Download Electron transport chain-2

Electron transport chain-2
Introduction
 The primary function of the citric acid cycle was
identified as the generation of NADH and FADH2 by
the oxidation of acetyl CoA.
 In oxidative phosphorylation, NADH and FADH2
are used to reduce molecular oxygen to water.
 The highly exergonic reduction of molecular oxygen
by NADH and FADH2 occurs in a number of
electrontransfer reactions, taking place in a set of
membrane proteins known as the electron-transport
chain.
oxidation-reduction potential
 High-energy electrons and redox potentials are of fundamental
importance in oxidative phosphorylation.
 In oxidative phosphorylation, the electron transfer potential of
NADH or FADH2 is converted into the phosphoryl transfer
potential of ATP.
 The measure of phosphoryl transfer potential is already
familiar to us: it is given by G°´ for the hydrolysis of the
activated phosphate compound.
 The corresponding expression for the electron transfer
potential is E´0, the reduction potential (also called the redox
potential or oxidation-reduction potential).
 A negative reduction potential means that the reduced
form of a substance has lower affinity for electrons
than does H2.
 A positive reduction potential means that the reduced
form of a substance has higher affinity for electrons
than does H2.
 Thus, a strong reducing agent (such as NADH) is
poised to donate electrons and has a negative
reduction potential, whereas a strong oxidizing agent
(such as O2 ) is ready to accept electrons and has a
positive reduction potential.
• Electrons tend to pass from the most negative carrier
to the most positive carrier (oxygen). This help
stepwise flow of electrons.”
• The standard free-energy change G°´ is related to the
change in reduction potential  E´° by:
Δ G°´= - n f Δ E°´
Where:
ΔG°´ = standard free energy
n
= number of electrons
F
= is Faraday constant (23.04 cal/volt)
ΔE°´ = the difference in the standard reduction potentials and its in volt.
• E´º in volts is measured by a responsive electrode
placed in solution containing both the electron
donor and its conjugate electron acceptor at
standard conditions.
1 M concentration,
25ºC and
pH 7
Example
• The free-energy change of an oxidation-reduction reaction can
be readily calculated from the reduction potentials of the
reactants. For example, consider the reduction of pyruvate by
NADH, catalyzed by lactate dehydrogenase.
The reduction potential of the NAD+:NADH couple, or halfreaction, is -0.32 V, whereas that of the pyruvate: lactate
couple is -0.19 V.
• To obtain reaction a from reactions b and c, we need to reverse
the direction of reaction c so that NADH appears on the left
side of the arrow. In doing so, the sign of E´0 must be
changed.
For reaction b, the free energy can be calculated with n = 2.
Likewise, for reaction d,
Thus, the free energy for reaction a is given by
Redox potential under non-standard conditions
(Nernst equation)
• Under standard conditions:
Δ G°= -n f Δ E°
• If we not operating under standard conditions we know
that
Δ G = Δ G° + RT ln Keq
Since :
ΔG =- nfE
Δ G°= -n f E°
These can be combined to give
-n f E = -n f E° +RT ln Keq
E = E° - RT / nf ln [ oxidant]/ [reductant]
Or
E = E° - RT/nf 2.303 log [ oxidant]/ [reductant]
Nernst equation is used to calculate redox potential E,
at any concentration of oxidant and reductant from Eº
When a system is at equilibrium, ∆E = 0.
We have:
∆E◦ = RT/nf ln Keq
Thus, the equilibrium constant and ∆E° are related
• The transfer of electrons down the respiratory
chain is energetically spontaneous because:
- NADH is a strong electron donor
- Oxygen is strong electron acceptor
How ATP becomes synthesized during the transfer of electrons to oxygen
The Components of the Electron Transport
Chain

The electron transport chain of the mitochondria is the
means by which electrons are removed from the reduced
carrier NADH and transferred to oxygen to yield H2O.

Electrons move along the electron transport chain going
from donor to acceptor until they reach oxygen the
ultimate electron acceptor.

The standard reduction potentials of the electron carriers
are between the NADH/NAD+ couple (-0.315V) and the
oxygen/H2O couple (0.816V).
Overview of the Electron Transport Chain
The components of the electron transport chain are organized
into 4 complexes. Each complex contains several different
electron carriers.
1. Complex I also known as the NADH-coenzyme Q reductase
or NADH dehydrogenase.
2. Complex II also known as succinate-coenzyme Q reductase
or succinate dehydrogenase.
3. Complex III also known as coenzyme Q reductase.
4. Complex IV also known as cytochrome c reductase.


Each of these complexes are large multisubunit complexes
embedded in the inner mitochondrial membrane.
Complex I:
• Also called NADH-Coenzyme Q reductase because this large
protein complex transfers 2 electrons from NADH to
coenzyme Q. Complex I was known as NADH dehydrogenase.
• Complex I (850,000 kD) contains a FMN prosthetic group
which is absolutely required for activity and seven or more FeS clusters.
• This complex binds NADH, transfers two electrons in the form
of a hydride to FMN to produce NAD+ and FMNH2.
• The subsequent steps involve the transfer of electrons one at a
time to a series of iron-sulfer complexes.
The importance of FMN.
First it functions as a 2 electron acceptor in the hydride transfer from NADH.
Second it functions as a 1 electron donor to the series of iron sulfur clusters.
 The
process of transferring electrons from
NADH to CoQ by complex I results in the
net transport of protons from the matrix
side of the inner mitochondrial membrane
to the inter membrane space where the H+
ions accumulate generating a proton motive
force.
stiochiometry is 4 H+ transported per
2 electrons.
 The
NADH + H+ + CoQ
NAD+ + CoQH2
ΔEo’ = 0.060 V – (−0.315V) = 0.375 V
ΔGo’ = −nFΔEo’ = −72.4 kJ/mol
Complex II:
• It is none other than succinate dehydrogenase,
the only enzyme of the citric acid cycle that is
an integral membrane protein, so its the only
membrane-bound enzyme in the citric acid
cycle
• This complex is composed of four subunits.
Two of which are iron-sulfur proteins and the
other two subunits together bind FAD through
a covalent link to a histidine residue.
• In the first step of this complex, succinate is bound and a
hydride is transferred to FAD to generate FADH2 and fumarate.
• FADH2 then transfers its electrons one at a time to the Fe-S
centers. Thus once again FAD functions as 2 electron acceptor
and a 1 electron donor. The final step of this complex is the
transfer of 2 electrons one at a time to coenzyme Q to produce
CoQH2.
• The overall reaction for this complex is:
Succinate + CoQ
Fumarate + CoQH2
ΔEo’ = 0.060 V – (+0.031V) = 0.029 V
ΔGo’ = −nFΔEo’ = −5.6 kJ/mol.
• For complex II the standard free energy change of the
overall reaction is too small to drive the transport of
protons across the inner mitochondrial membrane.
• This accounts for the 1.5 ATP’s generated per FADH2
compared with the 2.5 ATP’s generated per NADH.
Complex III:
• This complex is also known as coenzyme Qcytochrome c reductase because it passes the
electrons form CoQH2 to cyt c through a very
unique electron transport pathway called the
Q-cycle.
• In complex III we find two b-type
cytochromes and one c-type cytochrome.
Q-cycle:
 The Q-cycle is initiated when CoQH2 diffuses through the
bilipid layer to the CoQH2 binding site which is near the
intermembrane face. This CoQH2 binding site is called the QP
site.
 The electron transfer occurs in two steps. First one electron from
CoQH2 is transferred to the Fe-S protein which transfers the
electron to cytochrome c1. This process releases 2 protons to the
intermembrane space.
First half of
Q-cycle
 The second electron is transferred to the bL heme which
converts CoQH●- to CoQ. This re-oxidized CoQ can now
diffuse away from the QP binding site. The bL heme is near the
P-face. The bL heme transfers its electron to the bH heme
which is near the N-face. This electron is then transferred to
second molecule of CoQ bound at a second CoQ binding site
which is near the N-face and is called the QN binding site.
This electron transfer generates a CoQ ● - radical which
remains firmly bound to the QN binding site. This completes
the first half of the Q cycle.
First half of
Q-cycle
continue
 The second half of the Q-cycle is similar to the first half. A
second molecule of CoQH2 binds to the QP site. In the next
step, one electron from CoQH2 (bound at QP) is transferred to
the Rieske protein which transfers it to cytochrome c1. This
process releases another 2 protons to the intermembrane space.
second half
of Q-cycle
 The second electron is transferred to the bL heme to generate
a second molecule of re-oxidized CoQ. The bL heme transfers
its electron to the bH heme. This electron is then transferred to
the CoQ- radical still firmly bound to the QN binding site. The
take up of two protons from the N-face produces CoQH2
which diffuses from the QN binding site. This completes Q
cycle.
Second half
of Q-cycle
The net equation for the redox reactions of this Q cycle is:
QH2+ 2 cyt c1(oxidized) + 2H+
Q +2 cyt c1(reduced) +4H+
Cytochrome c is a soluble protein of the intermembrane
space. After its single heme accepts an electron from
Complex III, cytochrome c moves to Complex IV to donate
the electron
Complex IV:
 Complex IV is also known as cytochrome c oxidase because it
accepts the electrons from cytochrome c and directs them towards
the four electron reduction of O2 to form 2 molecules of H2O.
4 cyt c (Fe2+) + 4 H+ + O2
4 cyt c (Fe3+) + 2H2O
 Cytochrome c oxidase contains 2 heme
centres, cytochrome a and cytochrome a3
and two copper proteins.
 The reduction of oxygen involves the
transfer of four electrons. Four protons are
abstracted from the matrix and two protons
are released into the intermembrane space.
ATP synthetase: ATPase (Complex V)
• This enzyme complex synthesizes ATP , utilizing the energy of
the proton gradient (proton motive force) generated by the
electron transport chain.
• The Chemiosmotic theory proposes that after proton have
transferred to the cytosolic side of inner mitochondrial
membrane, they can re-enter the matrix by passing through the
proton channel in the ATPase (F0), resulting in the synthesis of
ATP in (F1) subunit.
• Coenzyme Q exists in mitochondria in the oxidized quinone
form under aerobic conditions and in the reduced quinol form
under anaerobic conditions.
• Structure is similar to vitamin K and E.
• All are characterized by the presence of polyisoprenoid side
chain:
[ CH2-CH=C-CH3-CH2]n.
• The ETC contains excess of Coenzyme Q. This is
compatible with Q acting as a mobile components of
the ETC that collects reducing equivalents from the
more fixed flavoprotein complexes and pass them to
cytochromes.
Cytochromes
• These are iron- containing electron transferring
proteins.
• They are heme proteins.
• 3 classes have been identified a,b and c
• Each cytochrome molecule in its ferric (Fe 3+) form
accepts one electron and reduced to the ferrous state
(Fe2+).
• In addition to iron, Cyt a3 also contain 2 bound
copper atom which undergo cupric (Cu 2+) to cuprous
(Cu+) redox changes during electron transfer.
Uncouplers
• Electron transport and phosphorylation can be
uncoupled by compounds that increase the
permeability of the innermitochondrial
membrane to protons in any place.
i.e Uncouplers causes electron transport to be
proceed at a rapid rate without the establishing
of proton gradient
e.g : 2,4 dinitrophenol
• The energy produced by the transport of
electrons is released as heat rather than being
used to synthesize ATP.
• In high doses, the drug aspirin uncouple
oxidative phosphorylation. This explain the
fever that accompanies toxic overdoses of
these drugs.
Electron transport inhibitors
• These compounds prevent the passage of electrons by
binding to chain components, blocking the
oxidation/reduction reaction
• Inhibition of electron transport also inhibits ATP synthesis.
e .g:
- Amytal and Rotenone block e- transport between FMN and
Co Q.
- Antimycin A blocks between Cyt b and Cyt c
- Sodium azide blocks between Cyt a a3 and oxygen
Ionophores
• Ionophores are termed because of their ability to
form complex with certain cations and facilitate
their transport across the mitochondrial membrane.
So ionophores are lipophilic
e. g: Valinomycin: allows penetration of K+ across
the mitochondrial membrane and then discharges
the membrane potential between outside and the
inside
( i.e: does not affect the pH potential).
Nigericin: also acts as ionophore for K+ but in
exchange with H+. It therefore abolishes the pH
gradient.