Biological ammonia (NH 3) oxidation to nitrate (NO 3-)-nitrification-is a critical pathway of the biogeochemical nitrogen cycle. Additional products and by-products of this pathway include nitrite (NO 2-), nitric oxide (NO), nitrous oxide (N 2 O), and nitrogen dioxide (NO 2), several of which are pollutants. How these species are generated during nitrification is not entirely clear, but pathways toward their generation have drawn substantial research effort. The cumulative evidence shows several parallel biological pathways comprising the net nitrification process. Bacteria were long thought to mediate all nitrification transformations; however, archaeal nitrifiers are now recognized. Furthermore, nitrification was thought to require two distinct microbial classes: NH 3 oxidizers to oxidize NH 3 to NO 2-, and NO 2 oxidizers that oxidize NO 2 to NO 3-. Comammox bacteria, which effect complete oxidation of NH 3 to NO 3-, were recently discovered. This Perspective summarizes the current understanding of nitrification biochemistry and highlights exciting opportunities for future research.

The metallobiochemistry underlying the formation of the inorganic N-N-bond-containing molecules nitrous oxide (N 2 O), dinitrogen (N 2 ), and hydrazine (N 2 H 4 ) is essential to the lifestyles of diverse organisms. Similar reactions hold promise as means to use N-based fuels as alternative, carbon-free energy sources. This review discusses research efforts to understand the mechanisms underlying biological N-N bond formation in primary metabolism and how the associated reactions are tied to energy transduction and organismal survival. These efforts comprise studies of both natural and engineered metalloenzymes, as well as synthetic model complexes.

One amino acid makes the difference between a metalloenzyme and a metalloprotein in two otherwise effectively identical cytochrome P460s.