Standard apparent reduction potential

When a biochemical half-reaction involves the production or consumption of hydrogen ions, the electrode potential depends on the pH. When reactants are weak acids or bases, the pH dependence may be complicated, but this dependence can be calculated if the pKs of both the oxidized and reduced reactants are known. Standard apparent reduction potentials E ° have been determined for a number of oxidation-reduction reactions of biochemical interest at various pH values, but the E ° values for many more biochemical reactions can be calculated from ArG ° values of reactants from the measured apparent equilibrium constants K. Some biochemical redox reactions can be studied potentiometrically, but often reversibility cannot be obtained. Therefore a great deal of the information on reduction potentials in this chapter has come from measurements of apparent equilibrium constants. 

Since tables of standard apparent reduction potentials and standard transformed Gibbs energies of formation contain the same basic information, there is a question as to whether this chapter is really needed. However, the consideration of standard apparent reduction potentials provides a more global view of the driving forces in redox reactions. There are two contributions to the apparent equilibrium constant for a biochemical redox reaction, namely the standard apparent reduction potentials of the two half-reactions. Therefore it is of interest to compare the standard apparent reduction potentials of various half reactions. 

An enzyme-catalyzed redox reaction can be divided into two half-reactions, one producing electrons and the other consuming electrons. The standard apparent reduction potentials Fr° and E ° for the two half-reactions in an enzyme-catalyzed redox reaction at a specified pH and ionic strength determine E ° for the overall reaction, which is positive for a reaction that can occur spontaneously. A biochemical redox reaction at a specified pH can be represented schematically by... 

Where Ox, Ox, Red, and Red are reactants (sums of species). The subscripts are abbreviations for right and left, but the two half-reactions could be distinguished in other ways. The half-reactions and their standard apparent reduction potentials at a specified pH are represented by... 

Comparison of equations 9.1-5 and 9.1-7 shows that the standard apparent reduction potentials for the half-reactions at specified pH are given by... 

Table 9.2 Standard Apparent Reduction Potentials E ° in Volts at 298.15 K and 1 bar as a Function of pH and Ionic Strength...

Table 9.3 Standard Transformed Gibbs Energies (in kJ moE ) of Reactions and Standard Apparent Reduction Potentials (in volts) at 289.15 K, 1 bar, pH 7, and Ionic Strength 0.25 M for Reactions Involved in the Methane Monooxygenase Reaction...

Table 9.4 Standard Apparent Reduction Potentials E ° (in volts) at 298.15 K of Half-reactions Involving Reactants with Multiple Species...

Figure 9.2 The pH dependence of the standard apparent reduction potentials at 298.15 K and 0.25 M ionic strength of the following biochemical half reactions, starting at the top of the ordinate [With permission from R. A. Alberty, Biophys. Chem. 389, 94-109 (2001). Copyright Academic Press.] ...

The effects of pH on the standard apparent reduction potentials of the half reactions involved in the nitrogenase reaction are shown in Table 9.5. The effects of pH on the apparent equilibrium constants of the reactions involved in the nitrogenase reaction as shown in Table 9.6. 

These tables can be used to calculate ArG ° and ATH ° at pH 7 and ionic strengths of 0, 0.10, and 0.25 M or at ionic strength 0.25 M and pHs of 5, 6, 7, 8, and 9 for any reaction for which all the reactants are in these tables. They can also be used to calculate standard apparent reduction potentials. The species data can be used to calculate average bindings of hydrogen ions by reactants. Mathematica programs for carrying out these calculations are provided. 

When oxidation and reduction are involved in an enzyme-catalyzed reaction, the standard apparent reduction potential for a half reaction can be calculated by typing the half reaction in calcappredpot and specifying the pHs and ionic strengths. 

Calculation of standard apparent reduction potentials for half reactions... 

The standard apparent reduction potential in volts at 298.15 K for a half reaction can be calculated using calcappredpots. As an example of a half reaction consider NADox + 2e = NADred ... 

Module[ energy),( Calculates the standard apparent reduction potential of a half reaction at specified pHs and ionic strengths for a biochemical half reaction typed in the form nadox+de==nadred. The names of the reactants call the corresponding functions of pH and ionic strength, nu is the number of electrons involved. pHlist and islist can be lists. ) energy = Solve[eq, de] ... 

Calcappredpot fails when the oxidized and reduced forms contain the same number of hydrogen atoms. In this case, there is no pH dependence, and the standard apparent reduction potential can be calculated by using... 

Calculates the standard apparent reduction potential of a half reaction at... 

Tables of Standard Apparent Reduction Potentials of Half Reactions... 

When the pH is specified, each biochemical half reaction makes an independent contribution to the apparent equilibrium constant K for the reaction written in terms of reactants rather than species. The studies of electochemical cells have played an important role in the development of biochemical thermodynamics, as indicated by the outstanding studies by W. Mansfield Clarke (1). The main source of tables of ° values for biochemical half reactions has been those of Segel (2). Although standard apparent reduction potentials ° can be measured for some half reactions of biochemical interest, their direct determination is usually not feasible because of the lack of reversibility of the electrode reactions. However, standard apparent reduction potentials can be calculated from for oxidoreductase reactions. Goldberg and coworkers (3) have compiled and evaluated the experimental determinations of apparent equilibrium constants and standard transformed enthalpies of oxidoreductase reactions, and their tables have made it possible to calculate ° values for about 60 half reactions as functions of pH and ionic strength at 298.15 K (4-8). 

Oxidizing agents with standard apparent reduction potentials above 0.807 V at pH 7 tend to oxidize H2 O to O2 (g), as illustrated by ... 

Table 8.1 Standard apparent reduction potentials in volts at 298.15 K, pH 6, and 0.25 M ionic strength...

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