Lecture 6: Membrane Proteins

Question 1

Most membrane proteins consist of

Answer 1

50% lipid and 50% protein by weight.

Question 2

Compare the MW of one protein and lipid molecule

Answer 2

• proteins are much larger than lipids.

- 1 protein molecule per 50-100 lipid molecules.

Question 3

The fundamental structural elements of membranes are

Answer 3

lipids.

Question 4

Which is responsible for carrying out specific membrane functions

Answer 4

proteins.

Question 5

Name four functions of membrane proteins

Answer 5

1-transporters.
2-linkers and anchors.
3-receptors.
4-enzymes.

Question 6

Define linkers and anchors

Answer 6

• linkers: link proteins on the cytosolic face of the membrane to proteins on the non-cytosolic face.
• anchors: anchor proteins on either face to the membrane.

Question 7

Define receptors

Answer 7

• detect chemical signals on one face of the membrane and relay the message on the other.
• interact with receptor–>change in configuration–>signal.

Question 8

Some membrane proteins function as enzymes to

Answer 8

catalyze specific reactions on either side of the membrane.

Question 9

Define the fluid mosaic model

Answer 9

• fluid because lipids are in constant movement.

- mosaic because proteins are inserted in “random” places throughout the membrane.

Question 10

Two classes of membrane-associated proteins

Answer 10

1-integral membrane proteins are directly attached to the membrane, physical contact with hydrophobic tails.
2-peripheral membrane proteins are bound to the membrane indirectly, only by electrostatic bonds between integral membrane proteins.

Question 11

Orientations in which transmembrane proteins span the lipid bilayer

Answer 11

• “in-out”: one-orientation (odd number of membrane spanning regions).
• “in-in”: two orientations (even number of membrane spanning regions).

Question 12

Describe the protein structure

Answer 12

• hydrophilic region consisting of amino acids with charged and polar side chains, two aqueous terminals.
• hydrophobic region containing amino acids with uncharged non-polar side chains.

Question 13

The hydrophobic region of the transmembrane protein is located

Answer 13

in the interior of the bilayer, in physical contact with tails.

Question 14

How can we dissolve transmembrane proteins

Answer 14

they can be solubilized, only by treatments that disrupt the lipid bilayer, thus hydrophobic interactions.

Question 15

Most commonly used reagent for solubilization are

Answer 15

detergents.

Question 16

Detergents are

Answer 16

small, amphipathic, lipid like molecules.

Question 17

Name two commonly used detergents and describe their abilities

Answer 17

SDS (sodium dodecyl sulfate): strong and ionic, extracts 100% of transmembrane proteins, however destroys conformation.
Triton X-100: mild non-ionic, as it has a non-ionized but polar groups at its hydrophobic ends, extracts 50% of transmembrane proteins but keeps 3D conformation.

Question 18

How do detergents differ from lipids

Answer 18

they only have ONE hydrophobic tail and are much smaller than lipid molecules.

Question 19

Detergents cluster into

Answer 19

micelles due to their cone shape.

Question 20

Describe the process of solubilization

Answer 20

1- mixed in great excess.
2- hydrophobic ends of detergents bind to hydrophobic region of transmembrane proteins and tails of lipids, thereby separating the proteins from lipids and createing protein-detergent complexes.

Question 21

Type of bond linking amino acids

Answer 21

peptide bond.

Question 22

Three types of amino acids found in protein molecules

Answer 22

• phenylalanine= hydrophobic (uncharged non-polar side chain).
• serine=uncharged polar side chain.
• glutamate=negatively charged side chain.

Question 23

Peptide bonds in a polypeptide chain are

Answer 23

-polar and form hydrogen bonds with each other.

Question 24

1st folding patter of membrane-spanning segments

Answer 24

a-helix, where the N-H group of every peptide bond is linked by a hydrogen bond to the C=O of a neighbouring peptide bond located 4 peptide bonds away *in the same chain.

Question 25

The a-helix is generated when

Answer 25

a single polypeptide chain turns around itself forming an inelastic cylinder.

Question 26

1st type of membrane-spanning a-helix

Answer 26

• all amino acid aside chains are uncharged and non-polar (hydrophobic).
• hydrophobic amino acid side chain exposed on the outside of the a-helix, in contact with tails of lipids.
• partial or complete charges will destabilize the bilayer.

Question 27

Receptors are

Answer 27

• transmembrane proteins with a single, hydrophobic a-helix.

- responsible for extracellular signals.

Question 28

The extracellular part of receptor proteins binds the _______ while there ___________ signals to the cell’s interior.

Answer 28

ligand, cytosolic part.

Question 29

2nd type of membrane-spanning a-helix

Answer 29

• hydrophobic amino acid side chains lie on the outside of the helix.
• hydrophilic amino acid side chains lie on the inside forming water-filled pores.

Question 30

How are water-filled pores created

Answer 30

by packing five 2nd type of membrane-spanning a-helices side by side in a ring within the hydrophobic region of the lipid bilayer.

Question 31

Water-filled pores function in

Answer 31

selective transport of large polar molecules (amino acids, nucleotides, sugars) and small charged molecules (ions).

Question 32

2nd folding patter of membrane-spanning segments

Answer 32

b-sheets, where individual polypeptide chains are held together by hydrogen-bonding.

Question 33

Adjacent polypeptide chains in a b-sheet can run either in

Answer 33

1- the same direction (parallel).

2- opposite direction (anti-parallel).

Question 34

A b-sheet can be curved into a

Answer 34

cylinder, forming an open-ended b-barrel.

Question 35

The water-filled pores inside the b-barrel function in

Answer 35

the selective transport of large polar molecules and small charged molecules, with weights up to 600 daltons.

Question 36

In contrast to transmembrane proteins some integral membrane proteins are

Answer 36

anchored by covalently attaching to lipids or glycolipids.

Question 37

How do we dissociate peripheral membrane proteins

Answer 37

by treatments with polar reagents, such as solution in high pH or high concentration of NaCl, that do not disrupt the lipid bilayer.