無機窒素の同化

Nitrate enters the cell and undergoes assimilatory nitrate reduction, a two-step enzymatic process. Nitrate reductase first reduces nitrate to nitrite using NADH or FAD as electron donors. Nitrite reductase then converts nitrite to ammonia with ferredoxin as a cofactor, making nitrogen available for cellular metabolism and biosynthesis.

The reductive amination pathway operates when ammonia is abundant; glutamate dehydrogenase rapidly converts α-ketoglutarate to glutamate. The GS-GOGAT system activates at low ammonia levels and is more energy-intensive. Glutamine synthetase forms glutamine from ammonia and glutamate, then glutamate synthase converts it into two glutamate molecules for amino acid and nucleotide synthesis.

Microorganisms switch pathways based on environmental ammonia availability. At high ammonia concentrations, the reductive amination pathway provides rapid nitrogen incorporation. At low ammonia levels, the GS-GOGAT system, though more energy-intensive, efficiently captures and assimilates scarce nitrogen into glutamate for biosynthesis of nucleic acids and other cellular components.

Nitrogen assimilation incorporates nitrate or ammonia into organic molecules for cellular metabolism. Nitrogen fixation, unique to certain prokaryotes, converts atmospheric nitrogen gas into bioavailable ammonia using the nitrogenase enzyme complex. Fixation is energy-intensive, requiring ATP and electrons, and replenishes biologically usable nitrogen in nitrogen-deficient ecosystems.

Glutamate is a central nitrogen-containing molecule in both ammonia assimilation pathways. In reductive amination, glutamate dehydrogenase produces glutamate directly from α-ketoglutarate. In the GS-GOGAT system, glutamate synthase generates two glutamate molecules from glutamine. Glutamate then serves as a nitrogen donor for biosynthesis of amino acids and other nitrogen-containing cellular constituents.

During assimilatory nitrate reduction, nitrate reductase uses NADH or FAD as electron donors to reduce nitrate to nitrite. Nitrite reductase then requires ferredoxin as a cofactor to complete the reduction to ammonia. The choice of electron donor depends on environmental and cellular conditions, ensuring efficient nitrogen incorporation into biomolecules.

The GS-GOGAT system has high affinity for ammonia, allowing glutamine synthetase to capture scarce nitrogen effectively. Although ATP-dependent and energy-intensive, this pathway ensures efficient nitrogen assimilation when ammonia is limited. The system's two-step process and regeneration of glutamate maximize nitrogen retention and utilization in biosynthesis in bacteria.