Biochemistry and Molecular Biology - Belyasova, N.A. 2002

Metabolism: Energy-Requiring Processes
Metabolism of Nitrogenous Compounds
Nitrogen Fixation and Its Incorporation into Organic Molecules

Molecular nitrogen is an extremely inert compound, and splitting the N₂ molecule requires a high energy input. Of all organisms on Earth, enzymatic Nitrogen Fixation is carried out by only a few prokaryotes. In their Cells, this process is catalyzed by the Nitrogenase system and proceeds via a pathway that can be described by the overall equation:

Class="center">N₂ + 6H⁺ + 6з + 12 ATP → 2NH₃ + 12ADP + 12P;      (16.1)

The bacterial nitrogenase system is localized in invaginations of Cell/30.html">The Plasma Membrane and consists of two components: an iron-sulfur protein (4Fe 4S^2-) and molybdoferredoxin (a molybdenum- and iron-containing protein). There are 2 molecules of the iron-sulfur protein per molecule of molybdoferredoxin (MoFe) in an active nitrogenase system. Flavin Cofactors and ferredoxin serve as accessory components involved in electron transfer to the nitrogenase system. The Role of the primary electron donor in this process is typically played by NADPH.

The process of electron transfer to molecular nitrogen (Fig. 16.2) proceeds as follows: the electron donor reduces ferredoxin, which transfers electrons to the iron-sulfur centers of the nitrogenase system either directly or via flavin. ATP then binds to the iron-sulfur Proteins and shifts their redox potential from -0.29 V to -0.4 V by inducing a conformational change in the protein. This increased reduction potential of the iron-sulfur proteins enables them to transfer electrons to molybdoferredoxin. In the next stage, ATP is hydrolyzed, MoFe is reduced, and the nitrogenase system dissociates into its component parts. It is believed that The transfer of a pair of electrons from the reduced form of the iron-sulfur proteins to molybdoferredoxin is coupled with the Hydrolysis of four ATP molecules.

The reduction of molybdoferredoxin is associated with the transition of the molybdenum atom from the oxidized Mo(VI) state to the reduced Mo(IV) state, from which electrons are transferred directly to N_2. Complete reduction of the nitrogen molecule to two molecules of NH₃ requires three consecutive 2-electron transfers coupled with the hydrolysis of 12 (4 × 3) ATP molecules.

Symbiotic nitrogen fixation, which involves certain plant structures In addition to the bacterial nitrogenase system, expends energy stored by both organisms. Evidence suggests that this process consumes up to one-fifth of all the energy stored by the plant.

The nitrogenase system reduces not only molecular nitrogen, but also acetylene, azide, nitrous oxide, cyanide, nitrites, and protons. A portion of the reducing equivalents is diverted to H⁺ by the nitrogenase system in a side reaction, which is why molecular hydrogen is always formed alongside ammonia during nitrogen fixation.

Molecular oxygen is a potent inhibitor of the nitrogenase system: in its presence, the essential Components of the system are rapidly inactivated. Consequently, nitrogen fixation takes place in anaerobic cellular microenvironments. For instance, in ROOT nodule Bacteria, a specialized form of Hemoglobin (leghemoglobin) protects the nitrogenase system from molecular oxygen, whereas in cyanobacteria, this function is performed by heterocyst walls.

The next stage in incorporating nitrogen into organic molecules is the amination of keto acids to yield Amino Acids. Reductive amination of keto acids is the most prevalent pathway. The primary product formed in this process is glutamate. Glutamate serves as a substrate for the incorporation of another ammonia molecule, resulting in the Synthesis of Glutamine (Fig. 16.3).

The reactions listed above are of particular importance in Amino acid Biosynthesis because the Enzymes catalyzing them are highly active, making these processes the primary pathways for incorporating ammonia nitrogen into amino acids. During subsequent transformations, the amino groups of glutamate and glutamine are incorporated into The Structure of most Other Amino Acids. Such reactions are termed Transamination reactions.

Fig. 16.2. Operation of the prokaryotic nitrogenase system during MOLECULAR NITROGEN FIXATION (explanation in text)

Fig. 16.3. Incorporation of ammonia into amino acids