Electron transport chain-2

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Electron transport chain-2
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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.

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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).

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• 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.

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• 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.

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• 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

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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.
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• 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
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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

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• 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
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• The transfer of electrons down the respiratory
  chain is energetically spontaneous because
• - NADH is a strong electron donor
• - Oxygen is strong electron acceptor

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

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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.

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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.

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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
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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.

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• 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.

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• 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.

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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.

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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
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• 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
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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
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• 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
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• 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

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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.

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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.

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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.
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• 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.

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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.

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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

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• 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.

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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

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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.