

Membrane

potentials

Energy Currency



"High energy" chemical

species



Redox reactions



Transport

works

Energy Utilization



Chemical

reactions



Mechanical

work

Oxidation of carbon atoms of

glucose is the major source of

energy in aerobic metabolism

Gain of electrons or hydrogen =

reduction =

energy increase

Loss of electrons or hydrogen =

oxidation =

energy decrease

Redox reactions

Oxidation of

carbon is

spontaneous.

In c re a s in g o x id atio n o f c a rb o n

H

C

H

H

H

H

C

H

O H

H

H

C

H

O

O

C

O

O H

C

H

O

]

[

]

[

ln

'

ox

red

o

A

A

nF

RT

E

E





Redox

potential - empirical measure of tendency to gain e's

Where [

A

red

] = [

A

ox

], E = E°'

E°'

is the

mid-point potential

, or standard redox

potential, the potential at which

[oxidant] =

[reductant]

for the half reaction.

ox

red

red

ox

B

A

B

A







Whenever one substance is oxidized, another substance is reduced

The more negative the potential

, the greater the

tendency of the reduced compound to donate electron
(reducing ability).

The more positive the potential

, the greater the

affinity for the electron of the oxidized compound
(oxidizing ability).

An electron transfer reaction is

spontaneous

(negative ΔG) if E°' is

positive

.

For an
electron
transfer:

'

'

'

'

)

(

'

)

(

'

)

(reductant

'

)

(

o

o

o

E

nF

G

E

E

E

E

E

o

donor

o

acceptor

o

o

oxidant



















ΔE, unlike ΔG, is not a state function and

depends on the path of the reaction.

For a series of sequential redox reactions

A  B  C  D

G

AD

= G

AB

+ G

BC

+

G

CD

The free energy is additive

E

AD

≠ E

AB

+ E

BC

+

E

CD

Possibility for

biological

modifications !!!

The location of the generation
site of ATP is the same as the
site

which

provided

the

energy for its generation.

Substrate level

phosphorylation

nATP

nP

nADP

i





O

H

e

H

O

2

2

2

4

4











The process in which ATP is formed as a result
of the transfer of electrons from NADH and
FADH

2

to O

2

by a series of electron carriers.

Oxidative Phosphorylation

Stages of

metabolism

Glycolysis -

found in all living organisms.

It occurs in the cytoplasm outside the

mitochondria.

When rapid production of ATP is needed.

The outer membrane

contains porins, voltage-

dependent anion channels which regulate metabolite
flux, ie., phosphate, Cl

-

, adenine nucleotides and

organic anions.

There are no pH and potential

gradients across the

mitochondrial outer membrane.

 porin proteins  large pores

(5,000 -10,000 D)

 permeable to ions and
molecules < 1 kDa

The inner membrane

is impermeable to ions and

polar molecules. Specific transporters shuttle
metabolites such as ATP, pyruvate, and citrate.

It serves two important functions in energy
generation.

capacitor

- it allows charge separation to

build up between the cytoplasm and the outside
of the cell

structural

- the membrane holds many of the

components involved in electron transport in the
an exact confirmation necessary to enable them
to perform their duties correctly

Krebs Cycle

Pyruvate

is carried into the mitochondria where it

is converted into Acetyl Co-A which enters the
Kreb's cycle.

Peter Mitchell

Nobel Prize in Chemistry,

1978

Chemiosmotic Hypothesis















n

p

i

nH

O

H

ATP

nH

P

ADP

2

The ATPase activity and the transmembrane
H

+

flux are reversible:

One of the great
unifying principles
of 20th century
biology

pH

RT

F

H













3

.

2

~



Electron transport (oxidation) and ATP

synthesis (phosphorylation) are

coupled by a proton gradient across

the inner mitochondrial membrane.

Energy Transformations

Electron–motive force

(NADH & FADH

2

)

Proton-motive force (a

proton gradient (ΔpH = 1.0

units; 8.0 matrix vs. 7.0

peri-mito]) and a membrane

potential (140mV; in(-)

out(+)).

Phosphoryl-transfer potential

in the form of ATP

The chemiosmotic process

the movement of substances across
a membrane is coupled to chemical

reactions.

Mitochondrial respiratory chain

Complex I

- Transfers e- from NADH to quinone pool &

pumps H

+

.

Complex II

- Transfers e

-

from succinate to

quinone pool.

Complex III

- Transfers e

-

from quinol to cyt. c &

pumps H

+

.

Complex IV

- Accepts e

-

from cyt. c, reduces O

2

to H

2

O

& pumps H

+

.

Complex V

- Harvests H

+

gradient &

regenerates ATP.

Electronic

energy

gradient

in

mitochon

dria

pH

F

RT

F

p

H

















3

.

2

~



“Proton-motive

force”,



p.



H+

= 0.224 V = ~22

kJ/mol

CHEMICAL

- H

+

concentration gradient

pH (0.5) – 37%

ELECTRICAL

-

membrane

potential 

(

150mV

) – 63%

The voltage gradient experienced by each of

proton pumps is about 30x10

6

volts/m.

The mitochondrial

membranes is rich

in an acidic

phospholipid -

cardiolipin which

makes it

impermeable to

proton leaks.

ATP synthesis

ATP Synthase

- makes 100 ATP per 300

H

+

per sec

Synthesis of ATP from ADP and orthophosphate is
coupled to a proton flux.















n

p

i

nH

O

H

ATP

nH

P

ADP

2

C

6

H

12

O

6

+ 6O

2

+ 36 ADPs + 36 P

i

 6CO

2

+ 6H

2

O +

36 ATPs

36 ATPs can be generated from one

molecule of glucose



























out

in

H

H

H

F

RT

]

[

]

[

ln





















out

in

in

out

ADP

ATP

ADP

ATP

F

RT

]

[

]

[

]

[

]

[

ln



The membrane potential
will

drive

ATP/ADP

exchange

in

the

direction of ATP efflux
and

ADP

influx

–

electrically dissipative.

Adenine nucleotide translocase

catalyzes

1:1 exchange of ADP for ATP.



























out

in

H

H

H

F

RT

]

[

]

[

ln





PO

PO

4

4

–

–

–

–

enters mitochondria via PO

enters mitochondria via PO

4

4

–

–

/OH

/OH

–

–

exchange (electroneutral).

exchange (electroneutral).

in

out

out

in

out

in

H

H

OH

OH

PO

PO

]

[

]

[

]

[

]

[

]

[

]

[

4

4

















PO

4

–

will accumulate in

mitochondria because of
the higher internal OH

–

concentration.

Biological question

How does a molecular motor convert
chemical energy, a scalar quantity, into
directed motion, a vector?

Physical idea

Mechanochemical coupling arises from
a free energy landscape with a direction
set by the geometry of the motor and its
track. The motor executes a biased
random walk on this landscape.

„Active” transport system

The motor, the transport complex and

the link between the two.

Molecular motors transduce chemical

energy into mechanical motion.

Motors are driven by ATP

ATP hydrolysis ~ 25k

B

T = 1×10

-

19

J.

Force ~ 1 to 100 pN

Thermal energy k

B

T ~ 4.1 × 10

-21

J

Thermal force ~ 1 pN

Molecular machines are encoded by a

genetic material and have the potential

to evolve.

The relation

between

molecular device

and other

devices

determines its

mode of action.

The Feynman Thermal

Ratchet

T

1

T

2

Works only if

T

1

>T

2

!!

The decay of temperature gradient

over 10 nm disappears within ns.

Wrong

model !!

Motor protein

conformational

change take place

within µs.

Molecular machine is an

isothermal engine

(not heat engines)

Principles of directed motion

of molecular motors and

pumps



Moving between wells occurs by thermal activation

over barriers.

Machine operates even after

macroscopic equilibrium has been

reached.



Time scales are determined

by depth of energy wells and
height of barriers.

T

k

G

B

e

time

waiting









They operate at energies close to kT and

fluctuations

play a central role.



Molecular motors operate in an environment

where viscosity dominates inertia (velocity if
proportional to force).

Only potential energy is
stored.

Friction dominates inertia.

1.

The chemical cycle is

autonomous.

2.

The binding energy to the track depends on

the state of the chemical cycle.

3.

The motor feels an asymmetric potential

energy as a function of position.

The structure of a molecular machine



A track with spatial

asymmetry.

Track

Machine



A site which binds to the track.



Out-of-equilibrium process coupled to a location.



A

catalitic

site

hydrolyzing ATP.

ATP



The allosteric interaction coupluing the

ATPase cycle to the track binding.

Molecular devices found in cells

Catalists

– enhance the rate of a chemical

reaction (enzymes)

Machines

– actively reverse the natural

flow of some chemical or mechanical
process by coupling it to another one.

Cyclic machines

–

process some external
source of free energy.

One-shot machines

–

exhaust some internal
source of free energy

Protein

translocat

ion

Enzymes work

by reduces the

activation

energy of a

reaction.

An enzyme

cannot alter the

net



G of the

reaction.

An enzyme

binds to the

transition

state.

A large c

P

can

reverse

the sign of G

reversing the
reaction.

Enzymes can be regarded as

a cyclic machines.

High c

S

rises the left end of the free

energy landscape.

It operates in cycles, but the system’s free energy

falls by G in each step.

The potential energy surface
is periodic.

Energy balance on a molecular

motor

The mechanism of the flashing

Brownian ratchet

The ratchet potential
exhibits

broken

spatial symmetry

V(x + L) = V(x)

Molecular motors

F

0

F

1

-ATPase

Myosin

Flagella motor

Dynein

Kinesin