Membrane Proteins

Question 1

True or False:

Lipids are generally larger than proteins

Answer 1

FALSE

Proteins are much larger than lipids, corresponding to the 1:50-1:100 protein-to-lipid ratio that is found within membranes.

Question 2

List seven different functional types of membrane proteins

Answer 2

1) Channel transport
2) Carrier transport
3) Receptor
4) Enzyme (catalysis)
5) Anchor (ECM needs anchoring to cell)
6) Cell-cell joining (desmosomes, tight junctions, etc.)
7) Cell-cell recognition

Question 3

List the two different types of membrane proteins (relating to structure, not function)

Answer 3

1) Peripheral proteins

2) Integral membrane proteins

Question 4

Which type of membrane protein is easily separated from the membrane? Why is it so easy to separate?

Answer 4

Peripheral membrane proteins are easiest to separate from the membrane. This is because they are not anchored in the membrane, but rather, temporarily associated with the internal or external side

Question 5

Integral membrane proteins are attached to the cell membrane by ____________

Answer 5

Covalently linked lipid or glycolipid molecules

Question 6

Peripheral membrane proteins are attached to the cell membrane by ____________

Answer 6

Electrostatic interaction

Question 7

Peripheral membrane proteins can become disassociated from the membrane following treatment with _____________

Answer 7

Polar reagents (solutions of extremely low pH [H+ high] or salt concentrations that do not disrupt the lipid bilayer)

Question 8

Integral proteins that span the entire membrane are called _______________

Answer 8

Transmembrane proteins

Question 9

Integral proteins must have at least one _______________, often an alpha helix

Answer 9

Non-polar region. This region is often coiled in an alpha helix, but can also exist as beta pleated sheets

Question 10

Transmembrane proteins with an EVEN number of membrane-spanning regions can have _______________ number of orientations.

Answer 10

TWO orientations:

In-In & Out-Out

Question 11

Transmembrane proteins with an ODD number of membrane-spanning regions can have _______________ number of orientations.

Answer 11

ONE orientation:

In-Out

Question 12

How are integral membranes studied?

Answer 12

The membrane is solubilized by a reagent, which disrupts the bilayer (hydrophobic interactions)

Question 13

What is most frequently used as a reagent to study transmembrane proteins?

Answer 13

Detergents are used. These are very small amphipathic molecules, containing both hydrophobic and hydrophilic groups

Question 14

What are the two most common detergents used in membrane solubilization?

Answer 14

SDS (sodium dodecyl sulfate) is a strong ionic detergent, with an ionized Na+ / O- group at its hydrophilic end.

Triton X-100 is a mild non-ionic detergent, with a non-ionized polar group at its hydrophilic end

Question 15

How do detergents differ from lipids? Why is this difference important?

Answer 15

They are smaller, and only have a single hydrophobic tail. This makes them shaped more like cones than cylinders, allowing them to tend towards clusters called micelles rather than forming a bilayer.

Question 16

How to detergents solubilize a membrane?

Answer 16

When mixed in excess, their hydrophobic tails aggregate around the hydrophobic core region of transmembrane proteins, and the hydrophobic tails of lipid molecules. This separates proteins from the lipid bilayer.

Question 17

Why are type-1 α-helices?

Answer 17

Alpha-helices that are hydrophobic on all sides. This is because the hydrophobic side chain of an amino acid is exposed on the outside of the α-helix.

Question 18

How do type-1 alpha helices function within the membrane?

Answer 18

They give rise to receptors for extracellular signalling

Question 19

What are the two domains that make up a type-1 α-helix?

Answer 19

The LIGAND-BINDING domain (ECM, ligand is signal molecule) and the REGULATORY domain

Question 20

What are type-2 α-helices?

Answer 20

They are helices where the hydrophobic amino acid side chains are concentrated on one side, while the other is hydrophilic (charged).

Question 21

How do type 2 α-helices function within the membrane?

Answer 21

The type-2 α-helices link together with other type-2, and form hydrophilic pores created by packing many helices side by side

Question 22

Do water-filled pores function in transporting only water?

Answer 22

No, they function in transporting large polar molecules (amino acids, nucleotides, and sugars) and small charged molecules across the membrane

Question 23

How to β-sheets interact with the membrane to form pores?

Answer 23

The hydrophobic amino acid side chains interact on the outside with the phospholipid bilayer, while the hydrophilic side chains form the inside of the pore.

Question 24

Why do both types of pores (alpha and beta) function as selective transporters?

Answer 24

Because they only allow certain HYDROPHILIC molecules across