Page 166 - Thermodynamics of Biochemical Reactions

9.5 Nitrogenase Reaction 165

Table 9.4 Continued

GSSG2- + 2H' + 2e- = 2GS-
glutathioneox + 2e- = 2glutathione,,,
I=OM - 0.1565 -0.2156 -0.2737 - 0.3243 - 0.3491
I = 0.25 M - 0.1692 -0.2280 - 0.2845 -0.3261 -0.3395
I = 0.25 M - 0.1732 -0.2320 - 0.2876 - 0.3254 -0.3359
TRox + 2e- = TRre,H02-
Thioredoxinox + 2e- = thioredoxinre,
/=OM - 0.1643 -0.2234 -0.281 5 -0.3323 -0.3570
I = 0.25 M -0.1707 - 0.2296 -0.2871 -0.3325 - 0.3474
I = 0.25 M -0.1727 -0.2317 -0.2888 -0.3318 - 0.3438

Source: With permission from R. A. Alberty, Arch. Biochern. Biophys. 389, 94-109 (2001).
Copyright Academic Press.

9.5 NITROGENASE REACTION

The nitrogenase reaction (EC 1.18.6.1) involves three biochemical reactions: (1)
the fixation of molecular nitrogen, (2) the hydrogenase reaction when molecular
nitrogen is absent (EC 1.18.99.1), and (3) the hydrolysis of ATP to ADP. About
15 moles of ATP are hydrolyzed per mole of nitrogen fixed (Burris, 1991), but this
amount varies with the pH and temperature. This is why it is not based on
conservation of atoms. It has been suggested (Alberty, 1994) that the role of the
hydrolysis of ATP is to supply the hydrogen ions required in the fixation reaction
so that the catalytic site does not become alkaline. The apparent reduction
potentials in the nitrogenase reaction are of special interest because of the
importance of nitrogen fixation and because of the extraordinarily large effect of
pH on the apparent equilibrium constant for the fixation of nitrogen. The
chemical reaction for the fixation reaction is
N,(g or aq) + 10H' + 8Fd,,, = 2NHl + H,(g or as) + 8Fd0: (9.5-1)

where Fd,,, and Fd,; represent the reactive site of the protein ferredoxin. Note
that a mole of H, is produced for each mole of N, converted to ammonia. N,
and H, can be in gaseous or aqueous states. Strombaugh et al. (1976) found that
the standard apparent reduction potentials of eight ferredoxins at 298.15 K and
pH 7 ranged from -0.377 V to -0.434 V, and so apparent equilibrium constants
for biochemical reactions involving ferredoxin will depend on the ferredoxin used.
The calculations here have been made with E'O = - 0.403 V, which was obtained
for Claustridium pasteurianum. Since other ferredoxins have different values of E'O,
different apparent equilibrium constants will be obtained. Since E'O for this
ferredoxin is independent of pH in the range 6.1 to 7.4 (Tagawa and Arnon, 1968),
it is assumed that there are no acid groups in the reactive site with pKs in the
range considered here. Reaction 9.5-1 is referred to as a reference reaction, and it
can be balanced with NH, rather than NH;. When the pH is in the neighbor-
hood of 9 and higher, it is necessary to include the acid dissociation of NH;
(pK = 9.25) in calculating the equilibrium composition.
When the pH is specified, the fixation of molecular nitrogen is represented by
the following biochemical reaction:
N,(g or as) + 8 ferredoxin,,, = 2 ammonia + H,(g or as) + 8 ferredoxin,,

(9.5-2)