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SCIENCE
Although nitrogen gas makes up 80% of the air, this abundant source of nitrogen is inaccessible to plants. Instead, they rely on nitrogenase, an enzyme produced by nitrogen-fixing bacteria to convert unreactive atmospheric dinitrogen into metabolically useful ammonia.
When Rees's group first reported the 2.8-Å-resolution X-ray crystal structure of the MoFe protein of nitrogenase, the detailed structure of its unique iron molybdenum cofactor met with some surprise: The cluster's six central irons were each coordinated to only three other atoms--not four, as had been expected--in a trigonal prismatic arrangement. Subsequent structures with resolutions as high as 1.6 Å, however, confirmed Rees's findings. But at an improved resolution of 1.16 Å, Rees's group has found a hexacoordinated atom inside the trigonal prismatic cage of iron atoms, making each of the six irons approximately tetrahedral. The central atom was missed, Rees suggests, because the mathematical series used to calculate experimental electron-density maps are truncated to include crystallographic data only up to the available resolution, resulting in resolution-dependent artifacts around the atoms in the final map. At low resolution, the combination of such artifacts from the surrounding electron-dense iron and sulfur atoms creates a hole in the electron-density map sufficient to hide the electron density from the relatively electron-poor central atom. From the magnitude of observed electron density in the 1.16-Å-resolution structure, the central atom could be carbon, oxygen, or nitrogen, Rees says. "But nitrogenase's enzymatic activity makes nitrogen most plausible," he tells C&EN. His group is collaborating with R. David Britt of the University of California, Davis, to use electron paramagnetic resonance to nail down the atom's identity. "The results underscore the importance of obtaining metal active-site structures at high resolution," says crystallographer Amy C. Rosenzweig of Northwestern University, "especially since these crystal structures are often used as starting points for computational, mechanistic, and synthetic-modeling studies."
Fritz Haber (1868–1934) first synthesized NH3 from N2 and H2 in 1909. Four years later, Haber and fellow German Carl Bosch of BASF modified the process for commercial production of NH3. Although early production was devoted primarily to making German explosives in World Wars I and II, the Haber-Bosch process later allowed large-scale production of nitrogen fertilizers and revolutionized modern agriculture. The demands of modern agriculture have overburdened plants' nitrogenase-based route to biologically useful nitrogen, requiring farmers to supplement their soils with nitrogen fertilizers. The NH3 required to make such fertilizers is produced industrially using the Haber-Bosch process, in which an iron oxide catalyst is used to convert N2 and hydrogen gas to NH3 at high temperatures and pressures. Many chemists have tried to create a synthetic nitrogenase-like catalyst as a greener alternative to the Haber-Bosch process. Their lack of success could be because they were missing the nitrogen atom. "We hope this structure will give synthetic chemists some fresh insights," Rees says. |
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