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Electron transport chain-2

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Title: Electron transport chain-2


1
Electron transport chain-2
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.

3
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 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

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 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
9
Redox 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

12
How ATP becomes synthesized during the transfer
of electrons to oxygen
13
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).

14
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.

15
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 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
17
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.

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.

20
Complex 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.

21
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
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
23
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
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

26
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.

27
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.

28
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  • 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.

32
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.

33
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

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.

35
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

36
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.
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