Section 9: Oxidative Phosphorylation

• Chapter 20: The Electron‐Transport Chain • Chapter 21: The Proton‐Motive Force

Learning Objectives

By the end of this section, you should be able to:  Describe the key components of the electron transport chain and how they are arranged  Explain the benefits of having the electron‐transport chain located in a membrane  Described how the proton‐motive force is converted into ATP  Identify the ultimate determinant of the rate of cellular respiration

1 Lecture: 11‐09‐2016

CHAPTER 20 The Electron‐Transport Chain

2 Similarities between Bicycling and the Electron‐Transport Chain

A bicycle chain The electron‐transport converts the energy chain transfers the from the rider’s legs energy of the oxidation into forward of carbon fuels to the movement of the rider energy of a proton and the bike. gradient

3 Chapter 20 Outline

4 cellular respiration • Oxidative phosphorylation captures the energy of high‐energy electrons to synthesize ATP.

• The flow of electrons from NADH and FADH2 to O2 occurs 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.

5 An overview of oxidative phosphorylation

• The electron‐transport chain and ATP synthesis occur in the mitochondria

• Recall that the citric acid cycle occurs in the mitochondrial matrix

• Oxidation and ATP synthesis are coupled by transmembrane proton fluxes.

• The respiratory chain (yellow structure) transfers electrons from NADH and

FADH2 to oxygen and simultaneously generates a proton gradient.

• ATP synthase (red structure) converts the energy of the proton gradient into ATP. 6 • 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.

Biological Insight: Sequence data suggest that all mitochondria are descendants of an ancestor of Rickettsia prowazekii, which was Electron micrograph (A) engulfed by another cell. and diagram (B) of a mitochondrion 7 The reduction potential E0′, or redox potential, is a measure of a molecule’s tendency to donate or accept electrons.

A strong reducing agent readily donates electrons and has a negative E0′.

A strong oxidizing agent readily accepts electrons and has a positive E0′.

The standard free‐energy change is related to the change in reduction potential.

where n is the number of electrons transferred and F is the Faraday constant.8 The measurement of redox potential.

Apparatus for the measurement of the standard oxidation–reduction potential of a redox couple. Electrons flow through the wire connecting the cells, whereas ions flow through the agar bridge 9 Energy is released when high‐energy electrons are transferred to oxygen.

The energy is used to establish a proton gradient.

10 • The electron‐transport chain is composed of four large protein complexes.

• The electrons donated by NADH and FADH2 are passed to electron carriers in the protein complexes.

• The carriers include flavin mononucleotide (FMN), iron associated with sulfur in proteins (iron‐sulfur proteins), iron incorporated into hemes that are embedded in proteins called cytochromes, and a mobile electron carrier called coenzyme Q (Q).

• Electron flow within the complexes in the inner‐mitochondrial membrane generate a proton gradient.

• These complexes appear to be associated with one another in what is called the respirasome.

11 Components of the electron-transport chain.

Electrons flow down an energy gradient from

NADH to O2.

The flow is catalyzed by four protein complexes.

Iron is a component of all of the complexes as well as cytochrome c.

12 Iron–sulfur clusters

(A) A single iron ion bound by four cysteine residues. (B) 2Fe-2S cluster with iron ions bridged by sulfide ions. (C) 4Fe-4S cluster. Each of these clusters can undergo oxidation–reduction reactions.

• Frataxin is a mitochondrial protein required for the synthesis of iron‐sulfur clusters. • Loss of frataxin results in Friedreich’s ataxia, a disease that affects the nervous system as well as the heart and skeletal system. 13 • 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 the inner mitochondrial membrane in what is called the Q pool.

14 • Electrons flow from NADH to O2 through 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: NADH‐Q oxidoreductase (Complex I) Q‐cytochrome c oxidoreductase (Complex III) Cytochrome c oxidase (Complex IV)

• An additional complex, succinate Q‐reductase

(Complex II), delivers electrons from FADH2 to Complex III.

• Succinate‐Q reductase is not a proton pump. 15 16 • The electrons from NADH are passed along to Q to form QH2 by Complex I. QH2 leaves the enzyme 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.

17 • Succinate dehydrogenase of the citric acid cycle is a part of the succinate‐Q reductase complex (Complex II).

• The FADH2 generated in the citric acid cycle reduces Q to QH2, which then enters the Q pool.

18 Electrons from QH2 are used to reduce two molecules of cytochrome c in a reaction catalyzed by the Q‐cytochrome c oxidoreductase or Complex III. Complex III is also a proton pump.

• QH2 carries two electrons while cytochrome c carries only one electron.

• The mechanism for coupling electron transfer from QH2 to cytochrome c is called the Q cycle.

• In one cycle, four protons are pumped out of the mitochondria and two more are removed from the matrix.

19 • QH2 carries two electrons while cytochrome c carries only one electron.

• The mechanism for coupling electron transfer from QH2 to cytochrome c is called the Q cycle.

Definition: • The Q cycle is aset of reactions in which coenzyme Q cycles between the fully reduced state and the fully oxidized state through one‐electron transfer reactions in which one of the electrons is temporarily stored in; provides a means of passing the two electrons of coenzyme Q to the single‐electron carrier cytochrome c, one electron at a time.

• In one cycle, four protons are pumped out of the mitochondria and two more are removed from the matrix.

20 The Q cycle

In the first half of the cycle, two electrons of a bound QH2 are transferred, one to cytochrome c and 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 QH2 also gives up its electrons, one to a second molecule of – cytochrome c and the other to reduce Q• to QH2. This second electron transfer results in the uptake of two protons from the matrix. The path of electron transfer is shown in red. 21 The cytochrome c oxidase mechanism.

• Cytochrome c oxidase accepts four electrons from four molecules of cytochrome c in order to catalyze the

reduction of O2 to two molecules of H2O.

• In the cytochrome c oxidase 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.

The cycle begins and ends with all prosthetic groups in their oxidized forms (shown in blue). Reduced forms are in red.

Four cytochrome c molecules donate four electrons, which, in allowing the binding and cleavage of an O2 molecule, also + makes possible the import of four H from the matrix to form two molecules of H2O, which are released from the enzyme to regenerate the initial state. 22 Proton transport by cytochrome c oxidase.

Four protons are taken up from the matrix side to reduce one molecule of O2 to two molecules of H2O.

These protons are called “chemical protons” because they participate in a clearly defined reaction with O2.

Four additional “pumped” protons are transported out of the matrix and released on the cytoplasmic side in the course of the reaction.

The pumped protons double the efficiency of free‐energy storage in the form of a proton gradient for this final step in the electron‐ transport chain.

23 The electron‐transport chain

High‐energy electrons in the form of NADH and FADH2 are generated by the citric acid cycle. These electrons flow through the respiratory chain, which powers proton pumping and results in the reduction of O2.

24 Answer:

The reduction potential of FADH2 is less than that of NADH (Table 20.1). As a result, when those electrons are passed along to oxygen, less energy is released. The consequence of the difference is that electron flow from FADH2 to O2 pumps fewer protons than does the electron flow from NADH.

25 Answer: Complex I would be reduced, whereas Complexes II, III, and IV would be oxidized. The citric acid cycle would become reduced because it has no way to oxidize NADH.

26 The area at the mouth of the Mississippi is nutrient rich because of agricultural runoff. Phytoplankton and aerobic bacteria proliferate to such an extent that the oxygen concentration in the water falls. Fish cannot survive under these conditions.

Consequently, the area is called a dead zone.

The Gulf of Mexico dead zone. The size of the dead zone in the Gulf of Mexico off Louisiana varies annually but may extend from the Louisiana and Alabama coasts to the westernmost coast of Texas. Reds and oranges represent high concentrations of phytoplankton and river sediment.

27 • Partial reduction of O2 generates 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.

28 Superoxide dismutase mechanism

• Superoxide dismutase and catalase help protect against ROS damage.

• The oxidized form of superoxide

dismutase (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 protons to form hydrogen peroxide and regenerate the oxidized form of the enzyme. 29 ROS Associated Conditions

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