Post on 31-Dec-2015
description
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+]