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