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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
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