Page 168 - Thermodynamics of Biochemical Reactions
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9.6 Changes in the Binding of Hydrogen Ions in Half Reactions at Specified pH 167
Table 9.6 Apparent Equilibrium Constants K' for Nitrogenase Reactions at 298.15 K
PH 5 PH 6 PH 7 PH 8 PH 9
N,(g) + 10H' + 8Fdre, = 2NHz + H2(g) + 8FdcL
N,(g) + 8 ferredoxinrcd = 2 ammonia + H,(g) f 8 ferredoxinox
1.40 x lo3' 1.40 x 10" 1.41 x 10" 15.6 3.40 10-9
N2(aq) + IOH' f 8 Fdre, = 2NHl + H2(aq) + 8Fd<L
N,(aq) + 8 ferredoxinrcd = 2 ammonia + H,(aq) + 8 ferredoxino,
2.18 x lo3' 2.18 x lo2' 2.20 x 10" 24.3 5.30 10-9
2H+ + 2Fdre, = H,(g) + 2Fd0:
2 ferredoxinre, = H2(g) + 2 ferredoxinox
2.180 21.8 0.2 18 2.18 x 2.18 x
2H' + 2Fdl,, = H2(aq) + 2Fd0:
2 ferredoxinre, = H,(aq) + 2 ferredoxinox
1.80 1.80 x 10-2 1.80 x 10-4 1.80~ 10-' 1.80 x
Source: From R. A. Alberty, Arch. Biorhein. Biophys. 389, 94~ 109 (2001). Copyright Academic Press.
Note: The apparent equilibrium constants for these reactions do not depend on ionic strength
because the equilibrium constants for the chemical reference reactions and acid dissociation do not
depend on ionic strength.
where ammonia represents the sum of NH; and NH,. Different abbreviations are
used for the two forms of ferredoxin in chemical reactions and biochemical
reactions to make it clear that the pH is held constant in the biochemical reaction.
This distinction is especially important when there are acid groups in the reactive
site of a protein, since the site at a specified pH consists of a sum of species
(Alberty, 2000d).
In the absence of molecular nitrogen, nitrogenase produces molecular hydro-
gen. This hydrogenase reaction (EC 1.18.99.1) is represented by the following
chemical reaction:
2H+ + 2Fd,,, = H,(g or aq) + 2Fd0: (9.5-3)
The corresponding biochemical reaction at specified pH is
2 ferredoxin,,, = H,(g or as) + 2 ferredoxin,, (9.5-4)
It may seem strange not to show H+ as a reactant, but [H'] is not involved in
the expression for the apparent equilibrium constant for reaction 9.5-4 because it
is held constant.
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.
The effect of pH on K' for the nitrogen fixation reaction (9.5-2) is so striking
(a change of 10" per pH unit) that it is shown in Fig. 9.3. Note that nitrogen
cannot be fixed by this reaction above pH 8 when ferredoxin from Cluustridium
pastcuriununz is used.
9.6 CHANGES IN THE BINDING OF HYDROGEN IONS
IN HALF-REACTIONS AT SPECIFIED pH
The change in binding of hydrogen ions in a biochemical reaction can be
calculated from the derivative of the standard transformed Gibbs energy of