3.21
Assimilação de Nitrogênio Inorgânico
O nitrogênio é um elemento essencial nos sistemas biológicos, compondo proteínas, ácidos nucleicos e outros constituintes celulares fundamentais. Muit…
O nitrogênio é um componente importante de proteínas, ácidos nucléicos, coenzimas e outros constituintes celulares.
A maioria das bactérias e archaea assimila nitrogênio na forma de nitrato e amônia.
O nitrato entra nas células e é sequencialmente reduzido a nitrito pela nitrato redutase, depois a amônia pela nitrito redutase, em um processo chamado redução assimilatória de nitrato.
A assimilação de amônia ocorre por meio de duas vias principais, dependendo dos níveis de amônia.
Em altos níveis de amônia, bactérias e fungos usam a via de aminação redutiva, onde a glutamato desidrogenase converte α-cetoglutarato em glutamato.
Em baixos níveis de amônia, a glutamina sintetase-glutamato sintase ou sistema GS-GOGAT é ativado.
Aqui, a glutamina sintetase, ou GS, converte amônia e glutamato em glutamina, que é então processada pela glutamato sintase, ou GOGAT, para produzir duas moléculas de glutamato.
A assimilação de nitrogênio difere da fixação de nitrogênio, um processo exclusivo de certos procariontes. Na fixação de nitrogênio, as enzimas nitrogenase convertem o nitrogênio atmosférico em amônia, tornando-o biologicamente disponível.
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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.