Carbon Monoxide. Toxic Gas and Fuel for Anaerobes and Aerobes: Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 59

electrons [ 107 , 122 ] has been proposed to occur before or with the methylation, but
how electrons are transferred to cluster A is still unclear.
Reduced, CO-treated ACS exhibits a characteristic EPR spectrum with g -values
of 2.074 and 2.028, referred to as NiFeC signal [ 138 , 139 ]. The corresponding
NiFeC species was suggested to be [4Fe4S] 2+ -(Ni p -CO)-(Ni d )[ 137 , 140 ] and its
relevance for the mechanism of ACS is controversially discussed. The formation
and decay of the NiFeC EPR signal are equal or faster than the overall rate of
acetyl-CoA formation, thus the NiFeC species is catalytically competent [ 137 ,
141 ]. Furthermore, NiFeC is the predominant metal-carbonyl species formed
when ACS reacts with CO [ 141 ]. A Ni p species that resembles the CO binding
state of ACS was generated by photolysis of the Ni p -CO state, exhibiting a low
barrier for recombination with CO [ 142 ]. On the other hand, Gencic and coworkers
[ 143 ] argued that the NiFeC species is due to a CO-inhibited state, which is in
equilibrium with the free form of cluster A.
The nature of the NiFeC species is important because it defines the possible
mechanism. Although the condensation reaction catalyzed by ACS does not
produce or consume electrons, the binding of a methyl cation temporarily oxidizes
the cluster A by two electrons. Two major mechanisms have been proposed to
account for the electronic states during catalysis (Figure 13 ).
The paramagnetic reaction mechanism includes the NiFeC species as a
CO-bound intermediate (Figure 13a ). CO and the methyl group bind in random
order to the reduced Ni p [ 101 ]. Bender et al. [ 142 ] proposed that during catalysis a
diamagnetic resting state (Ni 2 p ) predominates over the thermodynamically unfa-
vorable Ni p species, which is formed when the one-electron reduction is coupled to
the binding of CO, pulling the equilibrium towards Ni p -CO formation.
Transmethylation generates a reactive methyl-Ni 3 þ

p species [ 37 ] that readily
accepts an electron to generate the stable methyl-Ni 2 p state, which is in agreement
with the EPR-silent CH 3 -ACS species [ 122 ]. Carbonyl insertion into the Ni-C bond
generates an acetyl-Ni 2+ intermediate, which is nucleophilically attacked by CoA.
Thiolysis of the acetyl-Ni 2+ intermediate liberates two electrons. One electron is
used to regenerate the Ni 1 p state, while the other electron might go into an internal
electron transfer pathway to reduce the unstable methylated Ni 3+ -species [ 142 ].
In the diamagnetic reaction mechanism (Figure 13b )Ni p is supposed to cycle
between Ni 0 and Ni 2+ , while the [4Fe4S] cluster remains in the oxidized state,
rendering only diamagnetic species catalytically relevant [ 111 , 122 ]. The formation
of the two electron reduced Ni p state is in agreement with recent density functional
theory calculations, suggesting a proton coupled electron transfer mechanism prior
to methylation [ 144 ]. The methyl group and CO can bind in random order to Ni p ,
generating a Ni p -CO or Ni 2 p -CH 3 species. In contrast to the paramagnetic mech-
anism, no further reduction of the reactive Ni 3+ state is needed. The two electrons
liberated at thiolysis may be used to regenerate the Ni p state. One problem of this