Environmental Engineering Reference
In-Depth Information
Abstract Carbon monoxide (CO) pollutes the atmosphere and is toxic for
respiring organisms including man. ButCOisalsoanenergyandcarbonsource
for phylogenetically diverse microbes living under aerobic and anaerobic con-
ditions. Use of CO as metabolic fuel for microbes relies on enzymes like carbon
monoxide dehydrogenase (CODH) and acetyl-CoA synthase (ACS), which
catalyze conversions resembling processes that eventually initiated the dawn
of life.
CODHs catalyze the (reversible) oxidation of CO with water to CO 2 and come
in two different flavors with unprecedented active site architectures. Aerobic
bacteria employ a Cu- and Mo-containing CODH in which Cu activates CO and
Mo activates water and takes up the two electrons generated in the reaction.
Anaerobic bacteria and archaea use a Ni- and Fe-containing CODH, where Ni
activates CO and Fe provides the nucleophilic water. Ni- and Fe-containing
CODHs are frequently associated with ACS, where the CODH component
reduces CO 2 to CO and ACS condenses CO with a methyl group and CoA to
acetyl-CoA.
Our current state of knowledge on how the three enzymes catalyze these
reactions will be summarized and the different strategies of CODHs to achieve
the same task within different active site architectures compared.
Keywords acetyl-CoA synthase
carboxydotrophic
iron-sulfur
cluster
• molybdenum • molybdopterin • nickel
Please cite as: Met. Ions Life Sci . 14 (2014) 37-69
1
Introduction
1.1 Chemistry of Carbon Monoxide
Carbon monoxide (CO) is a colorless, odorless, flammable gas that burns with a
blue flame in air. Unlike the heavier carbon dioxide (CO 2 ), CO has a similar
molecular weight as N 2 and O 2 and readily mixes with air. CO has a low solubility
in water with approximately 0.35 L per L H 2 Oat0 C. The C-O distance of 1.06
Å
(solid CO) and 1.128
(gaseous CO) agrees with the presence of a C-O triple bond
making it isoelectronic with CN and N 2 . The triple bonded electronic structure of
CO results into a formal C-O polarization with a negative charge at C and a positive
charge at O. This polarization is almost exactly cancelled out by the counter acting
polarization due to the higher electronegativity of oxygen, making CO an unusual
“carbonyl”-type compound with an electron-rich carbon atom.
CO is produced by incomplete combustion of carbon-containing compounds.
CO can also be generated by adding concentrated sulfuric acid at 70-80 Cto
formic acid (HCOOH), a reaction, which is the formal dehydration of formic acid
to its anhydride CO. Conversely, CO can be converted to sodium formate by
Å
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