Reduction-oxidation (redox) couple: - PowerPoint Presentation

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4. Reduction-oxidation (redox) couple: pair of molecules of which one is reduced and the other is oxidized (e.g., lactate-pyruvate, NADH-NAD+, and FADH2-FAD each pair constitutes a half reaction the reductant of one pair donates electrons and the oxidant of the other pair accepts the electrons Red1 + Ox2  Ox1 + Red2REDOX FACTS

5. EO: standard reduction potential difference between two half reactions similar to the GO (standard free energy difference) EO is positive for favorable reactions (GO is negative for favorable reactions)Reduction potential (EO): how well one substance reduces another (donates electrons).

6. standard reduction potential of redox couples(a) sample/reference half-cell pair for measurement the ethanol/acetaldehyde couple Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically -0.197 V. In contrast, (b) the fumarate/succinate couple accepts electrons from the reference half-cell; that is, reduction occurs spontaneously in the system, and the reduction potential is thus positive. For each half-cell, a half-cell reaction describes the reaction taking place.

7. The more negative the reduction potential is, the easier a reductant can reduce an oxidant andThe more positive the reductive potential is, the easier an oxidant can oxidize a reductantnegative reduction potential: reducing agent- poised to donate e’s NADHpositive reduction potential: oxidizing agent- poised to accept e’s O2

8. The more negative the reduction potential is, the easier a reductant can reduce an oxidant andThe more positive the reductive potential is, the easier an oxidant can oxidize a reductantThe difference in reduction potential must be important Reduction Potential Difference =DEº  DEº = E° (acceptor) - E° (donor)measured in volts. The more positive the reduction potential difference is, the easier the redox reactionWork can be derived from the transfer of electrons and the ETC can be used to synthesize ATP.

9. For an electron transfer:DE°' = E°'(oxidant) – E°'(reductant) = E°'(acceptor) – E°'(donor)DGo' = – nFDE°'(E°' is the mid-point potential)An electron transfer reaction is spontaneous (negative DG) if E°' of the donor is more negative than E°' of the acceptor, i.e., when there is a positive DE°'.

10. The reduction potential can be related to free energy change by: Gº = -nFDEºwhere n = # electrons transferred = 1,2,3F = 96.5 kJ/volt Faraday constant

11. NADH (–) e therefore good e’ donor oxidants can oxidize every compound with less positive voltage -- (below it in the Table)Fumarate (+) e therefore good e’ acceptor…. High reduction potential, so good at taking e’s or being reduced..reductants can reduce every compound with a less negative voltage -- (above it in the Table)

12. strongest oxidantOxidants can oxidize (accept e’s) every compound with less positive voltage -- (below it in the Table)reductants can reduce (donate e’s) every compound with a less negative voltage -- (above it in the Table)Strongest reductant

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14. Electrons can move through a chain of donors and acceptorsIn the electron transport chain, electrons flow down a gradient.Electrons move from a carrier that is a reductant (high tendency to donate electrons) toward carriers that are oxidants (high tendency to accept electrons).

15. strongest oxidantOxidants can oxidize (accept e’s) every compound with less positive voltage -- (below it in the Table)reductants can reduce (donate e’s) every compound with a less negative voltage -- (above it in the Table)Strongest reductant

16. strongest oxidantOxidants can oxidize (accept e’s) every compound with less positive voltage -- (below it in the Table)reductants can reduce (donate e’s) every compound with a less negative voltage -- (above it in the Table)Strongest reductant

17. Thermodynamics Fundamentals Ox1 + e = Red1 ； E0'1Ox2 + e = Red2 ； E0'2If E0'1 > E0'2 for: Ox1 + e + Red2 = Red1 + Ox2 + eDE0' = E0‘1 - E0'2 > 0With Nernst equation: DG0' = -nFDE0' therefore DG0' < 0Therefore it is spontaneous

18. Consider transfer of 2 electrons from NADH to oxygen:a. ½ O2 + 2H+ + 2e-  H2O E°' = +0.815 Vb. NAD+ + 2H+ + 2e-  NADH + H+ E°' = -0.315 VSubtracting reaction b from a: c. ½ O2 + NADH + H+  H2O + NAD+ DE°'= +1.13 VDG = - nFDEo' = – 2(96494)(1.13) = – 218 kJ/molSince ATP requires 30.5 kJ/mole to form from ADP, more than enough energy is available to synthesize 3 ATPs from the oxidation of NADH.

19. strongest oxidantOxidants can oxidize (accept e’s) every compound with less positive voltage -- (below it in the Table)reductants can reduce (donate e’s) every compound with a less negative voltage -- (above it in the Table)Strongest reductant

20. ETC is avery efficient process- efficiency is ~ 42%, compared to about 3% efficiency when burning oil or gasolineHOW?Separating carbohydrates, lipids, etc. from oxygen to optimize recover of energyStepwise recovery of energy from oxidation of NADH and FADH2 during respiration to generate ATP

21. Electrons can move through a chain of donors and acceptorsIn the electron transport chain, electrons flow down a gradient.Electrons move from a carrier that is a reductant (high tendency to donate electrons) toward carriers that are oxidants (high tendency to accept electrons).

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24. strongest oxidantStrongest reductant

25. oxidative phosphorylation NADH and FADH2. are molecules that are oxidized (i.e., give up electrons) spontaneously. STRONG reductants!!The body uses these reducing agents (NADH and FADH2) in an oxidation-reduction reaction and it is the free energy from these redox reactions that is used to drive the production of ATP.

26. oxidative phosphorylation: oxidation of NADH) reaction coupled to a phosphorylation of ADP the reduction reaction (gaining of electrons) that accompanies the oxidation of NADH. In this case, molecular oxygen (O2) is the electron acceptor, and the oxygen is phosphorylationADP3- + HPO42- + H+ --> ATP4- + H2O DGo= +30.5 kJ (nonspontaneous)oxidation  NADH --> NAD+ + H+ +  2e- DGo= -158.2 kJ (spontaneous)reduction  1/2 O2 + 2H+ + 2e- --> H2O DGo= -61.9 kJ (spontaneous)Net reactionADP3- + HPO42- + NADH + 1/2 O2 + 2H+ --> ATP4- + NAD+ + 2 H2O DGo= -189.6 kJ (spontaneous)

27. cells use oxygen to break down the glucose and store its energy in molecules of ATP. the energy in glucose cannot be used by cells until it is stored in ATPWithout oxygen, cellular respiration could not occur because oxygen serves as the final electron acceptor in the electron transport system. The electron transport system would therefore not be available.

28. couple the redox and phosphorylation reactions: mechanism linking the reactions together. proton-pumping system that occurs inside double-membrane of mitochondriaSynthesis of ATP is coupled with the oxidation of NADH and the reduction of O2 There three key steps in this process:1. Electrons are transferred from NADH, through a series of electron carriers, to O2. The electron carriers are proteins embedded in the inner mitochondrial membrane. 2. Transfer of electrons by these carriers generates a proton (H+) gradient across the inner mitochondrial membrane. 3.When H+ spontaneously diffuses back across the inner mitochondrial membrane, ATP is synthesized. The large positive free energy of ATP synthesis is overcome by the even larger negative free energy associated with proton flow down the concentration gradient.

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30. Electrons are not transferred directly from NADH to O2, but rather pass through a series of intermediate electron carriers in the inner membrane of the mitochondrion. Why? This allows the pumping of protons across the inner membrane of the mitochondrion. that is ultimately responsible for coupling the oxidation-reduction reaction to ATP synthesis. electron carriers: NADH-Q reductase, ubiquinone (Q), cytochrome reductase, cytochrome c, and cytochrome oxidase transport electrons in a stepwise fashion from NADH to O2.  proton pumps: Three of the carriers NADH-Q reductase, cytochrome reductase, and cytochrome oxidase simultaneously pump H+ ions from the matrix to the intermembrane space. The protons that are pumped across the membrane complete the redox reaction ATP synthetase allows H+ ions to diffuse back into the matrix and uses the free energy released to synthesize ATP from ADP and HPO42-.

31. Electron Transport Electron transport releases the energy your cells need to make the most of their ATP The molecules of electron transport chains are built into the inner membranes of mitochondriaThe chain functions as a chemical machine that uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membraneThese ions store potential energy

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33. Flavoproteins Have FMN or FAD as a prosthetic group, and can participate in one and two-electron transfers Co Q: Can also participate in one and two-electron transfers Cytchromes: Contain a heme (iron) prosthetic group. The iron can exist in either Fe2+ or Fe3+ oxidation states, and therefore can participate in one-electron transfers Iron-sulfur proteins: Also involve iron(II) and (III) oxidation states and one-electron transfers Copper: Copper can exist as Cu+ or Cu2+ and can therefore participate in one-electron transfers Players

34. Electron CarriersNAD+/NADH and FAD/FADH2 were introduced earlier.FMN (Flavin MonoNucleotide) is a prosthetic group of some flavoproteins. It is similar in structure to FAD (Flavin Adenine Dinucleotide), but lacking the adenine nucleotide. FMN (like FAD) can accept 2 e- + 2 H+ to form FMNH2.

35. Electron Transport ChainNADH oxidized to NAD+FAD reduced to FADHCytochromes shuffle electrons finally to O2H2O formedATP formed3 ATP / 1 NADH2 ATP / 1 FADH

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37. http://www.johnkyrk.com/mitochondrion.html

38. Step 1:NADH Dehydrogenase (NADH-Q oxidoreductase) transfers electrons from NADH to CoQ, an electron carrierUses two bound cofactors to accomplish this.FMN and iron-sulfur center

39. Flavin adenine dinucleotide = FADFlavin mononucleotide = FMNEach are: Riboflavin = ring + ribitolribitolsemiquinone

40. Flavoproteins Flavoproteins are proteins that contain FAD or FMN.Example of Flavoproteins NADH dehydrogenase succinate dehydrogenase D-amino acid oxidase glucose oxidaseTwo Classes of Flavoprotein Enzymes1. Oxidases: FAD is reduced by substrate and reoxidized by molecular oxygen directly. - D-amino acid oxidase - glucose oxidase2. Dehydrogenases -NADH dehydrogenase - succinate dehydrogenaseNot react directly with O2.....Instead react with iron sulfur centers in protein (ultimately with CoQ)

41. FMN, when bound at the active site of some enzymes, can accept 1 e- to form the half-reduced semiquinone radical. The semiquinone can accept a 2nd e- to yield FMNH2. Since it can accept/donate 1 or 2 e-, FMN has an important role mediating e- transfer between carriers that transfer 2e- (e.g., NADH) & those that can accept only 1e- (e.g., Fe+++).

42. Iron-sulfur centers (Fe-S) are prosthetic groups containing 2, 3, 4 or 8 iron atoms complexed to elemental & cysteine S.4-Fe centers have a tetrahedral structure, with Fe & S atoms alternating as vertices of a cube.Fe-S spacefill; cysteine ball & stick.Fe orange; S yellow. PDB 2FUG

43. Electron transfer proteins may contain multiple Fe-S centers.Iron-sulfur centers transfer only one electron, even if they contain two or more iron atoms, because of the close proximity of the iron atoms.

44. Complex I

45. Coenzyme Q (CoQ, Q, ubiquinone) is very hydrophobic.It dissolves in the hydrocarbon core of a membrane. It includes a long isoprenoid tail, with multiple units having a carbon skeleton comparable to that of isoprene. In human cells, most often n = 10. Q10’s isoprenoid tail is longer than the width of a bilayer.It may be folded to yield a more compact structure, & is postulated to reside in the central domain of a membrane, between the 2 lipid monolayers.

46. QuinonesQuinones are compounds having a fully conjugated cyclic dione structure

47. The quinone ring of coenzyme Q can be reduced to the quinol in a 2e- reaction:Q + 2 e- + 2 H+  QH2.

48. When bound to special sites in respiratory complexes, CoQ can accept 1 e- to form a semiquinone radical (Q·-).Thus CoQ, like FMN, can mediate between 1 e- & 2 e- donors/acceptors.

49. COMPLEX III = cytochrome reductase (Q-cytochrome c oxidoreductase Cytochrome c is mobile.

50. Cytochromes are proteins with heme prosthetic groups. They absorb light at characteristic wavelengths. Absorbance changes upon oxidation/reduction of the heme iron provide a basis for monitoring heme redox state. Some cytochromes are part of large integral membrane complexes, each consisting of several polypeptides & including multiple electron carriers.  Individual heme prosthetic groups may be separately designated as cytochromes, even if in the same protein. E.g., hemes a & a3 that are part of the respiratory chain complex IV are often referred to as cytochromes a & a3.Cytochrome c is instead a small, water-soluble protein with a single heme group.

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52. Complex III

53. Cytochromes - proteins in ETS: Carry electrons and contain hemeHeme is based on porphyrins with iron in center, usually as Fe(II), and is tightly bound at sidescarries electrons only: Fe(III) + e-  Fe(II) Only one electron is transferred at a time.

54. Heme is a prosthetic group of cytochromes. Heme contains an iron atom in a porphyrin ring system. The Fe is bonded to 4 N atoms of the porphyrin ring.

55. Hemes in the 3 classes of cytochrome (a, b, c) differ slightly in substituents on the porphyrin ring system. A common feature is 2 propionate side-chains. Only heme c is covalently linked to the protein via thioether bonds to cysteine residues.

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58. Positively charged lysine residues (in magenta) surround the heme crevice on the surface of cytochrome c. These may interact with anionic residues on membrane complexes to which cyt c binds, when receiving or donating an e-.

59. COMPLEX IV only component of ETS that can interact with O2Cu binds to oxygen and donates electrons to oxygen

60. Complex IV (or cytochrome c oxidase)Catalyses the transfer of e- from cytochrome c to O2.Energy liberated pumps protons through conformational changes.- Reduction of oxygen to water one of the most important reactions in biology. More H+s taken up from matrix side, which balances bc1-complex, for which more H+ released to cytoplasmic side.

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62. Structure of bacterial cytochrome c oxidaseDimer with total molecular weight of 260 kDa (130 kDa/monomer). 4 subunits (mitochondrial cytochrome c has 13 subunits). Twelve TM helices. Two subunits (subunit I and subunit II) involved in electron transfer.

63. Distribution of cofactors Subunit I: heme a and the binuclear centre (heme a3 & CuB).It is the binuclear centre which forms the active site for O2 reduction. Subunit II: dinuclear centre (CuA which is two Cu atoms).Electrons first transferred from cytochrome c to CuA.Passed onto the binuclear centre via heme a. Also a Mg ion present at the interface between subunits.

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66. FAD is the initial electron receptor. FAD is reduced to FADH2 during oxidation of succinate to fumarate. FADH2 is then reoxidized by transfer of electrons through a series of three iron-sulfur centers to Coenzyme Q, yielding QH2. Succinate Dehydrogenase of the Krebs Cycle is also called complex II or Succinate-CoQ Reductase.

67. FAD  FeScenter 1  FeScenter 2  FeScenter 3  CoQIn this crystal structure oxaloacetate (OAA) is bound in place of succinate.X-ray crystallographic analysis of E. coli complex II indicates a linear arrangement of electron carriers within complex II, consistent with the predicted sequence of electron transfers:

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69. http://www.johnkyrk.com/mitochondrion.html

70. Mid-point potentials for Complex III and IVComplex I can pump protons since DE ≈ .380 V from NADH couple to ubiquinone. Complex III can pump protons since DE ≈ .160 V from ubiquinone to cytochrome c.Complex IV can pump protons since DE ≈ .600 V from cytochrome c to oxygen.nmEm,7 (mV) NAD+/NADH21- 320Fumarate/succinate2230Ubiquinone/ubiquinol2260Cytochrome c oxidised/reduced10 220O2 (1 atm)/ 2H2O (55 M)44820

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74. At the end of the electron transport chain, oxygen receives the energy-spent electrons, resulting in the production of water. ½ O2 + 2 e- + 2 H+ → H2OOxygen is the final electron acceptor.Almost immediately oxidized into H2O

75. How is this coupling accomplished?was originally thought that ATP generation was somehow directly done at Complexes I, III and IV. We now know that the coupling is indirect in that a proton gradient is generated across the inner mitochondrial membrane which drives ATP synthesis.

76. Mitochondrial respiratory chain: Complex I:- Transfers e- from NADH to quinone pool & pumps H+. Complex II:- Transfers e- from succinate to quinone pool. Complex III:- Transfers e- from quinol to cytochrome c & pumps H+. Complex IV:- Accepts e- from cytochrome c, reduces O2 to H2O & pumps H+. Complex V:- Harvests H+ gradient & regenerates ATP.