| STRUCTURE |
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. IN FINE DETAIL High-resolution crystal structure of the MoFe protein of nitrogenase, which contains the iron-molybdenum cofactor that converts N2 to NH3, reveals a previously overlooked atom--thought to be a nitrogen (blue sphere)--centered in a cage of six iron atoms (gray). (Carbon is green; molybdenum, red sphere; oxygen, red; and sulfur, yellow.) COURTESY OF OLIVER EINSLE/CALTECH In nitrogenase, N2 is converted to NH3 by a peculiar cofactor containing seven iron atoms, nine sulfur atoms, and a single molybdenum. Authors now show that `R` decade-old structure of this cofactor is missing a crucial ligand. When `R` 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 `R` `s findings. But at an improved resolution of 1.16 Å, `R` 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, `R` 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. "But nitrogenase's enzymatic activity makes nitrogen most plausible. The central nitrogen atom could serve a structural role. But it could also be left over from a single catalytic turnover of N2 to NH3, he adds, citing theoretical studies showing that N2 and its reduction products can bind within the iron cavity. The demands of modern agriculture have overburdened plants' nitrogenase-based route to biologically useful nitrogen, requiring farmers to supplement their soils with nitrogen fertilizers 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. |
| UPDATE | 09.02 |
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