Page 156 - Thermodynamics of Biochemical Reactions

Thernwdyanamics of Biochemical Reactions. Robert A. Alberty
Copyright 0 2003 John Wiley & Sons, Inc.
ISBN 0-471-22851-6

*i 9.1 Basic Equations
:1 9.2 Oxidation-Reduction Reactions Involving
Single Species at Specified pH
9.3 Methane Monooxygenase Reaction
*+ 9.4 Half-reactions with Reactants Involving
Multiple Species at Specified pH
8r 9.5 Nitrogenase Reaction
2 9.6 Changes in the Binding of Hydrogen Ions in
Half-reactions at Specified pH

When electrons are transferred in a chemical reaction or a biochemical reaction,
the reactions can, in principle, be carried out separately in two half-cells of a
galvanic cell. Each half-cell reaction makes its independent contribution to the
equilibrium electromotive force of the galvanic cell. When chemical half-reactions
in terms of species are considered, their reduction potentials and standard
reduction potentials are represented by E and Eo, respectively. When biochemical
half-reactions in terms of reactants (sums of species) are considered at a specified
pH, their apparent reduction potentials and standard apparent reduction poten-
tials are represented by E' and E'O, respectively. Reduction potentials for chemical
half-reactions are measured in relation to the standard hydrogen electrode, which
by convention has a standard reduction potential of zero volts at each tempera-
ture at zero ionic strength. The standard hydrogen electrode consists of molecular
hydrogen at 1 bar bubbling over a platinum electrode immersed in a solution with
hydrogen ions at unit activity. For a chemical half-reaction the standard reduc-
tion potential Eo is obtained when the concentrations of the species are 1 M; this
potential and the corresponding thermodynamic properties are taken to be
functions of the ionic strength. Remember that in equation 3.1-10 the convention
was adopted that the Gibbs energy of formation of a species is expressed by
AfG,i = A,Gy + RTln[Bj], where A,Gj and A,,Gy are taken to be functions of ionic
strength. Chemical half-reactions balance all atoms and charges. At specified pH,
biochemical half reactions do not balance hydrogen atoms or charges. Standard
apparent reduction potentials are obtained when the reactants (sums of species)
are all at 1 M. The measurement of electromotive force at specified pH yields the
same type of information as the measurement of apparent equilibrium constants,
namely Af G:' values for reactants. Conversely, Af GIo values calculated from