Transporters & Ion Channels 1

Question 1

Lipid Membrane Permeability for polar molecules

Answer 1

Impermeable to most polar molecules

• Requires transporters (many drugs have similar structure to nutrients so still use transporters - e.g. Bestatin using peptide transporters)
• Some drugs diffuse using their hydrophobicity

Question 2

Thermodynamics of transport

diffusion vs active transport

Answer 2

Diffusion

• No ATP hydrolysis or co-transport
• Relies on concentration difference, transmembrane potential, osmotic pressure

Active Transport
- ATP driven against normal diffusion direction

Question 3

3 main Transmembrane transporters

Answer 3

1) Channels (ion channels, aquaporins, pores)
2) Transporters (Uni, sym or anti- porters)
3) ATP-powered pumps

Question 4

Exceptions to the classification of 3 transporters

Answer 4

1) Cystic Fibrosis Transmembrane conductance Regulator (CFTR)
- ion channel belonging the the ABC transporter family of ATP-powered pumps

2) Chloride Channles (ClC)
- Often uniporters, other times antiporters (engaging protons)
- A uniporter transporting ions = ion channel?!

Question 5

4 mechanisms of Transmembrane Transport

Answer 5

1) Simple Diffusion
- Diffuse across membrane/pores
- e.g. O2, steroid hormones, lipophilic drugs

2) Facilitated Diffusion
- Diffusion via a specific protein
- e.g. Glucose and amino acids through uniporters

3) Active Transport (primary)
- Solute transported against gradient using ATP hydrolysis
- Requires specific protein, no co-transport
- E.g. ions and small hydrophilic molecules

4) Co-transport (secondary active transport)
- Driven by movement of a co-transported ion downs its gradient, not coupled to ATP hydrolysis
- e.g. sucrose through antiporters

Question 6

Beta barrel proteins

Answer 6

• Beta barrel pores
• Beta barrel transmembrane (TM) proteins
• Adhesion molecules (e.g. OmpX)

Question 7

Transmembrane Beta-barrel structure

Answer 7

• 8-22 Beta-strands per monomer/barrel

- Hydrophobic external residues, hydrophilic internal residues

Question 8

OmpF Beta-barrel pores

Answer 8

Dominate outer membrane porins in E.coli

• Passive diffusion channels normally found in trimer form
• Offer rapid, low-selectivity diffusion
• Ionic environment determines pore size

Question 9

Low-level specificity Beta Barrel pores

Answer 9

PhoE transports phosphate

• Very similar structure to OmpF but with a change in the constricting loop
• Has 2 extra positively charged amino acids, somewhat increasing its specificity for negatively charged molecules

Question 10

Medium-level specificity Beta Barrel pores

Answer 10

LamB transports maltose and maltodextrins

• 3 loops with 6 aromatic residues are somewhat hydrophobic
• Allows for ‘guided diffusion’

Question 11

Fully-specific beta Barrel pores

Answer 11

FepA & FhuA uptake Siderophores (small iron-binding molecules)
- Very specific active uptake due to ‘plug domain’ coupled to TomB

Question 12

Gap Junctions/Connexins (pores) structure

Answer 12

Monomers = 4 TM alpha helices

Typically 6 monomers per bilayer spanning domain (2 hexamers connect)

Question 13

Gap Junction function and control

Answer 13

Found spanning 2 membranes in muticellular organisms
- used for communication as well as flow of nutrients and ions

Opening/closing controlled by phosphorylation
- pore size regulated by different connexin mixtures

Question 14

Pore diffusion kinetics

Answer 14

A (out) <=====> A (in)

Question 15

Facilitated diffusion: GLUT1 transporter

Answer 15

Transports glucose into RBC and across BBB

• mutation can cause De Vivo disease
• Uniporters belonging to the Major Facilitator Superfamily (MFS), the largest uniport/cotransporter family (typically 12 TM alpha helices)

Question 16

GLUT4 transporter

Answer 16

Strong connection with diabetes when dysfunctional

• often stored in intracellular vesicles

Question 17

GLUT’s ‘alternating access’ mechanism

Answer 17

GLUT transporters present slightly different binding sites on both sides of the membrane (each inhibited by different compounds)

1) Outward open
2) Ligand-bound occluded
3) Inward open
4) Ligand-free occluded

2 & 4 required to prevent unwanted diffusion of small molecules/ions/water

Question 18

ADP/ATP Carrier Proteins (AAC)

Answer 18

Exchange ADP/ATP in and out of mitochondria

• exclusive to eukaryotes
• 6 TM alpha helices

Neither cotransporters or antiporters

• Function by facilitated transport, acting as uniporters with 2 substrates
• Being negatively charged, transport is influenced by proton-motive force