Membranes II

Question 1

How many residues are there per turn of an alpha helix?

How many A/residues is this?

Answer 1

3. 6

1. 5

Question 2

How are peptide bonds linked in alpha helices?

Answer 2

Peptide bond 1 is H bonded to peptide bond 4

Peptide bond 2 is H bonded to peptide bond 5
…etc

Question 3

How are antiparallel beta-pleated sheets formed?

Answer 3

Adjacent beta-strands run in opposite direction

Every other side chain extends above or below the sheet

H bonds are perpendicular to chains

Question 4

How thick is the entire lipid bilayer?

How thick is the hydrophobic region?

Answer 4

50 Angstroms

30A

Question 5

How many amino acids are needed to span the lipid bilayer?

Answer 5

In an alpha helix:
1 residue = 1.5A

SO need approx 20 residues

Question 6

What are porins?

Answer 6

Beta-barrel membrane proteins

Question 7

How can you predict membrane protein structure from sequence?

Answer 7

Hydrophobicity analysis
- each individual amino acid has a hydrophobicity value

Look for regions of hydrophobic amino acids
-> indicative of alpha-helices
=integral membrane proteins

Question 8

What doesn’t hydrophobicity analysis predict?

Answer 8

Beta-barrel membrane proteins

Question 9

What contacts the hydrophobic lipid interior?

Answer 9

Hydrophobic amino acids

Question 10

What are the most common membrane-spanning segments?

Answer 10

Alpha-helices

Question 11

How are membrane proteins assembled?

Answer 11

Almost all proteins synthesis on ribosomes

Energetically unfavourable to move large hydrophilic portions through lipid bilayer

Question 12

How do hydrophilic parts of membrane proteins get across the membrane?

Answer 12

Assemble in a protein conducting channel

Question 13

What can transmembrane helices be surrounded by?

Answer 13

Lipid

Other helices - not as hydrophobic

Question 14

What are the 4 main types of membrane proteins?

Answer 14

Transporters
Linkers
Receptors
Enzymes

Question 15

What are the 2 types of membrane transport?

Answer 15

Carrier proteins:
transport small solutes
passive or active

Ion channels

Question 16

Describe carrier proteins

Answer 16

Have specific solute binding sites

Switch conformation as to which side is open

Question 17

Describe channel proteins

Answer 17

Open = continuous channel
Selective - only some ions allowed through
Gated - opening + closing is regulated

Question 18

What are the 3 types of transport?

Which 2 are coupled transport?

Answer 18

Uniport
Symport
Antiport

Symport + antiport

Question 19

What is uniport?

Answer 19

Transports 1 type of molecule

Question 20

What is symport?

Answer 20

Transports 2 different molecules in the same direction

Against conc grad

Question 21

What is antiport?

Answer 21

Transports 2 different molecules in different directions

Question 22

Briefly explain how symporters are used in glucose transport

Answer 22

Na+ binds to symporter

= increases affinity for glucose

Question 23

Give an example of a passive transporter

Answer 23

Facilitative glucose transporters

ie GLUT1

Question 24

What is lactose permeate an example of?

Answer 24

A symporter

- transports lactose + protons in the same direction

Question 25

How are electrically charged molecules transported?

Answer 25

Electrochemical gradient
= conc grad + membrane potential

For a + charge:
Large EG when MP = -ve inside

Small EG when MP = +ve inside

Medium EG when MP = 0

Question 26

Give an example of an antiporter

Answer 26

Na-K ATPase

• pumps3 Na+ out and 2K+ in using ATP