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 167

4.2.2 The Reduction of Carbon Monoxide

The fact that CO - an isoelectronic molecule to N 2 - is also a substrate of
nitrogenase was first discovered in the case of the V-nitrogenase. The observation
that the formation of H 2 by the V-nitrogenase was inhibited by CO led to the
hypothesis that a portion of the electrons flowing through V-nitrogenase were
“re-routed” for the reduction of CO. Subsequently, this hypothesis was proven by
GC-MS analyses, which demonstrated the ability of the V-nitrogenase to convert
CO to C1-C4 hydrocarbons (i.e., CH 4 ,C 2 H 4 ,C 2 H 6 ,C 3 H 6 ,C 3 H 8 , 1-C 4 H 8 , and n -
C 4 H 10 ) in a reaction analogous to the industrial Fischer-Tropsch process [ 22 , 103 ].
There are notable differences, however, between this enzymatic reaction and its
industrial counterpart, particularly with regard to the reducing power and energy
source in these reactions. The enzymatic reaction uses protons and electrons to
reduce CO to hydrocarbons, and it is driven by the chemical energy released from
ATP hydrolysis, whereas the industrial process uses H 2 to reduce CO, and it occurs
at high temperature and pressure [ 22 , 104 ]. Our current knowledge of CO reduction
by V-nitrogenase can be summarized by a somewhat “abstract” equation as
follows:

H þ þ e !

CO þ CH 4 þ C 2 H 4 þ C 2 H 6 þ C 3 H 6 þ C 3 H 8 þ 1-C 4 H 8

þ n -C 4 H 10 þ H 2

Apparently, much work needs to be done to establish the stoichiometry of the
reaction, trace the fate of the oxygen atom, and elucidate the mechanistic details of
this reaction.
Following the discovery of CO reduction by V-nitrogenase, the reactivity
of Mo-nitrogenase toward CO was re-examined, which revealed the ability of the
Mo-nitrogenase to reduce CO to C2-C3 hydrocarbons. However, the activity of
CO reduction by Mo-nitrogenase is less than 0.5 % compared to that of its
V-counterpart. The gap in reactivity closes significantly, though, between the two
nitrogenases when H 2 O is replaced by D 2 O (Figure 9 ), which enhances the
CO-reducing activity of the Mo-nitrogenase by more than 20-fold while hardly
impacting the activity of the V-nitrogenase (Figure 9 )[ 22 , 105 ]. The reduction of
CO by Mo-nitrogenase also differs from that by V-nitrogenase in terms of the
product distribution profile. While the V-nitrogenase produces almost exclusively
C 2 H 4 (~94 % of total products) from CO, the Mo-nitrogenase has a more evenly
distributed product profile despite the fact that it still produces C 2 H 4 (~54 % of total
products) as the major product of CO reduction (Figure 9 ). These differences
suggest that, even if the two nitrogenases share a common mechanism for CO
reduction, there is still room to fine tune the enzymatic reactivity and product
profile through the modulation of the structural and electronic properties of these
homologous systems.