Lecture 27

Electron Transfer in Biology

Mechanical Work Driven by Electrons

Biological Electron Flow Does Work

Reduced substrate(e.g. glucose)

electron carrier chain

O2

H2O

+ CO2

Couplingmechanisms

Transducers: Mitochondrion Flagellum Transport System

ADP ATP Motion Substrateaccummulation

Work: Chemical Mechanical Osmotic

Oxidation and Reduction of Carbon

LEO says GER– Lose electrons: oxidized. Gain electrons: reduced

Feº + O2 Fe2O3 (rust)

CH3CH2CH3 3CO2 + 4H2O

Electron-Sharing by Carbon

Element ElectronegativityH 2.1C 2.5S 2.5N 3.0O 3.5

C - H C - O

Electrochemistry: Half-Reactions

Fe3+ + e- Fe2+

H3C CO

H+ 2H+ + 2e- CH3CH2OH

Always written as reductions

Standard Reduction Potentials When 2 “half-cells” are connected, which

direction will electrons flow?

Fe+3 Fe+2 H+ 1/2 H2CH3COOH

CH3CHOEo' +0.77 -0.42 -0.47

(0.00 at 1M [H+])

Standard Reduction Potentials (Eo)

1/2 O2 + 2H+ + 2e- H2O

CH3CO

H+ 2H+ + 2e- CH3CH2OH

NAD+ + 2H+ + 2e- NADH + H+

CH3COOH + 2H+ + 2e- CH3CHO

+0.82 V

–0.16 V

–0.32 V

–0.60 V

The half-reaction with larger(positive) Eo' will go as reduction

Nernst Equation

E = Eo' + RTnF

ln[e- acceptor][e- donor]

(where n = number of electrons transferred)

At 25oC, this is:

E = Eo' + 0.059n log10

[e- acceptor][e- donor]

Compare These Equations

pH = pK + log[salt][acid]

∆G = ∆Go' + RTln[product][reactant]

E = Eo + RTnF

ln[acceptor]

[donor]

Characteristicof species

Dependent ofconcentrations

Relationship of ∆Eº and ∆Gº

∆Eo' = Eo'(acceptor) - Eo'(donor) .

∆Go' = -nF∆Eo'

F = 96,494 J/(V•mol) = 23,100 cal/(V•mol)

Biological Electron Carriers “Pyridine” nucleotides

– NAD+

– NADP+

Flavine nucleotides– FMN– FAD

Cytochromes Iron-sulfur proteins Quinones Lipoamide

Nicotinamide-Adenine Dinucleotide

O

N

NN

N

NH2

O

HOOH

HH

HH

OP

O

O

P

O

O

O

O

OHOH

HH

HH

N

H

C

O

NH2

Nicotinamide

AMP

P

O

O

O

(NADP+)

Reduction of NAD+ by Two Electrons

N

C

O

NH2

H

R

2e-

2H+

N

H HC

NH2

O

R +

H+

Dehydrogenases and NAD+

AH2 + NAD+ A + NADH + H+

Reducedsubstrate

Oxidizedproduct

CH3

CH2OH

CO

H

CO

O

Oxidation(loss of H)

Some Typical DehydrogenasesEnzyme PathIsocitrate DH Krebs Cycle

α-ketoglutarat e DH Kreb sCycle

Mala te DH Kreb sCycle

Glyceraldehyde-3-phosphate DH

Glycolysis

Lactat e DH Glycolysis

All use NAD+ as electron acceptor

Stereospecificity of H Transfer

H3C C

D

D

OH

+

N

R

H

CNH2

O

H3C CO

D

N

R

CNH2

OH D

+ Not

N

R

CNH2

OD H

Stereospecific NAD+ Reduction

ADP Ribose N H

C O

H2N

HC

HOB

ADP Ribose NH

C O

H2N

H

COBH

Flavin Nucleotides FMN, FAD

N

N

N

NHH3C

H3C

O

O

CH2

C OHH

C OHH

C OHH

H2C O P O

O

O

N

NN

N

NH2

O

OHOH

HH

HH

OP

O

O

AMPFMN

FAD

FlavinMono-Nucleotide

FlavinAdenineDinucleotide

Reduction of Flavin Nucleotides

N

N

N

NHH3C

H3C

O

O

R

N

HN

NH

NHH3C

H3C

O

O

R

AH2

A

Eo' = -0.06

Some Typical FlavoproteinsEnzyme Pathway

Fatty acyl-CoA DH Fat oxidationDihydrolipoyl DH Fat oxidationSuccinate DH* Krebs cycleNADH DH Mitrochondrial

oxidativephosphorylation

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?

Ethanol Acetaldehyde

NAD+ NADH

Spectral Change by Reduction of NAD+

So appearance of NADH peak of A340

Measuring Rate of NADH Production

A340

2 units

1 unitof enzyme

noenzyme

Time

From A340 and molar extinction coefficient of NADH, you can calculate moles of NADH produced per time

Direction of Redox Reaction

From table of Eo'

Acetaldehyde + 2e- + 2H+ ethanol

NAD+ + 2e- + 2H+ NADH + H+

Eo'

-0.197

-0.32

So:

Acetaldehyde + NADH ethanol + NAD+

How Energetic is this Reaction?∆Eo' = Eo'(acceptor) - Eo'(donor)

= Eo'(acetaldehyde) - Eo'(NADH)

= -0.197 - (-0.32)= +0.123 volts

∆Go' = -nF∆Eo'

= -(2)(96.5 kJ/(V•mol))(0.123 V)

= -23.7 kJ/mol

How can we Oxidize Ethanol?

1. Remove the product

Ethanolacetaldehydeacetate1 CO2

2. Have appropriate ratio of [NAD+][NADH]

∆G = ∆Go' + RT ln[acetate][NADH][ethanol][NAD+]