Vanadium-dependent nitrogenase can bind two CO molecules at the same time

The iron-vanadium (FeV) cofactor in the vanadium-dependent nitrogenase was reacted with carbon monoxide (CO) and then gassed under pressure, whereby two molecules of the substrate could be made visible in bound form. The FeV cofactor is one of the largest and most complex metal centers in proteins currently known. It consists of seven iron ions (gray), 9 sulfur ions (yellow), a central carbon (black) and a vanadium ion (green) and carries a carbonate ion and a homocitrate molecule as organic ligands. Photo credit: Oliver Einsle

The biological fixation of the element nitrogen by the enzyme nitrogenase gives organisms access to molecular nitrogen (N2) in the earth’s atmosphere, which is essential for building cellular structures. In addition, a vanadium-dependent variant of nitrogenase can reduce the toxic gas carbon monoxide (CO) to hydrocarbons. These reductions in N2 and CO are among the most important processes in industrial chemistry, as they are used to produce both fertilizers and synthetic fuels. However, the researchers have not yet been able to decipher the different reaction pathways of the two reactions.

Dr. Michael Rohde from the team of Prof. Dr. Oliver Einsle at the Institute for Biochemistry at the University of Freiburg, in collaboration with two research groups from the Free University of Berlin, was able to show how the active center of the vanadium-dependent nitrogenase is able to bind two CO molecules at the same time and thus creates the basis for the to connect spatially adjacent carbon atoms of both molecules in a reductive process. The researchers recently presented their results in the journal Science Advances.

The industrial reduction of N2 and CO – known as the Haber-Bosch or Fischer-Tropsch process – requires high temperatures and high pressure. While the 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 not only plays an important role in fuels but also in the manufacture of plastics. Although the cleavage of an NN bond during nitrogen fixation is chemically fundamentally different from the formation of a CC bond during CO reduction, scientists previously suspected that nitrogenase uses the same basic mechanistic principles for both reactions.

In an earlier work, Rohde and Einsle’s team used nitrogenase to react with CO gas, which resulted in the specific binding of a single molecule. In their current study based on this, the researchers show that they gassed crystals of this first state with CO under pressure and then subjected them to an X-ray crystallographic analysis. This enabled them to observe directly how a second CO molecule binds. “The form of nitrogenase obtained in this way with two CO molecules at the active center probably represents a blocked state,” explains Rohde, “but it provides direct information about the mechanism of the enzyme.” As a result, Einsle’s team can now identify a detailed mechanism of the CO – Sketch reduction by nitrogenase.

Researchers clarify how a nitrogen-fixing enzyme also produces hydrocarbons

More information:
Michael Rohde et al., Two ligand-binding sites in CO-reducing V Nitrogenase disclose a general mechanistic principle, Science Advances (2021). DOI: 10.1126 / sciadv.abg4474 Provided by the University of Freiburg

Quote: Vanadium-dependent nitrogenase can bind two CO molecules at the same time (2021, June 10), accessed on June 11, 2021 from .html

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