Electron Transfer in Biology Lecture 27 Electron Transfer in Biology Redox reactions are important biochemical mechanisms. These occur by electron transfer.
Mechanical Work Driven by Electrons Electric motors use electron flow to drive motors.
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.
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.
Electron-Sharing by Carbon Who has more of the electrons?
Electrochemistry: Half-Reactions For every reduction there is an oxidation - half-reaction and the convention is to write them as reductions.
Standard Reduction Potentials When 2 “half-cells” are connected, which direction will electrons flow? Goes to larger +E’o
Standard Reduction Potentials (Eo) >+ E’0 is the one that goes as reduction.
Nernst Equation Redox potential
Compare These Equations Similarity in equations for pH of a solution, free energy change, and electron potential in concentration dependence.
Relationship of ∆Eº and ∆Gº Electron potential is related to free energy.
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.
Nicotinamide-Adenine Dinucleotide NAD+ and NADP+
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.
Dehydrogenases and NAD+ An oxidation series
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
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.
Stereospecific NAD+ Reduction Details of an active site.
Flavin Nucleotides FMN, FAD FAD and FMN are from riboflavin and carry electrons.
Reduction of Flavin Nucleotides The reactive sites are shown with the carried H in red.
Some Typical Flavoproteins Examples of flavoproteins. Flavoproteins bind flavin nucleotides very tightly *Sometimes covalently
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.
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
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
Direction of Redox Reaction Remember the one with the greatest + electron potential is the reduction.
How Energetic is this Reaction? Energies transform from volt change to free energy change in kiloloules.
How can we Oxidize Ethanol? 1. Remove the product Ethanolacetaldehydeacetate1 CO2 2. Have appropriate ratio of A question of how to run the reaction in the opposite direction. Two solutions (1 or 2)..