Fundamentals of N2 Fixation

Decades of N2 fixation research have uncovered many details of the intricacies and complexity of Biological N2 Fixation (BNF), from the evolutionary path of the enzyme to the diversity of microorganisms, the biochemistry of the reaction, and the underlying genetics. The enzyme and its reaction are complex and require substantial energy and reductant, while being extremely sensitive to O2. The enzyme is comprised of two multi-subunit proteins with metal cofactors. There are Mo, V and Fe forms of nitrogenase, encoded by related but divergent genes. Nonetheless, diverse microorganisms of varied physiologies are capable of BNF, and have evolved multiple solutions to obtaining sufficient energy and protecting from O2. Diazotrophs include strict anaerobes, microaerophiles, and cyanobacteria, which are photosynthetic and evolve O2. There are multiple genes involved in N2 fixation, beyond the nifHD(G)K genes that encode nitrogenase that provide multiple functions including protein assembly, cofactor synthesis and metal transport. Many diazotrophs on land provide fixed N to multicellular plants and animals through symbiotic interactions, and there are specific genes involved in maintaining symbiosis and facilitating N2 fixation. The nitrogenase genes are highly regulated in order to conserve energy when N is available or when O2 is present, through complex regulatory cascades that effect cellular changes, protein modifications and transcriptional regulation.