General Microbiology - Schlegel, H. 1987

Molecular nitrogen fixation
Biochemistry of nitrogen fixation

N₂ fixation is a reductive process, and ammonia is its first detectable product. The reduction takes place on The Nitrogenase Enzyme complex. Nitrogenase consists of two components: a molybdenum-iron-sulfur protein and an iron-sulfur protein¹. Both the enzyme itself and the N₂ fixation process are extremely sensitive to molecular oxygen. This explains why both free-living nitrogen-fixing Bacteria and ROOT nodule Tissues possess specialized mechanisms that protect nitrogenase from high partial pressures of oxygen.

The fixation of molecular nitrogen requires both reducing power and energy (Fig. 13.2), which can be supplied by Photosynthesis, Fermentation, or Respiration. In model experiments with purified Components of the nitrogenase system (in vitro), energy can be delivered as ATP and reducing power in the form of reduced pyridine NUCLEOTIDES and ferredoxins, using flavodoxin-containing carriers. The ATP cost for this process is very high.

The nitrogenase system reduces not only molecular nitrogen (N≡N), but also acetylene (HC≡CH), azide, nitrous oxide, cyanide, nitrites, isonitriles, and protons. Acetylene reduction serves as the basis for the simplest method to detect nitrogenase activity. Acetylene is reduced exclusively to Ethylene, which is easily quantified using gas Chromatography. All nitrogen-fixing microorganisms and symbiotic systems studied to date are capable of reducing acetylene.

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Fig. 13.2. General scheme of Nitrogen Fixation. Fd - ferredoxin; Fl - flavodoxin.

¹ These are referred to as "Component I" (or Mo-Fe protein) and "Component II" (or Fe-S protein). - Ed. note.

In the absence of molecular nitrogen, the nitrogenase system reduces protons to molecular hydrogen. Thus, the nitrogenase system also exhibits The properties of an ATP-dependent H₂-evolving Hydrogenase. Because molecular hydrogen is also formed in the presence of N₂, the corresponding reaction can be included in the equation describing nitrogen fixation:

8[Н] + N₂ + 2Н⁺ - 2NH⁺₄ + Н₂

Most nitrogen-fixing bacteria contain a (classical) hydrogenase that activates H₂ In addition to nitrogenase. The function of this hydrogenase is presumably to utilize the hydrogen produced during MOLECULAR NITROGEN FIXATION.

Regulation of Nitrogen Fixation. In many bacteria, nitrogenase is synthesized only when needed, i.e., in the absence of a suitable source of fixed nitrogen. Ammonium ions repress the synthesis of nitrogenase. In Purple and green bacteria, these ions also reduce The activity of the enzyme that has already been synthesized. Glutamine Synthetase apparently plays a major role in regulating nitrogenase formation. Bacteria require glutamine synthetase and glutamate synthase to incorporate ammonium ions into Organic compounds when these ions are present only at low concentrations. This system has a high affinity for ammonium ions and maintains their intracellular concentration at a low level. An increase in the concentration of ammonium ions in The Cell's environment (and consequently within the cell) represses The formation of glutamine synthetase, and consequently, of nitrogenase as well.

Transfer of Nitrogen Fixation Genes (nif Genes). The ability to fix nitrogen can be transferred from one bacterium to another via direct cell-to-cell contact. The possibility of transferring nif genes from Klebsiella pneumoniae to Escherichia coli via conjugation, coupled with the fact that these genes are located on a plasmid, raises hopes that their transfer to other bacterial species¹—and perhaps even to eukaryotic organisms—will be achieved in the near future. However, because nitrogen fixation requires not only nitrogenase but also a specific iron-sulfur protein, as well as Protection of the enzyme from O₂, such experiments involve considerable difficulties.

¹Such transfer has already been accomplished. - Ed. note.