Electron Transport and Oxidative Phosphorylation

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Electron Transport and Oxidative Phosphorylation
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An Overview

• Biological oxidations are catalyzed by
  intracellular enzymes. The purpose of oxidation
  is to obtain energy.
• Electron Transport Electrons carried by reduced
  coenzymes (NADH or FADH2) are passed sequentially
  through a chain of proteins and coenzymes (so
  called electron transport chain)to O2 .
• Oxidative Phosphorylation Coupling e- Transport
  (Oxidation) and ATP synthesis (Phosphorylation)
  .
• It all happens in mitochondrion or at the inner
  mitochondrial membrane (eukaryotic cells)

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mitochondrion
the mitochondrion contained the enzymes
responsible for electron transport and oxidative
phosphorylation
In inner membrane knobs
Impermeable to ions and most other compounds
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Reduction Potentials
E0standard reduction potential.
Crucial equation ?Go' -nF
?Eo'
The relative tendency to accept e-s and become
reduced.
Number of electrons transferred in the redox
reaction
? Eo'(acceptor) - Eo'(donor)
Faradays constant (96485 J/volt/mole)
If ? Eo' is positive, an electron transfer
reaction is spontaneous (?Go' lt0)
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• Fumarate2H2e-? succinate 0.031
• FAD 2H2e-? FADH2 0
• NAD 2H2e-? NADHH -0.32
• SuccinateFAD ? Fumarate FADH2
• ?G' - n F ?E'
• Eo' Eo'(acceptor) - Eo'(donor)
• ?G -296485(0-0.031) 5.98KJ/mol
• Succinate NAD ? Fumarate NADHH
• ?G -296485(-0.32- 0.031) 67.7KJ/mol

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• Removal of H across a C-C bond is not
  sufficiently exergonic to reduce NAD,but it does
  yield enough energy to reduce FAD.
• Thats why succinate dehydrogenase uses FAD other
  than NAD as coenzyme.

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Electron Carriers
The transfer of electrons is not directly to
oxygen but through coenzymes
There are 2 sites of entry for electrons into the
electron transport chain
NAD or FAD
Both are coenzymes for dehydrogenase enzymes
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Nicotinamide coenzymes NAD
Always a 2-electron reaction transferring 2 e-
and 2 H
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The flavin coenzymes / flavoproteins
FAD Always a 2-electron reaction transferring 2
e- and 2 H
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Oxidation and reduction of flavin coenzymes
it can accept/donate 1 or 2 e-. FMN has an
important role in mediating e- transfer between
carriers that transfer 2 e- (e.g., NADH) and
those that transfer 1 e- (e.g., Fe).
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• Role of FMN mediating between 2e- 1e- carriers
• For example, when NADH donates electrons to the
  respiratory chain, the initial electron transfers
  are
• NADH H FMN ? NAD FMNH2
• FMNH2 Fe ? FMNH Fe H

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Iron-sulfur Centers (clusters)

• Iron-sulfur centers (Fe-S) are prosthetic groups
  containing 1-4 iron atoms
• Iron-sulfur centers transfer only one electron,
  even if they contain two or more iron atoms.
• E.g., a 4-Fe center might cycle between redox
  states
• Fe3, Fe1 (oxidized) 1 e- ?? Fe2, Fe2
  (reduced)

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Ubiquinone
Other names and abbreviations
Q
Coenzyme Q
CoQ
Most often n 10 Free CoQ can undergo a 2 e-
oxidation/reduction Q 2 e- 2 H ?? QH2.
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When bound to special sites in respiratory
complexes, CoQ can accept 1 e- to form a
semiquinone radical (Q-).
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• Coenzyme Q (CoQ, Q or ubiquinone) is
  lipid-soluble. It dissolves in the hydrocarbon
  core of a membrane.
• the only electron carrier not bound to a protein.
• it can accept/donate 1 or 2 e-. Q can mediate e-
  transfer between 2 e- that transfer and 1 e-
  carriers

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Cytochromes
proteins that accept electrons from QH2 or FeS
Ultimately transfers the electrons to oxygen
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Cytochromes

• Cytochromes are electron carriers containing
  hemes . Hemes in the 3 classes of cytochrome (a,
  b, c) differ in substituents on the porphyrin
  ring.
• Some cytochromes(b,c1,a,a3) are part of large
  integral membrane protein complexes.
• Cytochrome c is a small, water-soluble protein.

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Heme is a prosthetic group of cytochromes. Heme
contains an iron atom in a porphyrin ring system.

• The heme iron can undergo 1 e- transition between
  ferric and ferrous states Fe3 e- ?? Fe2
• Copper ions besides two heme A groups (a and a3)
  act as electron carriers in Cyta,a3
• Cu2e- ?? Cu

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Electron carriers
NAD, flavins and Q carry electrons and H
Cytochromes and non-haem iron proteins carry only
electrons
NAD FAD undergoes only a 2 e- reaction
cytochromes undergo only 1e- reactions FMN Q
undergoes 1e- and 2 e- reaction
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Electron Transport chain(respiratory chain)

• The electron transport chain in the inner
  mitochondrial membrane can be isolated in four
  proteins complexes(I, II, III, IV).
• A lipid soluble coenzyme (Q) and a water soluble
  protein (cyt c) shuttle between protein complexes
• Electrons transfer through the chain - from
  complexes I and II to complex IV

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The electron transport chain
Mitochondrial Complexes
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Mitochondrial Complexes
NADH Dehydrogenase
Succinate dehydrogenase
Cytochrome Oxidase
CoQ-cyt c Reductase
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Support for this order of events

1. Energetically favorable. electrons pass from
lower to higher standard reduction potentials .
2. Spectra the absorption spectrum for the
reduced carrier differs from that of its oxidized
form. carriers closer to oxygen are more
oxidized.
3. Specific inhibitors. Those before the blocked
step should be reduced and those after be
oxidized.
4. Assay of individual complexes. NADH can reduce
complex I but not the other complexes.
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Order and Reduction Potentials
-0.32
-0.3
0.045
0.03
0.077
0. 29
0. 55
0. 22
0. 25
0.82
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Drugs that inhibit the ETC
Rotenone helps natives of the Amazon rain forest
catch fish!
Amytal rotenone
CN- CO
Antimycin A
binding tightly to the ferric form (Fe3) of a3
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• When the chain is blocked, electron carriers will
  be in a reduced state before the block point and
  in an oxidized state after it.
• This can easily be monitored using difference
  spectra.

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Inhibitors and Artificial Electron Acceptors
Rotenone amytal
Antimycin A
CN-,CO
Methylene blue 0.01
Ferricyanide0.36
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H Transport

• Complex I, III, IV drive H transport from matrix
  to the cytosol When e- flow through, which
  creates proton gradient(electrochemical
  potential) across the inner membrane
• Complex I and Complex IV The mechanism of H
  transport is still not known.
• The mechanism of H transport in Complex III is Q
  cycle.

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• 4H are pumped per 2e- passing through complex
  III.
• The H/e- ratio is less certain for the other
  complexes probably 4H/2e- for complex I
  2H/2e- for complex IV.

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• Q Cycle The mechanism of H transport in Complex
  III

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• Electrons are transported along the inner
  mitochondrial membrane, through a series of
  electron carriers
• Protons (indicated by charge) are translocated
  across the membrane, from the matrix to the
  intermembrane space
• Oxygen is the terminal electron acceptor,
  combining with electrons and H ions to produce
  water
• 4. As NADH delivers more H and electrons into
  the ETS, the proton gradient increases, with H
  building up outside the inner mitochondrial
  membrane, and OH- inside the membrane.