Chapter 20 Standard reduction potentials provide a basis

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Chapter 20

Electron Transport and Oxidative Phosphorylation

SUMMARY

Section 20.1

 Electron transport from one carrier to another creates a proton gradient across

the inner mitochondrial membrane.

 The proton gradient is coupled to the production of ATP in aerobic metabolism.

Section 20.2

 Standard reduction potentials provide a basis for comparison among oxidation-

Section 20.3

 The electron transport chain consists of four multisubunit membrane-bound

complexes and two mobile electron carriers (coenzyme Q and cytochrome c).

Section 20.4

 The coupling of electron transport to oxidative phosphorylation requires a

multisubunit membrane- bound enzyme, ATP synthase. This enzyme has a

Section 20.5

bound ATP that has already been formed.

Section 20.6

 Shuttle mechanisms transfer electrons, but not NADH, from the cytosol across

Section 20.7

 In the complete oxidation of glucose, a total of 30 or 32 molecules of ATP are

produced for each molecule of glucose, depending on the shuttle mechanism.

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LECTURE NOTES

As is the case for much of central energy metabolism, students are likely to be at

least passingly familiar with the basics, but none of the details covered by this

LECTURE OUTLINE

I. Role of electron transport in metabolism

A. Production of proton gradient

III. Organization of electron transport complexes

A. Complex I

1. Transfer of electrons from NADH to CoQ

5. Contribution to proton gradient

B. Complex II

1. Identity as succinate dehydrogenase

C. Complex III

1. Transfer of electrons from CoQ to cytochrome c

2. Connection between two-electron and one-electron carriers

5. Proton pumping

D. Complex IV

1. Electron transfer from cytochrome c to oxygen

2. Proton pumping

3. Involvement of copper, cytochromes a and a3

E. Details of cytochromes and other iron-containing proteins

IV. Connection between electron transport and phosphorylation

Electron Transport and Oxidative Phosphorylation 3

2. FADH2 = 1.5

V. Mechanism of coupling

A. Chemiosmotic coupling — experimental evidence

B. Conformational aspects

ANSWERS TO PROBLEMS

20.1 The Role of Electron Transport in Metabolism

1. Electrons are passed from NADH to a flavin-containing protein to coenzyme Q.

2. Electron transport and oxidative phosphorylation are different processes.

Electron transport requires the respiratory complexes of the inner mitochondrial

3. In all reactions, electrons are passed from the reduced form of one reactant to

the oxidized form of the next reactant in the chain. The notation [Fe—S] refers to

any one of a number of iron–sulfur proteins.

4. When FADH2 is the starting point for electron transport, electrons are passed

from FADH2 to coenzyme Q in a reaction carried out by Complex II that bypasses

5. Mitochondrial structure confines the reduced electron carriers produced by the

citric acid cycle to the matrix. There they are close to the respiratory complexes

20.2 Reduction Potentials in the Electron Transport Chain

6. The electron transport chain translocates charged particles by chemical means.

Interconversion of chemical and electrical energy is exactly what a battery does.

8. G°′ = –60 kJ/mol

9. We fundamentally add the half reactions in Table 20.1.

10. We fundamentally add the half reactions in Table 20.1.

11. We fundamentally add the half reactions in Table 20.1.

12. The cytochrome is the electron donor, and the flavin moiety is the electron

acceptor. Once again, we add the half reactions in Table 20.1.

Electron Transport and Oxidative Phosphorylation 5

This was the maximum value for a bound flavin. The negative sign indicates that

this reaction will not take place as written because it is not energetically

favorable.

13. Here is an illustration based on standard reduction potentials.

Both reduction potentials indicate a reaction that is not energetically favorable,

but less so with FAD than with NAD+. Other factors enter into consideration,

however, in a living cell. The first is that the reactions do not take place under

standard conditions, altering the values of reduction potentials. The second is

that the reduced electron carriers (NADH and FADH2) are reoxidized. Coupling

the reactions we have looked at here to others also makes them less

unfavorable.

14. The half reaction of oxidation NADH + H+  NAD+ + 2H+ + 2e– is strongly

20.3 Organization of Electron Transport Complexes

15. They all contain the heme group, with minor differences in the heme side chains

in most cytochromes.

the inner mitochondrial membrane.

18. A part of Complex II catalyzes the conversion of succinate to fumarate in the

citric acid cycle.

19. Three of the four respiratory complexes generate enough energy to

phosphorylate ADP to ATP. Complex II is the sole exception.

20. Cytochrome c is not tightly bound to the mitochondrial membrane and can easily

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21. Succinate + ½ O2  Fumarate + H2O

22. The components are in the proper orientation for the electrons to be transferred

rapidly from one component to the next; if the components were in solution,

23. From an evolutionary standpoint, two different functions can be performed by

24. The key point here is not the active site, which has a low tolerance for mutations,

but the molecules with which the proteins in question are associated.

Cytochromes are membrane-bound and must associate with other members of

25. Having mobile electron carriers in addition to membrane-bound respiratory

complexes allows electron transport to use the most readily available complex

rather than to use the same one all the time.

26. The Q cycle allows for a smooth transition from two-electron carriers (NADH and

FADH2) to one-electron carriers (cytochromes).

27. The protein environment of the iron differs in each of the cytochromes, causing

20.4 The Connection between Electron Transport and Phosphorylation

31. The F1 portion of the mitochondrial ATP synthase, which projects into the matrix,

is the site of ATP synthesis.

32. The F0 portion of mitochondrial ATP synthase lies within the inner mitochondrial

membrane, but the F1 portion projects into the matrix.

Electron Transport and Oxidative Phosphorylation 7

34. The F1 part of mitochondrial ATP synthase has a stationary domain (the 33

domain) and a domain that rotates (the  domain). This is exactly the

arrangement needed for a motor.

35. A P/O ratio of 1.5 can be expected because oxidation of succinate passes

electrons to coenzyme Q via a flavoprotein intermediate, bypassing the first

20.5 The Mechanism of Coupling in Oxidative Phosphorylation

38. The chemiosmotic coupling mechanism is based on the difference in hydrogen

ion concentration between the intermembrane space and the matrix of actively

respiring mitochondria. The hydrogen ion gradient is created by the proton

pumping that accompanies the transfer of electrons. The flow of hydrogen ions

back into the matrix through a channel in the ATP synthase is directly coupled to

the phosphorylation of ADP.

42. Dinitrophenol is an uncoupler of oxidative phosphorylation. The rationale was to

dissipate energy as heat.

43. The energy released as protons pass through the F particles is actually used to

cause conformational changes in the F1 proteins, thereby releasing ATP. The

―tight‖ conformation (one of three) provides a hydrophobic environment in which

ADP is phosphorylated by adding Pi without requiring immediate energy.

44. Uncouplers and respiratory inhibitors act in different ways. Uncouplers lead to

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46. Experiments with model systems have shown that electron transport and proton

pumping can take place separately.

20.6 Shuttle Mechanisms

47. The complete oxidation of glucose produces 30 molecules of ATP in muscle and

48. The transport ―product‖ (in the matrix) of the malate–aspartate shuttle is NADH,

20.7 The ATP Yield from Complete Oxidation of Glucose

49.

(a) 34

(b) 32