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The great planetary nitrogen cycle, which includes the cyclic conversion of nitrogen gas (N2) into “fixed” nitrogen that can be used by plants, is in large part mediated by metalloenzymes that catalyze the elementary chemical reactions. On page 1666 of this issue, Hino et al. (1) take an important step toward understanding the chemical function and evolution of one of these enzymes. They describe the structure of a nitric oxide reductase (NOR) from a common bacterium that plays an important role in the nitrogen cycle—and in human disease.