Chemistry Reference
In-Depth Information
FIGURE 15.4
The coupling of CODH activity with hydrogenase activity in R. rubrum. The two subunits of CPDH are shown with light and
dark wire tracings. Electrons generated by the oxidation of CO at the C and C
0
clusters are transferred to the internal redox chain in CODH,
consisting of B (and B
0
) and D Fe
e
S clusters. The D-cluster, located at the interface between the two subunits, is proposed to transfer electrons
to the electron transfer protein (Coof), which is coupled to hydrogenase.
(From Ragsdale & Pierce, 2008. Copyright 2008 with permission from
Elsevier.)
The C-cluster of CODH reduces CO
2
to CO, which is then converted by the A-cluster of the ACS to acetyl-CoA
(
Figure 15.3
). The C-cluster is an unusual Fe
e
Ni
e
Fe
3
S
4
e
5
, which can be best viewed as a [Fe
3
S
4
] cluster bridged to
a dinuclear Ni
e
Fe centre (
Figure 15.5
). The catalytic efficacy of the C-cluster from C. hydrogenoformans
is
for the oxidation of CO the turnover number is 39,000 s
1
and the k
cat
/K
m
greater than 10
9
M
1
s
1
.
remarkable
e
FIGURE 15.5
The C-cluster of CODH and the A-cluster of ACS.
(From
Ragsdale, 2007
.
Copyright 2007 with permission from Elsevier.)
The active site A-cluster of the ACS is unusual in that it consists of a [4Fe-4S] cubane unit linked via a bridging
cysteine residue to a proximal metal ion which, in turn, is connected to a square-planar distal Ni via two cysteine
bridges as shown in
Figure 15.5
.
The identity of the catalytically active metal ion in the proximal site has been
the subject of some debate, but it now seems to be agreed that the catalytically active formof the enzyme has Ni in this
site (
Svetlitchnyi et al., 2004
). Cluster-Abinds bothCO, themethyl group fromCoFeSP andCoA. Twomechanisms of
acetyl-CoA synthesis at cluster-Ahave been proposed. In themononuclear mechanism, bothCOand themethyl group
bind at the proximal Ni, generating an acetyl group; subsequent attack of the carbonyl carbon by deprotonatedCoA-S
then gives acetyl-CoA. In the binuclear mechanism, CO binds at the proximal Ni and the methyl group at the distal Ni.
METHYL-COENZYME M REDUCTASE
It is estimated that more than 10
9
tons of methane per year are generated by methanogenic archaebacteria
functioning in anaerobic environments, all of which is derived from the catalytic activity of MCR (
Shima et al.,
