1. Electron Transfer in Biology
Lecture 27
Electron Transfer in Biology
Redox reactions are important biochemical mechanisms. These occur by electron transfer.

2. Mechanical Work Driven by Electrons
Electric motors use electron flow to drive motors.

3. Biological Electron Flow Does Work
Biological electron flow also does biochemical work: makes ATP by trapping energy, energy is supplied for movement, and substances are transport against gradients via active transport.

4. Oxidation and Reduction of Carbon
LEO says GER
Lose electrons: oxidized. Gain electrons: reduced
Feº + O2  Fe2O3 (rust)
CH3CH2CH3  3CO2 + 4H2O
Many biochemical compounds contain carbon that is oxidized or reduced.

5. Electron-Sharing by Carbon
Who has more of the electrons?

6. Electrochemistry: Half-Reactions
For every reduction there is an oxidation - half-reaction and the convention is to write them as reductions.

7. Standard Reduction Potentials
When 2 “half-cells” are connected, which direction will electrons flow?
Goes to larger +E’o

8. Standard Reduction Potentials (Eo)
>+ E’0 is the one that goes as reduction.

9. Nernst Equation
Redox potential

10. Compare These Equations
Similarity in equations for pH of a solution, free energy change, and electron potential in concentration dependence.

11. Relationship of ∆Eº and ∆Gº
Electron potential is related to free energy.

12. Biological Electron Carriers
“Pyridine” nucleotides
NAD+
NADP+
Flavine nucleotides
FMN
FAD
Cytochromes
Iron-sulfur proteins
Quinones
Lipoamide
There are a number of biological electron carriers. These have different potentials. The carriers can form chains that transfer electrons in steps.

13. Nicotinamide-Adenine Dinucleotide
NAD+ and NADP+

14. Reduction of NAD+ by Two Electrons
NAD+ is reduced by two electrons and there is a H+ produced. Please note that I consider indication of the charge as important.

15. Dehydrogenases and NAD+
An oxidation series

16. Some Typical Dehydrogenases
Typical NAD+-dependent dehydrogenases. When a common coenzyme is used the mechanism is usually similar which makes learning biochemistry easier.
All use NAD+ as electron acceptor

17. Stereospecificity of H Transfer
The NAD+ and substrate bind to the enzyme at the active site in a particular configuration. Thus a particular hydrogen is always transferred.

18. Stereospecific NAD+ Reduction
Details of an active site.

19. Flavin Nucleotides FMN, FAD
FAD and FMN are from riboflavin and carry electrons.

20. Reduction of Flavin Nucleotides
The reactive sites are shown with the carried H in red.

21. Some Typical Flavoproteins
Examples of flavoproteins.
Flavoproteins bind flavin nucleotides very tightly
*Sometimes covalently

22. Some Practical Applications
1. How to measure rate of reaction?
2. Which direction will it go?
3. How energetic is it?
The oxidation of ethyl alcohol by alcohol dehydrogenase is shown.
Three questions are asked.

23. Spectral Change by Reduction of NAD+
NAD+ and NADH absorb light differently as seen in the above spectra. Light is absorbed at 340 nm by NADH and not NAD+. This difference can be used to follow the reaction.
So appearance of NADH  peak of A340

24. Measuring Rate of NADH Production
Typical experimental results
From A340 and molar extinction coefficient of NADH, you can calculate moles of NADH produced per time

25. Direction of Redox Reaction
Remember the one with the greatest + electron potential is the reduction.

26. How Energetic is this Reaction?
Energies transform from volt change to free energy change in kiloloules.

27. How can we Oxidize Ethanol?
1. Remove the product
Ethanolacetaldehydeacetate1 CO2
2. Have appropriate ratio of
A question of how to run the reaction in the opposite direction. Two solutions (1 or 2)..