Page 163 - Thermodynamics of Biochemical Reactions

162 Chapter 9 Oxidation-Reduction Reactions

pH 7 that are less than -0.422 V tend to produce H,(g). The pH dependencies
of some of these half reactions are shown in Fig. 9.1.
Goldberg and Tewari (Goldberg et al., 1992; Goldberg and Tewari, 1994a, b.
1995a, b; Goldberg, 1999) have published critically evaluated data on experimen-
tal determinations of K' of 94 enzyme-catalyzed redox reactions. These measure-
ments can all be used to calculate E'' for half-reactions and A,Gf values for the
species involved. Thus Table 9.2 can be extended considerably on the basis of
experimental measurements. A,Gf values of species can also be determined from
enzyme-catalyzed reactions that are not redox reactions. When the AfGy value is
unknown for either the oxidant or reductant, the oxidized form can arbitrarily be
assigned A,G; = 0 at zero ionic strength.

w 9.3 METHANE MONOOXYGENASE REACTION

The methane monooxygenase reaction (EC 1.14.13.25) is an especially interesting
enzyme-catalyzed reaction for which all the reactants are single species. The
chemical and biochemical forms of this reaction are given in the first two lines of
Table 9.3. The methane monooxidase reaction is remarkable because it can be
divided into three half-reactions. The chemical and biochemical equations for
these three half-reactions are given in Table 9.3. These three half reactions are in
a sense independent because they do not share reactants except for H,O and H+.
which are everywhere. In other words, these half reactions could be catalyzed
independently if there were appropriate sites on the enzyme for them to deposit
and withdraw electrons at appropriate reduction potentials. The enzyme was able
to couple the three half-reactions to give reaction 1 in Table 9.3. The fact that the
methane monooxygenase reaction can be divided into three half-reactions is quite
remarkable considering the statement at the beginning of this chapter that a redox
reaction can be divided into two half reactions. The reason is that the methane
monooxygenase reaction is the sum of the following two biochemical reactions,

Table 9.3 Standard Transformed Gibbs Energies (in kJ mol- ') 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
Chemical and Biochemical Reactions Ar G"" lvcl E'",'V

1, CH, + NADPH"- + 0, + H' = CH,OH + NADP3- + H,O
2. rncthai1e-t NADP,,, + 0, =methanol + NADP,, +H,O -374.13 4 0.969
3. CH3OH+2H+t2e- =CH,+H,O
4. methanol + 2c.~ =methane + H,O -14.68 2 0.076
5. NADP3 + H + 2e- = NADPH4-
6. NADP,,,+2e-=NADPr,, 61.09 2 -0.317
7 02+4H++4e- =2H,O
8. 02+4e- =2H,O -327.72 4 0.849
9. CH30H+NADP3 +3H+-4e- =CH,(aq)+NADPH"- +H,O
~
10. methanol +NADP0,+4e- =methane+NADP,,,+H,O 46.41 4 -0.120
I I. CH, + (f)o, CH,OH
=
12. methane + (f)O, =methanol - 149.18 2 0.773
13. (f)02+NADPH4- +Hf=NADP3- +H,O
13. (f)0,+NADP,,,=NADP,,+H20 -224.95 2 1.166

Sourct.: With permission from R. A. Alberty, Arch. Bioclit.m. Biopkys. 389,94- 109 (2001). Copyright Academic Prcss.
:Vole: All of these specics arc in aqueous solution, but the formal electron e- is not. Thc convention is that
A, G"(e ) = 0. The first two reactions and the last four reactions are whole reactions in contrast with thc
half-reactions. For the whole rcactions the values givcn here can be used to calculate thc apparcnt cquilihrium
constants under these conditions.