This is the first of two chapters that describe respiration in the mitochondria. The word

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1 The Electron-Transport Chain Chapter 20 This is the first of two chapters that describe respiration in the mitochondria. The word respiration can mean breathing, and in fact mitochondrial electron transport is one of the main reasons why we need to breathe. Mitochondria use up large amounts of oxygen as electron acceptors. This chapter is about the mitochondrial respiratory chain, also known as the electron-transport chain. A series of reactions take place in the mitochondrial inner membrane, removing electrons from cofactors such as NADH and FADH 2, and moving the electrons through lower-energy electron carriers until finally they are transferred to oxygen to form water. The process of electron transport is inextricably linked with pumping of protons from the inner matrix side of the mitochondrial inner membrane over to the outside of the inner membrane, the intermembrane space. The pumped protons form a gradient which is a source of stored energy. The next chapter will complete the story of respiration by describing how ATP can be made using the energy of the proton gradient. This chapter also shows how to deal with the mathematical connection between reduction potentials and the free-energy change of reactions. The structures and actions of electron carriers including flavin mononucleotide, iron sulfur clusters, heme groups, and coenzyme Q are shown. And whenever molecular oxygen is involved there is some danger of producing free radicals via ROS or reactive oxygen species. This is discussed in some detail. 189

2 190 CHAPTER 20 LEARNING OBJECTIVES When you have mastered this chapter, you should be able to accomplish the following objectives. Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria (Text Section 20.1) 1. Compare the amount of ATP required for a typical day's worth of activity to that stored in a sedentary male. Recognize the implications of the disparity and how a human body compensates for it. 2. Describe the compartments and membranes of mitochondria and locate the respiratory assemblies and the N and P sides (or the matrix side of the cytoplasmic sides) of the inner membrane. 3. Provide a hypothesis for the evolutionary origin of mitochondria. Oxidative Phosphorylation Depends on Electron Transfer (Text Section 20.2) 4. Relate quantitatively redox potential (ΔE 0 ) and free-energy change (ΔGº ). 5. Describe the meaning and the measurement of the redox potential (E 0 ) for a redox couple, relative to the standard reference half-cell. 6. Explain the meaning of E 0 in electron transfer and how it is related to hydrogen ion concentration. 7. Calculate ΔGº for oxidation reduction reactions from the redox potentials for individual redox couples. 8. Identify the driving force of oxidative phosphorylation and be able to calculate the amount of chemical work that can be coupled to the reduction of O 2 with NADH. 9. Calculate the free energy associated with a proton gradient. The Respiratory Chain Consists of Proton Pumps and a Physical Link to the Citric Acid Cycle (Text Section 20.3) 10. List the components of the respiratory chain and the electron-carrying molecules. 11. Describe the entry of electrons from NADH into NADH-Q oxidoreductase (Complex I) and trace their path through this proton pump. State the roles of flavin mononucleotide (FMN), iron-sulfur clusters, and coenzyme Q. 12. Distinguish among the quinone, semiquinone, and ubiquinol forms of coenzyme Q. Explain how reduction of a quinone can consume two protons. 13. Discuss the role of coenzyme Q as a mobile electron carrier between NADH-Q oxidoreductase and cytochrome reductase (Complex III). 14. Describe the entry of electrons into the respiratory chain at the succinate-q reductase complex (Complex II) from flavoproteins such as succinate dehydrogenase (a component of Complex II), glycerol phosphate dehydrogenase, and fatty acyl CoA dehydrogenase by way of FADH 2. Appreciate that Complex II is not a proton pump. 15. Describe the prosthetic group and the functions of the cytochromes and contrast the features of cytochromes b, c 1, c, a, and a List the components of the Q-cytochrome c oxidoreductase complex, and explain how ubiquinol transfers its electrons to cytochromes c 1 and b and ultimately to cytochrome c. Describe the roles of heme in these processes. 17. Explain the origin of the nomenclature used in cytochrome b (heme b L and heme b H ). Give a physical explanation for the difference between redox potentials of the two hemes.

3 THE ELECTRON-TRANSPORT CHAIN Explain how a two-electron carrier, ubiquinol, can interact with a one-electron carrier, the Fe-S cluster. Describe the steps of the Q cycle and write a balanced reaction for the cycle. 19. Describe the cytochrome oxidase proton pump (Complex IV) and its electron-carrying groups. Write a balanced reaction for the process catalyzed by cytochrome c oxidase. 20. Outline the mechanism for the reduction of O 2 to H 2 O on cytochrome oxidase. Describe the path of the electrons from the heme a CuA cluster to the heme a 3 CuB cluster and state the changes in the oxidation states of Fe and O. Note the formation of a superoxide anion intermediate. 21. Describe the salient features of the three-dimensional structure of cytochrome c and relate them to its interaction with cytochrome reductase and cytochrome oxidase. 22. List the reactive oxygen species that are generated during electron transport. Explain why oxygen is a potentially toxic substance and how the safe reduction of O 2 is achieved by cytochrome c oxidase. Summarize the reactions and the biological roles of superoxide dismutase, catalase, and the peroxidases. SELF-TEST Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria 1. For a sedentary male weighing 70 kg, how much more ATP is needed each day than than is present in the body? (a) 83 kg (b) 250 g (c) kg (d) g (e) None of the above. The body contains the amount needed. 2. Which of the following statements regarding mitochondria and their components are correct? (a) Mitochondria are approximately 20 nm in diameter. (b) The matrix compartment contains the enzymes of glycolysis. (c) Mitochondria are bounded by two membrane systems: an inner membrane and an outer membrane. (d) The inner membrane contains pores and is readily permeable to most small metabolites. (e) The inner membrane has a large surface area because it is highly folded. 3. Which of the following answers complete the sentence correctly? Mitochondria (a) are found in all kingdoms of life. (b) are semiautonomous organelles. (c) all contain proteins encoded by their own genes and encoded by the nuclear genome. (d) likely arose from the engulfment of a virus by a bacterium. Oxidative Phosphorylation Depends on Electron Transfer 4. Which of the following statements about the redox potential for a reaction are correct? (a) It is used to describe phosphate group transfers. (b) It is unrelated to the free energy of the reaction. (c) It can be used to predict whether a given compound can reduce another. (d) It can be used to predict whether a given oxidation will provide sufficient energy for the formation of ATP from ADP and P i. (e) It can be used to predict the rate of O 2 uptake upon the oxidation of a given substrate.

4 192 CHAPTER The equation for the reduction of cytochrome a by cytochrome c is Cyt a ( 3) Cyt c ( 2) Cyt a ( 2) Cyt c ( 3) where Cyt a ( 3) e Cyt a ( 2): E V Cyt c ( 3) e Cyt c ( 2): E V Which of the following answers completes the sentence correctly? Under standard conditions ([products] [reactants] [1M]; ph 7), the reaction (a) proceeds spontaneously. (b) yields sufficient energy for ATP synthesis. (c) does not alter the absorption spectra of the cytochromes. (d) involves the transfer of two electrons. 6. What is the ΔGº value for the following reaction? Use Table 20.1 in the text for values of Eº. Note that 1 kcal kj. succinate FAD fumarate FADH 2 (a) 5.79 kj/mol (b) 2.89 kj/mol (c) 0.59 kj/mol (d) 2.89 kj/mol (e) 5.79 kj/mol 7. The parameters ΔGº and ΔE 0 can be used to predict the direction of chemical reactions in standard conditions. On the other hand, ΔG can be used for any concentration of reactants and products to predict in what direction a chemical reaction will proceed. Using the expressions ΔG = ΔG + RT ln [products] and ΔG = nfδe 0 [ reactan ts] derive an expression for ΔE. Explain the significance of this redox potential. 8. Obtain ΔG for the reaction given in question 6 when the succinate concentration is M, the fumarate concentration is M, the FAD concentration is M, the FADH 2 concentration is M, and the temperature is 37ºC. (R J/mol K) 9. For each proton transported out of the matrix across the inner membrane and into the inner membrane space of a mitochondrion, how much free-energy potential is generated across the inner membrane? (a) kj/mol (b) 30.6 kj/mol (c) 2.18 kj/mol (d) 21.8 kj/mol The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle 10. Place the following respiratory-chain components in their proper sequence. Also, indicate which are mobile carriers of electrons. (a) cytochrome c (d) ubiquinone (b) NADH-Q oxidoreductase (e) Q-cytochrome c oxidoreductase (c) cytochrome c oxidase

5 THE ELECTRON-TRANSPORT CHAIN Match the enzyme complexes of the respiratory chain in the left column with the appropriate electron-carrying groups in the right column. (a) cytochrome c oxidase (1) heme c 1 (b) Q-cytochrome c oxidoreductase (2) FAD (c) NADH-Q oxidoreductase (3) heme a 3 (d) succinate-q reductase (4) heme b L (5) iron-sulfur complexes (6) Cu A and Cu B (7) FMN (8) heme a (9) heme b H 12. Which of the following statements about the enzyme complexes of the electron transport system are correct? (a) They are located in the mitochondrial matrix. (b) They cannot be isolated from one another in functional form. (c) They have very similar visible spectra. (d) They are integral membrane proteins located in the inner mitochondrial membrane. (e) They transfer electrons to one another by means of mobile electron carriers. 13. Which of the following statements about ubiquinol are correct? (a) It is the mobile electron carrier between cytochrome c oxidoreductase and cytochrome c oxidase. (b) It is an integral membrane protein. (c) Its oxidation involves the simultaneous transfer of two electrons to the Fe-S center of cytochrome reductase. (d) It is oxidized to ubiquinone by way of a semiquinone intermediate. (e) It is a lipid-soluble molecule. 14. Which cytochrome has a protoporphyrin IX heme that is not covalently bound to protein? (a) cytochrome a (d) cytochrome c (b) cytochrome a 3 (e) cytochrome c 1 (c) cytochrome b 15. Explain the roles of cytochrome c 1 and the b cytochromes (b L and b H ) in the oxidation of ubiquinol to ubiquinone. Are protons pumped across the inner mitochondrial membrane during these reactions? 16. In the reduction of O 2 to H 2 O by cytochrome oxidase, four electrons and four protons are used. How can this occur when a single electron at a time is transferred by heme iron and by copper? 17. Which of the following answers correctly complete the sentance? Reactive oxygen species (ROS) (a) serve as substrates for enzymes that render them less reactive. (b) arise from intermediates generated in electron transport. (c) are transported out of the cell on specialized carriers. (d) include Superoxide dismutase. 2O 2 + 2H + O 2 + H 2 O How can the FADH 2 generated by the succinate-q-reductase complex participate in electron transport if it is not free to diffuse from the enzyme complex? Does the oxidation of succinate transport protons?

6 194 CHAPTER Which of the following statements about an aerated, functional mitochondrial preparation in which the reduced substrate is succinate are correct? (a) Approximately 1.5 ATP molecules will be formed per succinate oxidized to fumarate. (b) Approximately two protons will be pumped across the inner membrane by the succinate-q reductase complex. (c) The addition of CN will result in the synthesis of only one ATP per succinate. (d) Reduction of NADH-Q oxidoreductase will occur. (e) Reduction of Q-cytochrome oxidoreductase will occur. 20. Run-off of artificial fertilizers near the coast of Louisiana causes large blooms of phytoplankton. Phytoplankton are photosynthesizers, so they produce oxygen while they are alive. How can this lead to formation of a dead zone where there is not enough oxygen to support shrimp and crab populations? ANSWERS TO SELF-TEST 1. c. Approximately 83 kg of ATP is needed per day but only 250 g (0.250 kg) is present. That means there is a 83 kg 0.25 kg kg deficit. 2. c, e. Answer (a) is incorrect because mitochondria are ~500 nm in diameter. They are roughly the size of bacteria. 3. b, c. Answer (d) is incorrect because a bacterium, not a virus, was probably engulfed. 4. c, d 5. a. The ΔE 0 for the reaction is 0.05 V. Calculating ΔGº, ΔG = nfδe 0 where n, the number of electrons transferred, is 1, and F is kj/v mol. ΔGº 1(96.49 kj/v mol)(0.05 V) 4.82 kj/mol Therefore, the reaction will proceed spontaneously. However, when considered by itself, it is insufficiently exergonic to drive ATP synthesis, which requires 30.6 kj/mol under standard conditions. In the cell, this comparison is relatively meaningless because ATP is not synthesized during oxidative phosphorylation by direct chemical coupling of redox reactions to ATP formation, but rather by being coupled to a proton motive force. In addition, the concentrations of the reactants can alter the actual free-energy change observed in the reaction. Answer (c) is incorrect because the state of oxidation of a cytochrome alters its absorption spectrum. The heme group contributes significantly to the adsorption spectrum of cytochomes, and the state of oxidation of the heme affects its adsorption spectrum. 6. e. The ΔE 0 for this reaction is 0.03 V. Calculating ΔGº, ΔGº nfδe 0 2(96.49 kj/v mol)( 0.03 V) 5.79 kj/mol 7. The same proportionality constants that relate ΔGº and ΔE 0 can be used to relate ΔG and ΔE. Substituting ΔGº nfδe 0 and ΔG nfδe into the expression for ΔG, [products] nfδe = nfδe0 + RT ln [reactants] E = E + RT Δ Δ 0 nf l n [products] [reactants]

7 THE ELECTRON-TRANSPORT CHAIN ΔE is a measure of the direction in which an oxidation reduction reaction will proceed for any given concentration of reactants and products. If ΔE is positive the reaction is exergonic in the direction written. ΔG = ΔG + ln [Fumarate][FADH 2] RT [ Succinate][ FAD] 3 3 ( M)( M) ΔG = 579. kj/ mol + (8.314 J/ mol K)(310K) ln 3 3 ( 2 10 M)( 2 10 M) ΔG = kj / mol kj / mol = kj / mol 9. d. Answers (a) and (b) are incorrect because kj/mol is the free energy released by the oxidation of an NADH by 1 2 O 2, and 30.6 kj/mol is the free energy released by the hydrolysis of ATP to ADP P i. 10. The proper sequence is b, d, e, a, and c. The mobile carriers are (a) and (d). 11. (a) 3, 6, 8 (b) 1, 4, 5, 9 (c) 5, 7 (d) 2, d, e. Answer (c) is incorrect because each enzyme complex has a unique absorption spectrum that reflects the environment of its electron carriers and its oxidation state. 13. d, e 14. c 15. See the Q cycle in Figure in the text. Ubiquinol transfers one electron to cytochrome c 1 through a Rieske Fe-S cluster in cytochrome oxidoreductase. The semiquinone derived from the Q in this process donates an electron to cytochrome b L, giving rise to ubiquinone. In turn, the electron from cytochrome b L is transferred to cytochrome b H, which then reduces another semiquinone to ubiquinol. Thus, the b cytochromes act as a recycling device that allows ubiquinol, a two-electron carrier, to transfer its electrons, one at a time, to the Fe-S cluster of cytochrome oxidoreductase. Cytochrome c 1 accepts the electrons (one electron/cytochome c 1 ) from the Fe-S cluster and transfers them to cytochrome c through the b cytochromes. Protons pumping across the mitochondrial membrane is tightly coupled to the oxidation of ubiquinol. The complete oxidation of one QH 2 yields two reduced cytochome c molecules and removes two protons from the matrix. 16. See Figure in the text. Molecular O 2 is bound between the Fe 2 and Cu ions of the heme a 3 -Cu B center of cytochrome oxidase. The oxygen remains bound while four electrons and four protons are sequentially added to its various intermediates, resulting in the net release of two H 2 O. The heme a-cu A center supplies the electrons for this process. Although four electrons are used in the reduction of O 2 to 2 H 2 O, the individual steps of the reaction cycle involve single electron transfers. 17. a, b, d 18. The FADH 2 generated by the succinate-q-reductase complex upon oxidation of succinate transfers it electrons to iron-sulfur center and finally to ubiquinone. The system does not transport protons across the inner mitochondrial membrane. 19. a, e. Answer (b) is incorrect; no protons are pumped by Complex II. 20. As described in the text, it is not the living phytoplankton that cause the problem. The oxygen is all used up by aerobic bacteria that eat the dead phytoplankton that have filtered down to the bottom. Thus the most severely affected animals are the shrimp and crabs, which also live on the bottom of the sea.

8 196 CHAPTER 20 PROBLEMS 1. Nitrite (NO 2 ) is toxic to many microorganisms. It is therefore often used as a preservative in processed foods. However, members of the genus Nitrobacter oxidize nitrite to nitrate (NO 3 ), using the energy released by the transfer of electrons to oxygen to drive ATP synthesis. Given the following E 0 values, calculate the maximum ATP yield per mole of nitrate oxidized. NO 3 2H 2 e NO 2 H 2 O E V 1 2 O 2 2 H 2 e H 2 O E V 2. How are mitochondria thought to have arisen? What evidence suggests a particular bacterium as the origin of the mitochondria in all eukaryotes? 3. Suppose that a newly discovered compound called coenzyme U is isolated from mitochondria. (a) Several lines of evidence are presented in advancing the claim that coenzyme U is a previously unrecognized carrier in the electron transport chain. (1) When added to a mitochondrial suspension, coenzyme U is readily taken up by mitochondria. (2) Removal of coenzyme U from mitochondria results in a decreased rate of oxygen consumption. (3) Alternate oxidation and reduction of coenzyme U when it is bound to the mitochondrial membrane can be easily demonstrated. (4) The rate of oxidation and reduction of coenzyme U in mitochondria is the same as the overall rate of electron transport. Which of the lines of evidence do you find the most convincing? Why? Which are the least convincing? Why? (b) In addition to the evidence cited in (a), the following observations were recorded when coenzyme U was incubated with a suspension of submitochondrial particles. (1) The addition of NADH caused a rapid reduction of coenzyme U. (2) Reduced coenzyme U caused a rapid reduction of added cytochrome c. (3) In the presence of antimycin A, the reduction of coenzyme U by added NADH took place as rapidly as in the absence of antimycin A. However, the reduction of cytochrome c by reduced coenzyme U was blocked in the presence of the inhibitor. (4) The addition of succinate caused a rapid reduction of coenzyme U. Assign a tentative position for coenzyme U in the electron-transport chain. 4. Coenzyme Q can be selectively removed from mitochondria using lipid solvents. If these mitochondria are then incubated in the presence of oxygen with an electron donor that is capable of reducing NAD, what will be the redox state of each of the carriers in the electron transport chain? 5. Analysis of the electron-transport pathway in a pathogenic gram-negative bacterium reveals the presence of five electron-transport molecules with the redox potentials listed in Table Table 20.1 Reduction potentials for pathogenic gram-negative bacterium Oxidant Reductant Electrons transferred E 0 (V) NAD NADH Flavoprotein b (ox) Flavoprotein b (red) Cytochrome c ( 3) Cytochrome c ( 2) Ferroprotein (ox) Ferroprotein (red) Flavoprotein a (ox) Flavoprotein a (red)

9 THE ELECTRON-TRANSPORT CHAIN 197 (a) Predict the sequence of the carriers in the electron-transport chain. (b) How many molecules of ATP can be generated under standard conditions when a pair of electrons is transported along the pathway? (c) Why is it unlikely that oxygen is the terminal electron acceptor? 6. Calculate the minimum value of ΔE o that must be generated by a pair of electron carriers to provide sufficient energy for ATP synthesis. Assume that a pair of electrons is transferred. 7. The value of ΔE o for the reduction of NADP is 0.32 V. (a) Calculate the equilibrium constant for the reaction catalyzed by NADPH dehydrogenase: NADP NADH NADPH NAD (b) What function could NADPH dehydrogenase serve in the cell? 8. Why is it important for the value ΔE 0 for the NAD :NADH redox couple to be less negative than those for the redox couples of oxidizable compounds that are components of the glycolytic pathway and the citric acid cycle? 9. The hemes in cytochrome bc1 have different redox potentials because they are in different polypeptide environments. Speculate on what physical properties of the protein could lead to a higher or lower redox potential in the hemes. ANSWERS TO PROBLEMS 1. The two reactions that generate nitrate are NO 2 H 2 O NO 3 2 H 2 e E V 1 2 O 2 2 H 2 e H 2 O E V The net reaction is NO O 2 NO 3 E V The Nernst equation is used to calculate the free energy liberated by the oxidation of nitrite under standard conditions: ΔGº nfδe 0 2(96.49 kj/v mol)(0.40 V) 77.2 kj/mol Therefore, each mole of nitrite oxidized yields, in principle, energy sufficient to drive the formation of ~2.5 moles of ATP under standard conditions (ΔG 30.6 kj/mol). 2. Mitochondria probably arose when an ancient free-living organism capable of oxidative phosphorylation was engulfed by another cell, forming a symbiont with enhanced survival capabilities. Comparison of the sequences of mitochondrial DNAs from a number of eukaryotes indicates an evolutionary relationship. The existence of a common set of encoded proteins in all mitochondrial DNAs, each of which is in the genome of one bacterium, Rickettsia prowazekii, is strong evidence for this organism as the initial symbiont that gave rise to the mitochondria in all eukaryotes. 3. (a) If coenzyme U is a component of the electron-transport chain, it should undergo successive reduction and oxidation in the mitochondrion, and its overall rate of electron transfer should be close to the overall rate. Observations 3 and 4 are therefore the most convincing. In addition to electron carriers, other compounds can be taken up by mitochondria, and some of them, such as pyruvate, can affect the rate of oxygen consumption because they are substrates that donate electrons to carriers. Therefore, observations 1 and 2 are less convincing.

10 198 CHAPTER 20 (b) The first two observations show that coenzyme U lies along the electron transport chain between NADH, which can reduce it, and cytochrome c, which is reduced by it. The fact that antimycin A blocks cytochrome c reduction by coenzyme U suggests that the carrier lies before cytochrome reductase. Succinate, which can transfer electrons to Q, can also transfer electrons to coenzyme U, so the position of coenzyme U in the chain is similar to that of Q. 4. The removal of ubiquinone from the electron transport chain means that no electrons can be transferred beyond Q in the pathway. You would therefore expect all carriers preceding Q to be more reduced and those beyond Q to be more oxidized. 5. (a) The carrier with the most negative reduction potential has the weakest affinity for electrons and so transfers them most easily to an acceptor. The carrier with the most positive reduction potential will be the strongest oxidizing substance and will have the greatest affinity for electrons. A carrier should be able to pass electrons to any carrier having a more positive reduction potential. Thus, the probable order of the carriers in the chain is flavoprotein b, NADH, cytochrome c, flavoprotein a, and ferroprotein. (b) ΔE V ( 0.62 V) 1.47 V The total amount of free energy released by the transfer of two electrons is ΔGº nfδe 0 2(96.49 kj/v mol)(1.47 V) kj/mol Because kj/mol is required to drive ATP synthesis under standard conditions, the number of molecules of ATP synthesized per pair of electrons is 283.7/ (c) It is unlikely that oxygen is the terminal electron acceptor because the reduction potential for the ferroprotein is slightly more positive than that of oxygen (E α.082 V), so under standard conditions, the ferroprotein could not transfer electrons to oxygen. 6. The minimum amount of free energy that is needed to drive ATP synthesis under standard conditions is 30.6 kj/mol. The value of ΔE 0 needed to generate this amount of free energy can be determined using the equation If a pair of electrons is transferred, then ΔGº = nf ΔE 0 D D = G E 0 nf kj / mol Δ E 0 = = V (. kj / Vmol) 7. (a) The transfer of a pair of electrons from NADH to NADP + occurs with no release of free energy: NAD H 2e NADH NADP H 2e NADPH E 0 = 0.32 V E V

11 THE ELECTRON-TRANSPORT CHAIN 199 For the overall reaction, ΔE V. Because ΔGº = nf ΔE 0, ΔGº = 0 = RT ln K eq ln K eq = 0 K eq = 1 K eq 1 means that this reaction is at equilibrium when the ratio [NADPH] [NAD ]/ [NADP ] [NADH] equals 1. Any combination of concentrations that give a ratio of 1 in this quotient represents an equilibrium condition. (b) In the cell, NADPH dehydrogenase serves to replenish NADPH when the reduced cofactor is needed for biosynthetic reactions. On the other hand, metabolites such as isocitrate and glucose 6-phosphate are substrates for NADP -linked dehydrogenases. NAD + can accept reducing equivalents generated as NADPH through the action of these enzymes. 8. NADH is a primary source of electrons for the respiratory chain. Oxidizable substrates must have a more negative reduction potential to donate electrons to NAD. A more negative redox potential for the NAD :NADH couple would make it unsuitable as an electron acceptor. 9. The higher (or the more positive) the redox potential for a molecule, the higher its electron affinity. Reduction of cytochrome b converts the iron from the 3 state to 2, reducing the positive charge, and along with the proprionates, conferring a net neutral charge on the heme. Any physical property tending to increase that electron affinity (or to stabilize the reduced state) would increase the redox potential for the molecule. Placing a heme in an environment with a positive charge nearby would be expected to increase the redox potential of the group by providing a stabilizing force for the additional electron. Conversely, placing the heme in an environment where there is a nearby negative charge should tend to decrease the redox potential. A polar environment would tend to favor the oxidized or more charged state than the more neutral state.

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