Environmental Engineering Reference
In-Depth Information
carbon dioxide activation. The primary reaction being catalyzed is the half-cell
reaction that is written as equation (
1
).
Ð
2H
þ
þ
2e
CO
2
þ
CO
þ
H
2
O
ð
1
Þ
Two chemically distinct types of CODH are distinguished by the structure of
their active site. Some aerobes use enzymes containing a
-sulfido bridged
Cu-S-Mo(pyranopterin) center [
5
,
7
]: on the other hand, some anaerobic micro-
organisms, such as
Moorella thermoacetica
(
Mt
) and
Carboxydothermus
hydrogenoformans
use enzymes in which the active site is a distorted [Ni3Fe-4S]
cubane-like cluster with a pendant (dangling) Fe [
8
]. The entire [Ni4Fe-4S] cluster
is referred to as the 'C-cluster' and the corresponding enzymes are known as
Ni-CODHs. The Ni-CODH family is extended to include the class of enzymes
known as CODH/ACS, in which a Ni-CODH is strongly associated with an acetyl-
CoA synthase. In acetogenic bacteria, CODH-ACS catalyzes C-C bond formation
using CO (formed from CO
2
) and a methyl group (donated by a corrinoid iron sulfur
protein) to form acetyl-CoA (equation
2
). In methanogenic archaea, Ni-CODH
coupled to acetyl-CoA decarbonylase/synthase (ACDS) cleaves acetyl-CoA to
CO
2
and methane [
9
].
μ
2
þ
þ
þ
H
þ
ð
2
Þ
CO
þ
½
CH
3
-Co II
ð
FeSP
þ
CoASH
Ð
CH
3
COSCoA
þ
½
Co
ðÞ
FeSP
This article focuses on the anaerobic Ni-CODHs, which have been divided into
four classes in terms of the metabolic role and subunit components of the enzymes,
as depicted, cartoonwise, in Figure
1
[
10
]. The classification can be confusing, as
Roman numerals are used to denote both class and isozymes within a class, but
hopefully this will not present a problem.
Class I enzymes consist of five subunits (
) and are found in autotrophic
methanogens, where acetyl-CoA is synthesized from CO
2
and H
2
. Class II enzymes
are found in acetoclastic methanogens and catalyze the decarbonylation of acetyl-
CoA to CO
2
, coenzyme A (CoA), and a methyl group which is transferred to
tetrahydrosarcinapterin. Class III enzymes are found in homoacetogens and are
sub-classified as two independent enzymes, a
ʱʲʳʴʵ
ʳʴ
heterodimer (CoFeSP): the catalytic center for synthesizing acetyl-CoA in tetra-
meric CODH/ACS is the same as found in Class I enzymes. Finally, Class IV
enzymes consist only of the CODH entity, a monofunctional
ʱ
2
ʲ
2
tetramer (CODH/ACS) and a
ʱ
2
dimer that catalyzes
the conversion of CO to CO
2
in the anaerobic CO-dependent energy metabolism of
bacteria and archaea. The structure of a Class IV CODH is shown in Figure
2
, along
with the structure of a Class III CODH/ACS.
Important examples of all five Ni-CODHs are provided by the thermophilic
bacterium
Carboxydothermus hydrogenoformans
which can grow on CO as the
sole carbon and energy source [
11
]. The first of these, referred to as CODH I
Ch
(the Roman letter does not refer to the classification mentioned above) is involved
in energy conversion and delivers electrons derived from CO oxidation to a
hydrogenase that evolves H
2
[
12
]. In
Rhodospirillum rubrum
, an enzyme very
closely related to CODH I
Ch
forms part of a well-characterized CODH-hydrogenase
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