Ammonia Synthesis—Biological

Abstract

Dinitrogen (N 2 ) in the earth's atmosphere cannot be metabolized directly by most organisms and, consequently, cellular nitrogen is usually obtained as an already reactive form, such as ammonia or nitrate. These sources of “fixed” nitrogen are relatively scarce and most often limit agricultural production. Because organic nitrogen is incompletely recycled and because available ammonia and nitrate are continually metabolized to N 2 , the biological reduction of N 2 occupies a pivotal position in the biogeochemical nitrogen cycle. This process, called nitrogen fixation, is performed only by prokaryotic microorganisms and catalyzed by the enzyme, nitrogenase. N 2 ‐fixing microorganisms may be either free‐living or associated symbiotically with other microbes, plants, or infrequently animals. The most agriculturally significant are the symbiotic microorganisms that live within plant tissues. The most common nitrogenase is the molybdenum‐(Mo)based enzyme, but similar vanadium‐based and iron‐based enzymes are known plus a fourth unique system. The structure of its two component proteins and their 2:1 complex are known. Most nitrogenases catalyze the reduction of other substrates, such as C 2 H 2 and HCN, in reactions inhibited by CO. Substrate reduction involves association–dissociation cycles in which electrons (accompanied by MgATP hydrolysis) are transferred between component proteins. Each nitrogenase is genetically distinct, and its biosynthesis, maturation, and activity are regulated by the products of up to 19 genes. A fundamental understanding of the biochemical action and genetic regulation of the nitrogen‐fixation components has significant potential for improving the economy of nitrogen fixation, minimizing water pollution, and increasing global food supplies.