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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Chapter 20 ETC and Oxidative Phosph