Reducing Power
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Transcript of Reducing Power
Reducing PowerReducing Power
• Electrons of reduced coenzymes flow toward O2
• This produces a proton flow and a transmembrane potential
• Oxidative phosphorylation is the process by which the potential is coupled to the reaction: ADP + Pi ATP
Reduced Coenzymes Conserve Energy Reduced Coenzymes Conserve Energy from Biological Oxidations from Biological Oxidations
• 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
Diagram of an electrochemical cellDiagram of an electrochemical cell
• Electrons flow through external circuit from Zn electrode to the Cu electrode
Reduction PotentialsReduction Potentials
Cathode (Reduction)Half-Reaction
Standard PotentialE° (volts)
Li+(aq) + e- -> Li(s) -3.04
K+(aq) + e- -> K(s) -2.92
Ca2+(aq) + 2e- -> Ca(s) -2.76
Na+(aq) + e- -> Na(s) -2.71
Zn2+(aq) + 2e- -> Zn(s) -0.76
Cu2+(aq) + 2e- -> Cu(s) 0.34
O3(g) + 2H+(aq) + 2e- -> O2(g) + H2O(l) 2.07
F2(g) + 2e- -> 2F-(aq) 2.87
Standard reduction potentials Standard reduction potentials and free energyand free energy
• Relationship between standard free-energy change and the standard reduction potential:
Go’ = -nFEo’
n = # electrons transferred
F = Faraday constant (96.48 kJ V-1)
Eo’ = Eo’electron acceptor - Eo’
electron donor
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?
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
Example: Biological SystemsExample: Biological Systems
Both NAD+ and FAD are oxidizing agents
N
NH2
O
R
N
NH2
O
R
N
N
N
N
R
H
O
O N
HN
NH
N
R
H
O
O
2H 2e
H , 2e
,
NAD NADH
FAD FADH2
The question is which would oxidize which?
NAD + FADH2 NADH + FAD + H
OR
FAD + NADH + H NAD + FADH2
Which one of the above is the spontaneous reaction?
in which G is negative
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
NAD + 2H NADH + H Eo ' = -0.32 eV
+ 2HFAD FADH2
2e+
+ 2e Eo ' = -0.22 eV
Eo’ = Eo’electron acceptor - Eo’
electron donor
Remember,
NAD + 2H NADH + H Eo ' = -0.32 eV
+ 2HFAD FADH2
2e+
+ 2e Eo ' = -0.22 eV
For a spontaneous reaction Eo ’ must be positive
Therefore,Therefore,
NAD + 2H NADH + H Eo ' = -0.32 eV
+ 2HFAD FADH2
2e+
+ 2e Eo ' = -0.22 eV
rearrange
NADH + H NAD + 2H 2e+ Eo ' = +0.32 eV
+ 2HFAD FADH2+ 2e Eo ' = -0.22 eV
Add the two reactionsNADH + H+FAD FADH2 + NAD Eo ' = 0.10 eV
NADH + H+FAD FADH2 + NAD Eo ' = 0.10 eV
electronacceptor
electrondonor