Electron Transfer Chain

Electron Transfer Chain - Molecular Biochemistry I Electron...

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Electron Transfer Chain Copyright © 1999-2007 by Joyce J. Diwan. All rights reserved. Molecular Biochemistry I
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Electron Transfer An electron transfer reaction: Aox + Bred A Ared + Box Aox is the oxidized form of A (the oxidant) Bred is the reduced form of B (the reductant). For such an electron transfer, one may consider two half-cell reactions: Aox + n e- A Ared e.g., Fe+++ + e- A Fe++ Box + n e- A Bred
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Aox + n e- A Ared Box + n e- A Bred For each half reaction: E = E°' – RT/nF (ln [reduced]/[oxidized]) e.g., for the first half reaction: E = E°' – RT/nF (ln [ Ared ]/[ Aox ]) E = voltage, R = gas const., F = Faraday, n = # of e - . When [ Ared ] = [ Aox ], E = E°'. E°' is the mid-point potential , or standard redox potential, the potential at which [oxidant] = [reductant] for the half reaction.
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For an electron transfer: E°' = E°'(oxidant) – E°'(reductant) = E°'(acceptor) – E°'(donor) Go' = – nF E°' (E°' is the mid-point potential) An electron transfer reaction is spontaneous (negative G) if E°' of the donor is more negative than E°' of the acceptor, i.e., when there is a positive E°' .
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Consider transfer of 2 electrons from NADH to oxygen: a. ½ O2 + 2H+ + 2e- A H2O E°' = +0.815 V b. NAD+ + 2H+ + 2e- A NADH + H+ E°' = - 0.315 V Subtracting reaction b from a: c. ½ O2 + NADH + H+ A H2O + NAD+ E°'= +1.13 V G = - nF Eo' = – 2(96494)(1.13) = – 218 kJ/mol
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Electron Carriers NAD+/NADH and FAD/FADH2 were introduced earlier. FMN ( F lavin M ono N ucleotide) 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 .
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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++ +). C C C H C C H C N C C N N C NH C H 3 C H 3 C O O CH 2 HC HC HC H 2 C OH O P O- O O- OH OH C C C H C C H C N C C H N N C NH C H 3 C H 3 C O O CH 2 HC HC HC H 2 C OH O P O- O O- OH OH C C C H C C H C N C C H N N H C NH C H 3 C H 3 C O O CH 2 HC HC HC H 2 C OH O P O- O O- OH OH e - + H + e - + H + FMN FMNH 2 FMNH ·
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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
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This note was uploaded on 09/25/2010 for the course BCH 4024 taught by Professor Allison during the Spring '08 term at University of Florida.

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Electron Transfer Chain - Molecular Biochemistry I Electron...

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