Binding of a second CO molecule observed
Researchers clarify another step in the mechanism of reduction of carbon monoxide through nitrogenase
Oliver Einsle
Different reactions of nitrogenase
The industrial reductions of N2 and CO - known as the Haber-Bosch and Fischer-Tropsch processes, respectively - require high temperatures and pressure. While N2 reduction leads to the bioavailable product ammonium (NH4+), at least two carbon atoms combine during the conversion of CO. The predominant reaction product is ethylene (ethene, C2H4), a colorless gas that plays an important role not only in fuels but also in the production of plastics. Although the cleavage of an N-N bond in nitrogen fixation is chemically quite fundamentally different from the formation of a C-C bond in CO reduction, scientists previously suspected that nitrogenase uses the same basic mechanistic principles for both reactions.
Second CO molecule at the active site
In a previous work, the team led by Rohde and Einsle used nitrogenase to react with CO gas, resulting in the specific binding of a single molecule. In their current study, which builds on this work, the researchers show that they gassed crystals of this first state with CO under pressure and then subjected them to X-ray crystallographic analysis. This allowed them to directly observe how a second CO molecule binds. “The form of nitrogenase obtained in this way, with two CO molecules at the active site, probably represents a blocked state,” Rohde explains, “but it provides direct clues to the mechanism of the enzyme.” As a result, Einsle's team can now outline a detailed mechanism of CO reduction through nitrogenase.
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