Transport proteins control

Ion

In

Out

Potassium

140

mM

1 – 4.5

mM
Sodium

5 – 15

mM

145

mM

Magnesium

5

mM

1 – 2

mM

Calcium

>

0.5



M

2.5 – 5

mM

Chloride

4

mM

110

mM



Ionic composition of

intracellular fluid



osmolarity



Cell volume



Intracellular pH



Membrane potential



Ions gradients



Exchange of molecules

Passive flux down the gradient of chemical potential

The chemical potential is the

free energy per mole of

compound transported.

i

n

n

p

T

i

i

j

n

G























,

,

i

i

i

dn

dG





µ

0

, c

o

are the chemical potential and

concentration (1M) under standard
conditions.

]

[

]

[

ln

0

0

c

c

RT









A potential difference across a biological

membrane:

~

70 mV

The voltage gradient is

20,000,000

V/m

.

For e charged solute the electrochemical
potential is defined













zeF

c

c

RT

dn

dG

]

[

]

[

ln

~

0

0







o

and μ

o

are

for the standard

state.

The effect of changes in

external chloride ion
concentration on the

membrane potential of an

isolated frog muscle fibre

(Hodgkin & Horowicz, 1959)

One electrode monitors

membrane potential (V

m

)

and the other passes enough

current (I

m

) through the

membrane to clamp V

m

to a

predetermined command

voltage (V

command

).

Something controls the membrane potential

Movement of molecules across cell

membranes

3)

Facilitated diffusion (10

2

-10

4

ions/sec)

4) Active transport (1-1000 ions/sec)

5)

Bulk transport

A)

Exocytosis

B)

Endocytosis

1)

Diffusion through bilayer

2)

Difusion through a pore (10

7

-10

8

ions/sec)

Two requirements of membrane transport



Energy to move

substances



Route through the membrane



the lipid bilayer – nonspecific



facilitated by proteins –

specific

1.

Light

- powers

H

+

pumping

- bacteriorhodopsin

proteins undergo alternating cycles of

oxidation/reduction, which powers

H

+

pumping

2.

Electron transfer

(substrate oxidation)

during metabolism, electrons are passed along

the electron transport chain

3.

ATP

- large class of ATP-driven ion transporters.

Direct coupling of metabolism

to the transport process.

Inhibited by metabolic inhibitors such

as cyanide and dinitrophenol.

Active

transport

S

1

S

2

ATP

ADP + Pi

Side 1

Side 2

Active
Transport



ATP activates the protein by giving up a

phosphate



Transport protein must be activated

Primery active transport

(direct energy utilizing active transport)

Na

+

/K

+

-ATPase Ca

2+

-

ATPase

H

+

-ATPase

H

+

/K

+

-

ATPase



Binding of ATP changes protein shape and affinity

for solute

(a gene family exhibiting

sequence homology)

P-Class Pumps

ATP

C

O

O

P

O-

O-

O

C

O

OH

ADP

Enzyme-

Enzyme-

P

i

H

2

O

P-class ion pumps

Secondary Active Transport

(Coupled Transport)

Driven by chemical or electrochemical gradients

Expenditure of metabolic energy is

INDIRECTLY coupled to

translocation.

Uniport

– transport of a single solute driven only by ΔΨ

B

z

S

S

Z

O

Z

I











]

[

]

[

log

10

2.3RT/F = 59 mV



B at

25

0

C

Nerst equation

S

+z

Et the equilibrium

0

~

~



















Z

Z

S

H

S

n

G





n is the number of moles of H

+

that would

have to move down the



~

J





gradient to

generate the accumulation.

pH

RT

F

H













3

.

2

~

















zF

S

S

RT

Z

O

Z

I

S

Z

]

[

]

[

log

3

.

2

~

10



0

)

3

.

2

(

]

[

]

[

log

3

.

2

~

~

10





































pH

RT

F

n

zF

S

S

RT

G

Z

O

Z

I

H

S

S

Z

Z





B

z

n

pH

n

S

S

Z

O

Z

I















)

(

]

[

]

[

log

10

2.3RT/F = 59 mV



B at

25

0

C

Symport

(cotransport) amino acids

and sugers

S

+z

H

+

Antiport

(countertransport)

restricted to ions

If n = z, then the charge

movement would be neutral and
has no effect.

pH

n

S

S

Z

O

Z

I











10

log

Combining 

~

H

and



~

S+Z

pH

n

Z

z

n

S

S

Z

O

Z

I















)

(

log

10

n is the number of moles of
H

+

that would have to move

againsty the



~

J





gradient

to

generate

the

accumulation.















zF

S

S

RT

Z

I

Z

S

Z

0

10

log

3

.

2

~



pH

RT

F

H













3

.

2

~



S

+z

H

+

The consequence of the

transfer of charged

malecules

Electrogenic

Electroneutral

Master pump !!!

The master pump concept

Creates transmembrane gradient of a

selected ion.

The electrochemical potential energy is

stored only across the membrane in which
the pump is located.

Other ions and molecules are transported

across the membrane by coupling their
movement to the movement of the selected
ion.

Ion gradients generally store smaller

packets of energy than ATP - coupled
transporters (increased efficiency).

Coupling transport to a single master pump

serve a

control function

.

Atributes of a master pump

High efficiency

Low dissipation

(leakage current)

is

the reason that pumps almost exclusively
transport

the

relatively

impermeant

inorganic cations.

High capacity

– the ion gradient involve

concentrations that are relatively large
compared to the concentrations of the
compounds that are to be transported.

Cytosol

David Stokes,
Univ. Virginia

Na

+

,K

+

-ATPase

Abundance reflects

importance

– Erythrocyte = 20-30 copies
– Heart cell or neuron > 100,000

copies

Substrates

– 1 ATP

(intracellular)

– 3 Na

+

per cycle

- obligatory, no other ion can

substitute.

– 2 K

+

per cycle

- an extracellular cation is

obligatory, but K

+

and Rb

+

both work well.

– Other monovalent ions have finite but low
activity (NH

4

+

> Cs

+

> Li

+

)

Pump Activity is Electrogenic

•

Maintenance of high intracellular K

+

needed

for optimal intracellular enzyme activity.

Na

+

,K

+

-ATPase Functions

•

Maintenece of osmotic stability and cell volume.

•

Restoration of potentials in

excitable cells.

•

Generates anergy for transport in the form of Na

+

gradient.

•

Generation of heat



20% of body heat

in mammals is from
the basal activity of
Na

+

,K

+

-ATPase.



> 30% of metabolic

energy in resting
mammals is consumed
by Na

+

,K

+

-ATPase.

Na

+

and K

+

bind to separate sites.

When the transported

substrate

serves

a

regulatory function, then it
may be desirable to control
its

concentration

separately.

A transport system might not be

coupled to the master pump

When the transport

system has a high capacity
itself, it may adversely
affect the ion gradients
established by the master
pump.

Ca

2+

-ATPase

Integration of a transport systems !!!

Accumulation of ions and sucrose in the plant vacuole.

Two types of proton pumps:



V-class H

+

ATPase



a pyrophosphate-hydrolyzing proton pump

They generate a lowered luminal pH and an inside-
positive electric potential – the inward pumping of
H

+

ions.

The inside-positive

potential powers the

movement of Cl

−

and

NO

3

−

from the cytosol

through separate

channels.

Proton antiporters,

powered by the H

+

gradient, accumulate

Na

+

, Ca

2+

, and sucrose

inside the vacuole.

Na

+

/glucose

cotransporter

Glucose

transport

requires Na

+

gradient

Coupling is 2:1













































i

m

i

out

in

i

i

out

i

in

i

i

i

i

i

F

z

C

C

RT

n

n

n

G









ln

)

(

At
equilibrium:

0

]

[

]

[

ln

]

[

]

[

ln







































m

out

in

out

in

F

Na

Na

RT

G

G

RT

G



)

/

exp(

]

[

]

[

]

[

]

[

RT

F

Na

Na

G

G

m

in

out

out

in























High

C[glucose]

Low

Low

The neutral

pH and

electroneutrali

ty of the

cytosol is

continuously

maintained.

Acidification of the

stomach lumen

The role of H

+

/K

+

ATPase

This is the largest

concentration

gradient across a

membrane in

eukaryotic

organisms!

Ion channels are

enzymes that catalyze

the flow of ions across

cell membranes

causing picoamp

current.

The catalytic rate is on the order of 10

7

per second.

ions/s

10

C

10

1.6

ion

1

s

C

10

A

10

1pA

7

19

12

12

















How much is a picoamp of current?

Channels

(

gated pore)

secondary active transport

Counter-transport

:

Na

+

/H

+

,

HCO

3



/Cl



,

K

+

/H

+

,

Ca

2+

/H

+

, Na

+

/Ca

2+

Co-transport:

Na

+

/glucose, Na

+

/amino acid,

Na

+

/K

+

/Cl

-

•

Cystic

fibrosis

•

Epilepsy

•

Diabetes

•

Migraines

•

Neuro-toxins

Channels
malfuncti
on

Properties of Ion Channels

Membrane-spanning protein

Hydrophilic ion conductive pathway



water-filled



traversing ion must lose hydration shell

Gating



Mechanical gating

(MscL)



Voltage gated channels



Ligand-gated channels



Both voltage and ligand gating

Selective



size



charge



charge distribution

Voltage Gated Sodium

Channel

Voltage-dependent
gating

Receives acetyl choline

released from the

presynaptic cleft and

reinitiates an action

potential by allowing Na

+

and K

+

ions to pass through

the channel

Ligand-gated channel –

acetyl choline

gated channel

1.

Five membrane spanning

subunits [

2

] all similar.

2.

An allosteric protein (three

conformations; open, closed, and
inactive).

3.

Acetylcholine binding promotes opening the closed channel

4.

Open channels allow Na

+

but not

Cl

–

to pass.

Methods for Studying Ion Channels

Biochemistry

– agonist, antagonist or drug

binding
– isolation and purification
– reconstitution
– radioactive ion flux

Molecular
biology

– sequencing,
cloning,
mutagenesis

Structural biology

– microscopy, crystallography,
NMR, ...

Electrophysiology

– tissue slice
– extracellular recording
– intracellular recording
– whole-cell recording
– single channel

recording

Voltage-gated potassium channels

Membrane voltage determines

whether channels are open – provide

a way for the membrane voltage to

feed back onto itself.

It has a diffusion rate of

10

8

ions per second.

One K

+

ion is dehydrated,

transfered, and rehydrated

every 10 ns.

Roux & McKinnon, Science (1999)



















w

m

r

Q

E





1

1

2

1

2

Hydrophobic barrier

Born-Formula

There are about 7 water

molecules in the first
hydration

shell

of

potassium ion.

Each water molecule

stabilizes

the

ion

by

approximately 24 kT.

Voltage-gated K

+

channels

mediate outward K

+

currents during nerve action

potentials.

Selectivity filter



K

+

ions encounter four

layers of carbonyl oxygen
atoms & a layer of threonine
hydroxyl oxygen atoms.



Four K

+

ion binding sites.



K

+

is surrounded by eight oxygen atoms from

the protein

- four ‘above’ and four ‘below’.
- very similar to water molecules around
hydrated K

+

.

C=O atoms of the protein backbone

form selectivity filter (4



Tyr-Val-Gly-

Tyr-Gly

).

The sequence is conserved in all K

+

-

channels.

Zhou et al. Nature (2001)

The inner

pore is lined

with

hydrophobic

residues.

Central cavity contains

K

+

ion that is surrounded

by 8 water molecules

Helices represent

dipoles which attract

cations

Why does the ion coordination required for high

selectivity not cause the ions to bind too tightly

& prevent rapid diffusion through the pore?

An ion enters the queue

from one side of the

filter while a diferent ion

exits from the opposite

side.

Selectivity filter contains more than one ion –

repulsion between closely spaced ions will helps

overcome the intrinsic binding site affnity.

On average, two K

+

ions

present at a given time

separated by one water

molecule.



The Val and Tyr hold the

selectivity filter at a certain
diameter by hydrogen bonding
with the inner helix.



They form hydrogen bonds

which acts as tight springs that
will not allow the pore to
collapse.



The "springs" prevent the selectivity

filter from interacting with cations
smaller than K

+

.

Radius(Å) 1.33 1.48 1.69 0.95
0.60

Ion

K

+

~ Rb

+

> Cs

+

>> Na

+

> Li

+

The selectivity is based on the size

difference between K

+

and Na

+

The conductive conformation of the filter requires the two K

+

.

Entry of the second K+ ion induces a conformational change.

A simple thermodynamic consequence



Some fraction of the ion binding energy is used

to change the filter’s structure.



Consequently ions bind less tightly than if a

conformational change did not occur.



Weak binding is a prerequisite for high conduction rates.

KcsA: crystal structures at high and

low K

+

concentration.

Zhou et al. Nature
(2001)

Transfer is isoenergetic



conductivity close to

diffusion limit.