Membrane channels and pumps

Stryer ch 13

Concentration & potential differences across cell membranes

MEMBRANE TRANSPORT SYSTEMS

Regulate the cellular volume

Control the transfer of metabolites across membranes

Maintain ionic and molecular gradients across membranes

DIFFERENT TYPES OF TRANSPORT SYSTEMS

Passive diffusion e.g, hydrophobic molecules

Facilitated diffusion by channels e.g. transfer of

metabolites and ions

Active transport

µ’’

!’’, c’’

!’, c’

µ’

The thermodynamics of transport across membranes

µ’ = µ0 + RT ln c’ + zF!’

µ’’ = µ0 + RT ln c’’ + zF!’’

"G = µ’’ - µ’ = RT ln c’’/c’ + zF (!’’ - !’)

"G > 0 active transport

"G < 0 passive transport

[Na+] [K+]

Outside cell 140 mM 5 mM

Inside cell 10 mM 100 mM

"G = RT ln c’’/c’ + zF "!

Transport of 3 mol Na+ from inside to outside of a cell at a

transmembrane potential of 70 mV

"G = 3 RT ln [Na+]out / [Na+]in + 3 F (!out - !in)

= (20.5 + 20.5) kJ = 41 kJ

Transport of 2 mol K+ from outside to inside of a cell

"G = 2 RT ln [K+]in / [K+]out + 2 F (!in - !out)

= (15.5 - 13.5) kJ = 2 kJ

At physiological conditions ATP ADP + P "G = -60 kJ

Conditions required for a protein to work as a

transmembrane pump

• Protein must contain binding sites to fix substances

which should be transported

• Protein must exist in two conformations

• Binding sites must have different affinities in the two

conformations

Free energy changes for transport processes

"G = RT ln c’’/c’ + zF "!

Structure of the Ca2+ ATPase

The transport mechanism of the Ca2+ ATPase

The transport mechanism of the Ca2+ ATPase

The transport mechanism

of the Na+- K+ ATPase

[Na+] [K+]

Outside cell 140 mM 5 mM

Inside cell 10 mM 100 mM

Some co-transport systems

Molecule Ion gradient Organism

transported or tissue

Glucose Na+ Intestine, kidney

Amino acids Na+ Mouse tumor cells

Lactose H+ E. coli

Scheme of active transport by Lactose Permease

Scheme of active transport by Lactose Permease

X-ray structure of Lactose Permease

Structural changes of Lactose Permease during active transport

Ion Channels

Morphology of two types of mammalian neurons

Transport rate of ions across membranes

Pumps ca 1000 ions/second

Channels 106 - 107 ions/second

Some properties of ion channels

1. Ion channels can be highly selective

2. They exist in an open and a closed state

3. The transition between open / closed stae is regulated

4. Open states often spontaneously convert to inactivated form

Schematic representation of a synapse

Voltage-gated ion channels

Ligand-gated ion channels

Synaptic vesicles: 104 Ach

300 vesicles release Ach

[Ach] 10 nM > 500 µM in ms

Ach binds to postsynaptic

membrane

Membrane depolarization

Patch clamp technologyE Neher & B Sakmann 1976

Patch clamp recording of acetylcholine receptor channel

+

Electrical eel: source of AchR

Torpedo Marmorata

Action potentials are mediated by transient changes of Na+

and K+ permeabilities

Opening of

AchR channel

Opening of K+ channels &

restoration of resting potential

Topology of Na+ channel

Sequence relationships of ion channels

Structure of potassium channel

Details of structure of

potassium channel

Considerations on the

energy of ion selectivity

Summary

Specific channels rapidly transport ions across membranes

Ions flow down their concentration gradient

Responsible for nerve impulses

- Specificity for certain ions

- Existence of open & closed states

- Regulated by ligands or trans-membrane potential