Cleaving the N,N Triple Bond: The Transformation of Dinitrogen to Ammonia by Nitrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 166

Compared to the Mo-nitrogenase, the V-nitrogenase reduces substrates at lower
efficiencies despite having an electron flux similar to that through its
Mo-counterpart during substrate turnover (Table 2 )[ 5 , 92 ]. In the case of
C 2 H 2 reduction, the V- and Mo-nitrogenases use ~60 % and

10 %, respectively,
of the total electron flux for the formation of H 2 , a side product of the reaction
(Table 2 ).

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Table 2 Specific substrate-reducing activities of Mo- and V-nitrogenases of A. vinelandii . a,b
H 2 Formation
under Ar

H 2 Formation
under N 2

NH 3 Formation
under N 2

Electron Partitioning
Ratio of NH 3 /H 2 c

Mo-Nitrogenase 489 133 205 2.3

V-Nitrogenase 419 192 111 1.1

a This table was adapted from [ 96 ].
b All activities are expressed in nmol product per nmol protein per min.
c Calculations are based on the assumption that the formation of each H 2 requires two electrons,
while the formation of each NH 3 requires three electrons.

Similarly, in the case of N 2 reduction, the V- and Mo-nitrogenases produce H 2
and NH 3 at ratios of 1:1 and 1:2, respectively, again showing a clear favoritism of
the V-nitrogenase toward H 2 formation (Table 2 )[ 92 , 99 , 102 ]. As such, the
stoichiometry of N 2 reduction by the V-nitrogenase could be tentatively depicted
as follows:

10 H þ þ 10 e ! N 2 þ 2NH 3 þ 2H 2

Such a stoichiometry could be physiologically relevant, since the in vivo
measurements of N 2 reduction by V-nitrogenase also yielded similar ratios to
those obtained in the in vitro assay [ 5 ]. The elevated level of H 2 formation by
the V-nitrogenase suggests the possible presence of additional sites in the nitro-
genase that are dedicated to H 2 evolution, which may or may not be related to the
reduction of N 2 . Alternatively, the increased H 2 :NH 3 ratio in the case of
the V-nitrogenase may reflect the utilization of a specific “VFe protein cycle”
by this nitrogenase, in which two molecules of H 2 (instead of one in the case of
the Mo-nitrogenase) need to be released in order to make ready the FeVco
for the activation/reduction of N 2 . Finally, it was reported earlier that a
sub-stoichiometric amount of N 2 H 4 could be generated at elevated temperatures
by the V-nitrogenase of A. chroococcum during the turnover of N 2 [ 85 , 86 ].
The formation of N 2 H 4 by this V-nitrogenase correlated positively with the
decrease of NH 3 formation between 40 Cand50 C, which was interpreted by
a looser association of the reaction intermediate(s) to the V-nitrogenase, render-
ing this nitrogenase more “leaky” than its Mo-counterpart during substrate
turnover [ 85 ].