3.21
Asimilación de nitrógeno inorgánico
El nitrógeno es un elemento esencial en los sistemas biológicos, constituyendo un componente fundamental de las proteínas, los ácidos nucleicos y otro…
El nitrógeno es un componente principal de las proteínas, los ácidos nucleicos, las coenzimas y otros constituyentes celulares.
La mayoría de las bacterias y arqueas asimilan el nitrógeno en forma de nitrato y amoníaco.
El nitrato entra en las células y se reduce secuencialmente a nitrito por la nitrato reductasa, y luego a amoníaco por la nitrito reductasa, en un proceso llamado reducción asimiladora de nitratos.
La asimilación del amoníaco se produce a través de dos vías principales, dependiendo de los niveles de amoníaco.
A altos niveles de amoníaco, las bacterias y los hongos utilizan la vía de aminación reductora, donde la glutamato deshidrogenasa convierte el α-cetoglutarato en glutamato.
A niveles bajos de amoníaco, se activa el sistema glutamina sintetasa-glutamato sintasa o GS-GOGAT.
Aquí, la glutamina sintetasa, o GS, convierte el amoníaco y el glutamato en glutamina, que luego es procesada por la glutamato sintasa, o GOGAT, para producir dos moléculas de glutamato.
La asimilación de nitrógeno difiere de la fijación de nitrógeno, un proceso exclusivo de ciertos procariotas. En la fijación de nitrógeno, las enzimas nitrogenasas convierten el nitrógeno atmosférico en amoníaco, lo que lo hace biológicamente disponible.
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Q1: How does nitrate get converted to ammonia in bacterial cells?
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.
Q2: What is the difference between the reductive amination pathway and the GS-GOGAT system?
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.
Q3: Why do microorganisms use different ammonia assimilation pathways?
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.
Q4: How does nitrogen fixation differ from nitrogen assimilation?
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.
Q5: What role does glutamate play in nitrogen assimilation?
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.
Q6: What electron donors are used during assimilatory nitrate reduction?
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.
Q7: Why is the GS-GOGAT system more efficient at low ammonia concentrations?
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.