Carbon Monoxide. Toxic Gas and Fuel for Anaerobes and Aerobes: Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 58

The Ni ion proximal to the [4Fe4S]-cluster (Ni p ) is labile [ 113 ] and readily
substituted by Cu and Zn [ 114 ]. Ni p is thiolate-bridged to the distal Ni (Ni d ) that is
coordinated by two cysteines and a glycine residue in a square planar thiolato- and
carboxamido-type N 2 S 2 coordination environment.
Due to the long distance between clusters A and C (~70

)[ 111 , 112 ] a direct
electron transfer between both clusters is unlikely. A potential gas channel has been
characterized that connects cluster C and A by crossing the N-terminal domain of
ACS (Figure 12b )[ 111 , 112 , 115 - 121 ]. This channel facilitates the diffusion of CO
between both clusters and prevents the loss of CO [ 119 , 120 ]. However, ACS can
also directly take up CO from the solvent [ 116 ]. A direct uptake is also consistent
with the presence of a monomeric ACS in C. hydrogenoformans , which is produced
when exogenous CO is available as substrate [ 81 ].
An “open” and a “closed” conformation of ACS have been reported, that directly
affect the state of the gas channel [ 81 , 111 , 112 ]. Compared to the closed state, in
which cluster A is buried within the ACS domain interface, the middle and
C-terminal domains are rotated by ~50 in the open state [ 111 ]. On the one hand
this rotation exposes cluster A to the solvent and makes Ni p accessible to methyl-
ated CoFeSP. On the other hand the gas channel is blocked by a single helix of
the N-terminal ACS domain, restricting CO diffusion. The gas channel is open in
the closed conformation of ACS and allows condensation of CO, CoA, and the
ACS-bound methyl group to occur.

Å

2.3.3 Substrate Binding and Reaction Mechanism

Different mechanisms for the condensation reaction have been proposed, varying
in the oxidation states of the Ni ions and the sequence of substrate binding.
We have currently no direct evidence where the substrates bind. However, some
indirect evidence is pointing to Ni p as the presumable position of CO activation.
The gas channel for CO diffusions opens directly above the proximal Ni site [ 111 ,
112 ] and there is evidence that Ni p must be present for methyl group transfer from
CoFeSP to ACS [ 122 , 123 ]. The Arg-rich interdomain cavity formed by the three
ACS subunits was proposed as CoA binding site [ 124 , 125 ].
A mechanism in which both Ni ions serve as binding sites for the substrates and
the [4Fe4S] cluster acts as an electron reservoir appears plausible based on the
composition of cluster A [ 112 , 126 ]. However, mechanisms favoring one Ni are
also supported by computational studies [ 127 ] and model complexes [ 128 - 130 ].
Although a role of the [4Fe4S] cluster for electron transfer appears plausible it is
unlikely, as electron transfer to and from the [4Fe4S] cluster was shown to be about
200-fold slower than methyl group transfer [ 131 ].
In the absence of low potential reductants, oxidized ACS is inactive and
EPR-silent [ 87 ]. The [4Fe4S] cluster of oxidized ACS is in the [4Fe4S] 2+ oxidation
state [ 90 , 132 , 133 ]. Both Ni ions have a square-planar coordination and are likely
present as Ni 2+ [ 111 - 113 , 133 - 136 ]. Reductive activation by one [ 101 , 137 ] or two