Environmental Bioinorganic Chemistry

Life on CO and CO2 -- Phototrophic anaerobes such as Rhodospirillum rubrum have the ability to utilize the gaseous pollutant CO as their sole carbon and energy source. This ability derives from the oxidation of CO to CO2 catalyzed at the Ni-Fe-S (C-clusters) of the enzyme carbon monoxide dehydrogenase (CODH). Alternatively, acetogens such as Moorella thermoacetica use bifunctional CODH/acetyl-CoA synthases (ACSs) and metallocenters known as A-clusters to convert the greenhouse gas CO2 to acetyl-CoA. CODH/ACSs also catalyze the degradation of acetyl-CoA, which ultimately forms another greenhouse gas, methane.

Collectively, CODH/ACSs play a major role in the global carbon cycle as well as in the formation and removal of greenhouse gases and CO in our environment. It has been estimated that 1 x 108 tons of CO are removed by bacteria from the lower atmosphere and earth annually. A better understanding of the CODH mechanism could lead to the development of biomimetic catalysts capable of lowering the concentration of CO in heavily polluted areas.

Pathway of CO2 Fixation -- One molecule of CO2 (blue, top right) is converted to formate, then completely reduced to a methyl moiety in a series of steps that are catalyzed by folate dependent enzymes. This methyl moiety is transferred by a methyltranferase (MeTr) to the corrinoid-iron-sulfur protein (CFeSP). In turn, CFeSP transfers the methyl group to the ACS subunit of bifunctional CODH/ACS. The other molecule of CO2 (red, top right) is reduced to CO by the C-cluster of the CODH subunit of CODH/ACS. The CO intermediate is then transferred to the A-cluster of the ACS subunit through a long channel in the enzyme. Finally, the methyl group, CO, and CoA are assembled to form acetyl-CoA.