Reducing Power

1. Electrons of reduced coenzymes flow toward O2 • This produces a protonflow and a transmembranepotential • Oxidative phosphorylationis the process by which the potential is coupled to the reaction: ADP + Pi ATP Reducing Power

2. Amino acids, monosaccharides and lipids are oxidized in the catabolic pathways • Oxidizing agent - accepts electrons, is reduced • Reducing agent - loses electrons, is oxidized • Oxidation of one molecule must be coupled with the reduction of another molecule • Ared + Box Aox + Bred Reduced Coenzymes Conserve Energy from Biological Oxidations

3. Diagram of an electrochemical cell • Electrons flow through external circuit from Zn electrode to the Cu electrode

4. Reduction Potentials

5. Standard reduction potentials and free energy • Relationship between standard free-energy change and the standard reduction potential: DGo’ = -nFDEo’ n = # electrons transferred F = Faraday constant (96.48 kJ V-1) DEo’= Eo’electron acceptor - Eo’electron donor

6. Example Suppose we had the following voltaic cell at 25o C: Cu(s)/Cu+2 (1.0 M) // Ag+(1.0 M)/ Ag (s) What would be the cell potential under these conditions?

7. Example Suppose we had the following voltaic cell at 25o C: Cu(s)/Cu+2 (1.0 M) // Ag+(1.0 M)/ Ag (s) What would be the cell potential under these conditions? Ag+ + e- ---> Ag0 E0red = + 0.80 v Cu+2 + 2e- ----> Cu0 E0red = + 0.337 v

8. Example: Biological Systems Both NAD+ and FAD are oxidizing agents

9. The question is which would oxidize which? OR Which one of the above is the spontaneous reaction? in which DG is negative

10. To be able to answer the question We must look into the “electron donation” capabilities of NADH and FADH2 i.e. reduction potentials of NADH and FADH2

11. Remember, DEo’= Eo’electron acceptor - Eo’electron donor For a spontaneous reaction DEo’must be positive

12. Therefore, rearrange Add the two reactions

13. electron acceptor electron donor