascade of subsequent reactions.

Question 15

What is glucagon?

Answer 15

The hormone made in the pancease that breaks stored glygogen into glucose.

Question 16

Give an example of a partially embedded protein in the plasma membrane.

Answer 16

Cyclooxygenase-1 (catalyzes the synthesis of prostaglandins), which has alpha-helices with hydrophobic side chains embedded in the membrane

Question 17

What is a prostaglandin?

Answer 17

A cyclic fatty acid with hormone-like effects (responsible for labor contractions)

Question 18

Describe integral proteins with covalently-attached lipids.

Answer 18

Lipids help anchor the protein to the membrane and are attached after synthesis of said protein; can anchor to intracellular or extracellular surface.

Question 19

List three common lipid anchors of transmembrane proteins.

Answer 19

1. Fatty acids (like myristate (14:0) or palmitate (16:0)
2. Poly-prenyl (prenyl = 2-methyl-2-butyl-R) groups like farnesyl
3. GPI, which stands for glycophosphotidylinitosol (a glycolipid)

Question 20

How are peripheral proteins bound to the plasma membrane?

Answer 20

By non-covalent interactions with other proteins (on both sides of the membrane)

Question 21

What is protein translocation?

Answer 21

The modification of a protein via intracellular signals in order for it to switch from cytosolic association to membrane association.

Question 22

Give two examples of protein translocation.

Answer 22

1. Protein kinase C binds the lipid diacylglycerol, which anchors it to the membrane.
2. Phospholipase A2 is phosphorylated, which enhances its binding to a specific integral membrane protein.

Question 23

What are two specialized membrane domains and what are they made out of?

Answer 23

Caveolae and rafts, which contain many cholesterol and sphingolipids (which have saturated hydrocarbon chains like sphingomyelin and glycosphingolipids) for orderly state as well as specific proteins which contribute to membrane functions like intracellular signaling (protein kinase C), extracellular signal reception (LDL receptor), and catalysis (andothelial nitric oxide synthase)

Question 24

How can motion of membrane proteins be visualized?

Answer 24

By forming heterokaryons (a cell with multiple, genetically different nuclei)

Question 25

Why is the movement of some membrane proteins restricted?

Answer 25

Because these integral proteins are linked to peripheral cytoplasmic proteins on the cytoskeleton (like on red blood cells, which limits the lateral movement).

Question 26

How do epithelial cells limit protein movement in specific membrane domains?

Answer 26

Tight junctions.

Question 27

Where are epithelial cells found?

Answer 27

Lining the cavities of the body to form flat surfaces, as well as skin, and it is the first type of cell to differentiate in an embryo during the eight-cell stage.

Question 28

What is FRAP and why is it used?

Answer 28

1. Florescence recovery after photobleaching
2. Another way to see movement of proteins within a membrane (to determine which are anchored and which are free to move)

Question 29

What is glycocalyx?

Answer 29

A slimy/fuzzy glycoprotein (or glycolipid) with oligosaccharides attached that is produced by epithelial cells and some bacteria

Question 30

What are two important things to remember about glycocalyx?

Answer 30

1. Oligosaccharide chains are found on both membrane glycoproteins and glycolipids.
2. Both integral and peripheral glycoproteins contribute to the gycocalyx.

Question 31

The glycocalyx is associated with what three things that form the extracellular matrix?

Answer 31

1. Proteins
2. Glycoproteins
3. Proteoglycans