Download - Chapter 20 ETC and Oxidative Phosph
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Biochemistry: A Short Course
Second Edition
Tymoczko Berg Stryer
2013 W. H. Freeman and Company
CHAPTER 20
The Electron-Transport Chain
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Oxidative phosphorylation captures the energy of high-energy electrons to
synthesize ATP.
The flow of electrons from NADH and FADH2to O2occurs in the electron-
transport chain or respiratory chain.
This exergonic set of oxidation-reduction reactions generates a proton gradient.
The proton gradient is used to power the synthesis of ATP.
Collectively, the citric acid cycle and oxidative phosphorylation are called cellular
respiration or simply respiration.
Respiration is an ATP-generating process in which an
inorganic compound (such as molecular oxygen) serves
as the ultimate electron acceptor. The electron donor can
be either an organic compound or an inorganic one.
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Outer membrane (w/ porin channels)
Inner Mem
NADH: Complex I CoQ Complex III Cyt C Complex IV
FADH: Complex II CoQ Complex III Cyt C Complex IV
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An overview of oxidative phosphorylation.
Oxidation and ATP synthesis are coupled by transmembrane proton fluxes. The
respiratory chain (yellow structure) transfers electrons from NADH and FADH2to
oxygen and simultaneously generates a proton gradient. ATP synthase (red structure)
converts the energy of the proton gradient into ATP.
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The outer mitochondrial membrane is permeable to most small ions and molecules
because of the channel protein mitochondrial porin.
The inner membrane, which is folded into ridges called cristae, is impermeable to
most molecules.
The inner membrane is the site of electron transport and ATP synthesis.
The citric acid cycle and fatty acid oxidation occur in the matrix.
The electron-transport chain and ATP synthesis occur in the mitochondria.
Recall that the citric acid cycle occurs in the mitochondrial matrix.
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The electron-transport chain is a series of coupled redox reactions that transfer
electrons from NADH and FADH2to oxygen.
Describe the key components of the electron-transport chain and how
they are arranged.
Energy is released when high-energy electrons are transferred to oxygen.
The energy is used to establish a proton gradient.
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The electron-transport chain is composed of four large protein complexes.
The electrons donated by NADH and FADH2are passed to electron carriers in
the protein complexes.
Complex I, Complex II, Complex III, cytochrome, CoQ
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Electrons flow down an energy gradient from NADH to O2. The flow is catalyzed by fourprotein complexes. Iron is a component of all of the complexes as well as cytochrome c.
Components of the electron-transport chain
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Coenzyme Q is derived from isoprene.
Coenzyme Q binds protons (QH2) as well as
electrons, and can exist in several oxidation
states.
Oxidized and reduced Q are present in theinner mitochondrial membrane in what is
called the Q pool.
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The reduction of ubiquinone (Q) to ubiquinol (QH2) proceeds through a semiquinone
intermediate (QH).
Oxidation states of quinones
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Electrons flow from NADH to O2through three large protein complexes embedded in
the inner mitochondria membrane.
These complexes pump protons out of the mitochondria, generating a proton gradient.
The complexes are:
1. NADH-Q oxidoreductase (Complex I)
3. Q-cytochrome c oxidoreductase (Complex III)
4. Cytochrome coxidase (Complex IV)
An additional complex, succinate Q-reductase (Complex II), delivers electrons from
FADH2to Complex III.
Succinate-Q reductase is not a proton pump.
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Oxidation states of quinones
Notice that the electron affinity of the
components increases as electrons move
down the chain.
The complexes shown in yellow boxes are
proton pumps. Cyt cstands for cytochrome c.
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I
I
III
IV
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Electron flow within the complexes in the inner-mitochondrial membranegenerate a proton gradient.
These complexes appear to be associated with one another in what is called the
respirasome.
Explain the benefits of having the electron-transport chain
located in a membrane.
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The electrons from NADH are passed along to Q to form QH2by Complex I. QH2leaves theenzyme for the Q pool in the hydrophobic interior of the inner-mitochondrial membrane.
Four protons are simultaneously pumped out of the mitochondria by Complex I.
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COMPLEX I
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Succinate dehydrogenase of the citric acid cycle is a part of the succinate-Q reductase complex (Complex II).
The FADH2generated in the citric acid cycle reduces Q to QH2, which then
enters the Q pool.
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Electrons from QH2are used to reduce two
molecules of cytochrome cin a reaction catalyzed
by the Q-cytochrome coxidoreductase or Complex
III. Complex III is also a proton pump.
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QH2carries two electrons while cytochrome ccarries only one electron.
The mechanism for coupling electron transfer from QH2to cytochrome cis
called the Q cycle.
In one cycle, four protons are pumped out of the mitochondria and two
more are removed from the matrix.
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The Q cycle
In the first half of the cycle, two electrons of a bound QH2are transferred, one tocytochrome cand the other to a bound Q in a second binding site to form the
semiquinone radical anion Q. The newly formed Q dissociates and enters the Q
pool. In the second half of the cycle, a second QH2also gives up its electrons, one
to a second molecule of cytochrome cand the other to reduce Qto QH2. This
second electron transfer results in the uptake of two protons from the matrix. The
path of electron transfer is shown in red.
Complex III: Cyt C Oxidase
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Cytochrome coxidase (Complex III) accepts four electrons from four molecules of
cytochrome cin order to catalyze the reduction of O2to two molecules of H2O.
In the cytochrome coxidase reaction, eight protons are removed from the matrix.
Four protons, called chemical protons, are used to reduce oxygen. In addition, four
protons are pumped into the intermembrane space.
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High-energy electrons in the form of NADH and FADH2are generated by the citric acid cycle.
These electrons flow through the respiratory chain, which powers proton pumping and results in
the reduction of O2.
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Partial reduction of O2generates highly reactive
oxygen derivatives, called reactive oxygen species
(ROS).
ROS are implicated in many pathological conditions.
ROS include superoxide ion, peroxide ion, and
hydroxyl radical.
Two to four percent of oxygen molecules consumed
by mitochondria are converted into superoxide ions.
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Superoxide dismutase and catalase help protect against ROS damage.
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Superoxide dismutase mechanism
The oxidized form of SOD
(Mox) reacts with one
superoxide ion to form O2
and generate the reduced
form of the enzyme (Mred).
The reduced form then
reacts with a second
superoxide ion and two H
to form hydrogen peroxide
and regenerate the
oxidized form of the
enzyme.
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