Transcript Lecture 27

Lecture 27
Electron Transfer
in Biology
Mechanical Work Driven by Electrons
Biological Electron Flow Does Work
electron carrier
Reduced substrate
chain
(e.g. glucose)
Coupling
mechanisms
Transducers: Mitochondrion
ADP
Work:
ATP
Chemical
Flagellum
Motion
Mechanical
H2O
+ CO2
O2
Transport System
Substrate
accummulation
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
C-H
C-O
Element Electronegativity
H
2.1
C
2.5
S
2.5
N
3.0
O
3.5
Electrochemistry: Half-Reactions
3+
Fe
-
+e
O
+
H3C C + 2H + 2e
H
2+
Fe
CH3CH2OH
Always written as reductions
Standard Reduction Potentials
When 2 “half-cells” are connected, which
direction will electrons flow?

+3
Fe
Eo'
+2
Fe
+0.77
+
H
1
/2 H2
-0.42
(0.00 at 1M [H +])
CH3COOH
CH3CHO
-0.47
Standard Reduction Potentials (Eo)
1
/2 O2 + 2H+ + 2eO
+
CH3C + 2H + 2e
H
+
+
NAD + 2H + 2e
+
-
CH3COOH + 2H + 2e
H2O
+0.82 V
CH3CH2OH –0.16 V
NADH + H
+
CH3CHO
The half-reaction with larger
(positive) E o' will go as reduction
–0.32 V
–0.60 V
Nernst Equation
-
[e acceptor]
RT
E = E o' +
ln nF
[e donor]
(where n = number of electrons transferred)
o
At 25 C, this is:
-
[e acceptor]
0.059
E = Eo' + n log 10
[e donor]
Compare These Equations
[salt]
pH = pK + log
[acid]
[product]
²G = ²G ' + RTln
[reactant]
o
}
[acceptor]
RT
E = Eo +
ln
nF
[donor]
Characteristic Dependent of
of species concentrations
Relationship of ∆Eº and ∆Gº
²E o' = Eo' (acceptor) - Eo' (donor)
o
²G ' = -nF²E o'
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
H
O
Nicotinamide
C
NH 2
O
O
H
H
O
P
O
O
NH 2
N
N
H
OH
O
P
H
OH
N
O
H
H
N
AMP
O
O
N
H
OH HO
O
H
P
(NADP +) O
O
Reduction of NAD+ by Two Electrons
O
H
H
C
H
O
NH 2
-
2e
C
NH 2
N
R
N
R
+
2H
+
+
H
Dehydrogenases and
AH 2 + NAD+
Reduced
substrate
A + NADH + H+
Oxidized
product
CH3
CH2OH
O
C
H
O
C
O
+
NAD
Oxidation
(loss of H)
Some Typical Dehydrogenases
Enzyme
Isocitrate DH
Path
Krebs Cycle
-ketoglutarate DH Krebs Cycle
Malate DH
Krebs Cycle
Glyceraldehyde-3phosphate DH
Lactate DH
Glycolysis
Glycolysis
All use NAD+ as electron acceptor
Stereospecificity of H Transfer
D
H3C
C
O
OH
H3C
D
D
H
C
Not
+
+ O
H
D
C
O
D
H
C
C
NH 2
O
NH 2
NH 2
N
N
N
R
R
R
Stereospecific
+
NAD
Reduction
HO
B
C
ADP
Ribose
H
H
N
C
O
H 2N
ADP
BH
Ribose
O C
H
N
H
C
H 2N
O
Flavin Nucleotides FMN, FAD
O
H3C
N
H3C
N
NH
N
FAD
Flavin
Adenine
Dinucleotide
O
CH2
H
C
OH
NH 2
N
FMN
H C OH
Flavin
H C OH O
O
MonoNucleotide H2C O P O P O
O
N
AMP
N
O
O
H
H
H
OH
H
OH
N
Reduction of Flavin Nucleotides
O
H3C
N
H3C
N
NH
AH2
N
O
R
Eo' = -0.06
O
H3C
H3C
A
H
N
NH
N
R
N
H
O
Some Typical Flavoproteins
Enzyme
Fatty acyl-CoA DH
Dihydrolipoyl DH
Succinate DH*
NADH DH
Pathway
Fat oxidation
Fat oxidation
Krebs cycle
Mitrochondrial
oxidative
phosphorylation
Flavoproteins bind flavin nucleotides very tightly
*Sometimes covalently
Some Practical Applications
+
NAD
NADH
Ethanol
Acetaldehyde
1. How to measure rate of reaction?
2. Which direction will it go?
3. How energetic is it?
Spectral Change by Reduction of NAD+
So appearance of NADH  peak of A340
Measuring Rate of NADH Production
2 units
A340
1 unit
of enzyme
Time
no
enzyme
From A340 and molar extinction
coefficient of NADH, you can calculate
moles of NADH produced per time
Direction of Redox Reaction
From table of E o'
-
Acetaldehyde + 2e + 2H
NAD+ + 2e- + 2H+
+
ethanol
E o'
-0.197
NADH + H +
-0.32
So:
Acetaldehyde + NADH
ethanol + NAD
+
How Energetic is this Reaction?
²E o' = Eo' (acceptor) - Eo' (donor)
= Eo'(acetaldehyde) - Eo' (NADH)
= -0.197 - (-0.32)
= +0.123 volts
o
²G ' = -nF²E o'
= -(2)(96.5 kJ/(V•mol))(0.123 V)
= -23.7 kJ/mol
How can we Oxidize Ethanol?
1. Remove the product
Ethanolacetaldehydeacetate1 CO2
+
[NAD ]
2. Have appropriate ratio of [NADH]
[acetate][NADH]
²G = ²G ' + RT ln
+
[ethanol][NAD ]
o