Page 157 - Thermodynamics of Biochemical Reactions

156 Chapter 9 Oxidation-Reduction Reactions

measurements of K’ can be used to calculate E’O values. The relation between E
and ArG for a chemical half reaction is E = -A,G/~Y~~F‘, where Ivej is the number
of electrons involved and F is the Faraday constant (the product of the Avogadro
constant and the proton charge, 96 485.309 C mol-I). The relation between E’
and A,G‘ for a biochemical half-reaction at specified pH is E‘ = -A,G‘/Iv,IF. The
classical book in this field is W. M. Clarke, Oxidation-Reduction Potentids of
Organic Systems (1 961). The use of transformed thermodynamic properties in this
field was introduced by Alberty (1993c, 1998d, 2001b).
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’O have been determined for a
number of oxidation-reduction reactions of biochemical interest at various pH
values, but the E’O values for many more biochemical reactions can be calculated
from A,-G’O 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 trans-
formed 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.

9.1 BASIC EQUATIONS

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 ELo and EF for the two half-reactions in an enzyme-catalyzed
redox reaction at a specified pH and ionic strength determine E’O 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

Ox + Red‘ = Red + Ox’ E’O = E: ~ E? (9.1-1)

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

Ox + (v,(e- = Red ELo (9.1-2)

Ox’ + lvele- = Red’ EF (9.1-3)

Of course. these reactions may be very much more complicated. Since the pH is
specified, H+ is not included as a reactant, and a reactant may be a sum of species
if the reactant has pKs in the pH region of interest. These biochemical reactions
do balance atoms of elements other than hydrogen, but they do not balance
electric charges. When the half-reactions occur in half-cells connected by a KCI
salt bridge, the difference E’ in electric potential between the metallic electrodes