Chemistry Reference
In-Depth Information
THE Wood-Ljungdahl Pathway
CO
2
"H
2
"
HCOOH
H
4
folate
HCO
-H
4
folate
"Eastern"
or Methyl
Branch
CO
2
"Western" or
Carbonyl Branch
CH
+
=H
4
folate
"H
2
"
CODH
"H
2
"
CO
CH
3
-Co
(III)
CH
2
=H
4
folate
MeTr
CoA
"H
2
"
O
Biomass
(Cell Carbon)
Co
(I)
CFeSP
CH
3
=H
4
folate
C
H
3
C
SCoA
Acetate
+ ATP
4H
2
+ 2CO
2
®
CH
3
COOH + 2H
2
O
FIGURE 15.3
The Wood
e
Ljungdahl pathway. “H
2
” is used in a general sense to designate the requirement for two electrons and two protons
in the reaction.
(From Ragsdale & Pierce, 2008. Copyright 2008 with permission from Elsevier.)
are both thought to be ancient enzymes, which possibly allowed primitive organisms to live in the anaerobic,
CO
2
-rich atmosphere. In this pathway, carbon dioxide isreducedtocarbonmonoxidebyCODH,whichisthen
converted to acetyl coenzyme A by ACS, using a methyl group which is itself derived from CO
2
.Themethyl
group is then transferred from methyltetrahydrofolate by a methyltransferase (MeTr) to a corrinoid iron
e
sulfur protein (CFeSP), which in turn transfers the methyl group to the A-cluster of ACS.
Microorganisms which contain CODH/ACS enzymes are found in all locations where anaerobic metabolism is
the only means of survival, from peat bogs to the rumen of the cow, to the human intestine.
2
The so-called
C-cluster of CODHs allow organisms to use CO as a source of energy and carbon, while other acetogenic and
methanogenic bacteria use bifunctional CODH/ACS enzymes to convert the greenhouse gas CO
2
to acetyl-CoA.
Collectively, CODH/ACS enzymes play a key role in the C1 metabolism of anaerobic organisms and represent
a major component of the global carbon cycle.
CODHs catalyse the oxidation of carbon monoxide in a reversible, two-electron process. They are homodi-
meric enzymes with five metal clusters, two C-clusters which catalyse the oxidation of CO to CO
2
and three
typical [Fe
4
S
4
] cubane clusters (
Figure 15.4
)
. In R. rubrum, electrons are transferred from the D-cluster to
a membrane-associated Fe
S protein designated CooF, which transfers electrons to a hydrogenase, coupling CO
oxidation with H
2
production.
e
2. Where it is supplied by the microbial 'guest workers' who profit from our prolific source of food, and, in return, perhaps inadvertently,
return the hospitality with a few vitamins and other essential nutrients that we are unable to make.
