Title: Electron transport chain-2
1Electron transport chain-2
2Introduction
- 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.
3oxidation-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 E0, the reduction
potential (also called the redox potential or
oxidation-reduction potential).
4- 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.
5- 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 - ? G - n f ? E
- Where
- ?G standard free energy
- n number of electrons
- F is Faraday constant (23.04 cal/volt)
- ?E the difference in the standard reduction
potentials and its in volt.
6- Eº 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
7Example
- 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 NADNADH couple,
or half-reaction, is -0.32 V, whereas that of the
pyruvate lactate couple is -0.19 V.
8- 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 E0 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
9Redox potential under non-standard conditions
(Nernst equation)
- Under standard conditions
- ? G -n f ? E
- If we not operating under standard conditions we
know that - ? G ? G RT ln Keq
- Since
- ? G - n f E
- ? G -n f E
- These can be combined to give
- -n f E -n f E RT ln Keq
-
10- 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
11- The transfer of electrons down the respiratory
chain is energetically spontaneous because - - NADH is a strong electron donor
- - Oxygen is strong electron acceptor
12How ATP becomes synthesized during the transfer
of electrons to oxygen
13The 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).
14Overview 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.
15Complex 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 Fe-S 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.
16 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. - The stiochiometry is 4 H transported per 2
electrons.
NADH H CoQ
NAD CoQH2 ?Eo 0.060 V (-0.315V) 0.375
V ?Go -nF?Eo -72.4 kJ/mol
17Complex 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.
18- 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.
19- The overall reaction for this complex is
- Succinate CoQ Fumarate
CoQH2
?Eo 0.060 V (0.031V)
0.029 V - ?Go -nF?Eo -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 ATPs generated per
FADH2 - compared with the 2.5 ATPs generated per
NADH.
20Complex III
- This complex is also known as coenzyme
Q-cytochrome 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.
21Q-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
22- 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
23continue
- 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
24- 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
25- 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
26Complex 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.
27ATP 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.
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30- 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-CHC-CH3-CH2n.
31- 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.
32Cytochromes
- 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.
33Uncouplers
- 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
34- 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.
35Electron 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
36Ionophores
- 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.