Garrett R.H., Grisham C.M. - Biochemistry (1999)(2nd ed.)(en)
.pdf
Oxaloacetate
2 676 Chapter 21 ● Electron Transport and Oxidative Phosphorylation
(Together, the oxidized and reduced forms of the substance are referred to as a redox couple.) Such a sample half-cell is connected to a reference half-cell and electrode via a conductive bridge (usually a salt-containing agar gel). A sensitive potentiometer (voltmeter) connects the two electrodes so that the electrical potential (voltage) between them can be measured. The reference half-cell normally contains 1 M H in equilibrium with H2 gas at a pressure of 1 atm. The H /H2 reference half-cell is arbitrarily assigned a standard reduction potential of 0.0 V. The standard reduction potentials of all other redox couples are defined relative to the H /H2 reference half-cell on the basis of the sign and magnitude of the voltage (electromotive force, emf) registered on the potentiometer (Figure 21.2).
If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, ° , is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 M H . The hydrogen half-cell measured at pH 7.0 has an ° of 0.421 V.)
Several Examples
Figure 21.2a shows a sample/reference half-cell pair for measurement of the standard reduction potential of the acetaldehyde/ethanol couple. Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically 0.197 V. In contrast, the fumarate/succinate couple and the Fe3 /Fe2 couple both cause electrons to flow from the reference half-cell to the sample half-cell; that is, reduction occurs spontaneously in each system, and the reduction potentials of both are thus positive. The standard reduction potential for the Fe3 /Fe2 half-cell is much larger than that for the fumarate/succinate half-cell, with values of0.771 V and 0.031 V, respectively. For each half-cell, a half-cell reaction describes the reaction taking place. For the fumarate/succinate half-cell coupled to a H H2 reference half-cell, the reaction occurring is indeed a reduction of fumarate.
Fumarate 2 H 2 e 88n succinate |
0.031 V |
(21.3) |
||
Similarly, for the Fe3 /Fe2 half-cell, |
|
|
° |
|
|
|
|
|
|
Fe3 e 88n Fe2 |
|
0.771 V |
(21.4) |
|
|
° |
|
|
|
However, the reaction occurring in the acetaldehyde/ethanol half-cell is the oxidation of ethanol:
Ethanol 88n acetaldehyde 2 H 2 e |
0.197 V (21.5) |
° |
|
The Significance of °
Some typical half-cell reactions and their respective standard reduction potentials are listed in Table 21.1. Whenever reactions of this type are tabulated, they are uniformly written as reduction reactions, regardless of what occurs in the given half-cell. The sign of the standard reduction potential indicates which reaction really occurs when the given half-cell is combined with the reference hydrogen half-cell. Redox couples that have large positive reduction potentials
21.2 ● Reduction Potentials—An Accounting Device for Free Energy Changes in Redox Reactions |
677 |
Table 21.1
Standard Reduction Potentials for Several
Biological Reduction Half-Reactions
Reduction Half-Reaction (V)
1 O2 2 H 2 e 88n H2O |
0.816 |
2 |
|
Fe3 e 88n Fe2 |
0.771 |
Photosystem P700 |
0.430 |
NO3 2 H 2 e 88n NO2 H2O |
0.421 |
Cytochrome f (Fe3 ) e 88n cytochrome f (Fe2 ) |
0.365 |
Cytochrome a3(Fe3 ) e 88n cytochrome a3(Fe2 ) |
0.350 |
Cytochrome a(Fe3 ) e 88n cytochrome a(Fe2 ) |
0.290 |
Rieske Fe-S(Fe3 ) e 88n Rieske Fe-S(Fe2 ) |
0.280 |
Cytochrome c (Fe3 ) e 88n cytochrome c (Fe2 ) |
0.254 |
Cytochrome c1(Fe3 ) e 88n cytochrome c1(Fe2 ) |
0.220 |
UQH H e 88n UQH2 (UQ coenzyme Q) |
0.190 |
UQ 2 H 2 e 88n UQH2 |
0.060 |
Cytochrome b H(Fe3 ) e 88n cytochrome b H(Fe2 ) |
0.050 |
Fumarate 2 H 2 e 88n succinate |
0.031 |
UQ H e 88n UQH |
0.030 |
Cytochrome b 5(Fe3 ) e 88n cytochrome b 5 (Fe2 ) |
0.020 |
[FAD] 2 H 2 e 88n [FADH2] |
0.003–0.091* |
Cytochrome b L(Fe3 ) e 88n cytochrome b L(Fe2 ) |
0.100 |
Oxaloacetate 2 H 2 e 88n malate |
0.166 |
Pyruvate 2 H 2 e 88n lactate |
0.185 |
Acetaldehyde 2 H 2 e 88n ethanol |
0.197 |
FMN 2 H 2 e 88n FMNH2 |
0.219 |
FAD 2 H 2 e 88n FADH2 |
0.219 |
Glutathione (oxidized) 2 H 2 e 88n 2 glutathione (reduced) |
0.230 |
Lipoic acid 2 H 2 e 88n dihydrolipoic acid |
0.290 |
1,3-Bisphosphoglycerate 2 H 2 e 88n |
|
glyceraldehyde-3-phosphate Pi |
0.290 |
NAD 2 H 2 e 88n NADH H |
0.320 |
NADP 2 H 2 e 88n NADPH H |
0.320 |
Lipoyl dehydrogenase [FAD] 2 H 2 e 88n |
|
lipoyl dehydrogenase [FADH2] |
0.340 |
-Ketoglutarate CO2 2 H 2 e 88n isocitrate |
0.380 |
2 H 2 e 88n H2 |
0.421 |
Ferredoxin (spinach) (Fe3 ) e 88n ferredoxin (spinach) (Fe2 ) |
0.430 |
Succinate CO2 2 H 2 e 88n -ketoglutarate H2O |
0.670 |
*Typical values for reduction of bound FAD in flavoproteins such as succinate dehydrogenase (see Bonomi, F., Pagani, S., Cerletti, P., and Giori, C., 1983. European Journal of Biochemistry 134:439–445).
have a strong tendency to accept electrons, and the oxidized form of such a couple (O2, for example) is a strong oxidizing agent. Redox couples with large negative reduction potentials have a strong tendency to undergo oxidation (that is, donate electrons), and the reduced form of such a couple (NADPH, for example) is a strong reducing agent.
