Nitrogen assimilation converts nitrate, nitrite, or ammonium into glutamine and glutamate, thereby supplying nitrogen for the synthesis of amino acids, nucleotides, and other nitrogen-containing compounds. Different key enzymes have clearly defined roles in substrate affinity, energy consumption, and tissue localization.

Keywords: nitrogen assimilation; glutamine synthetase; glutamate synthase; glutamate dehydrogenase; nitrate reductase; nitrite reductase; GS/GOGAT cycle; ammonium assimilation; nitrogen metabolism

1 Basic Processes of Nitrogen Assimilation Metabolism in Organisms

1.1 Uptake and Conversion of Inorganic Nitrogen

Plants, algae, fungi, and microorganisms can use ammonium, nitrate, and other nitrogen-containing compounds as nitrogen sources. Ammonium can directly enter amino acid biosynthetic pathways, whereas nitrate must first be reduced to nitrite by nitrate reductase and then further reduced to ammonium by nitrite reductase before entering glutamine and glutamate synthesis. Different organisms do not use nitrogen sources in exactly the same order. Because ammonium is already in a relatively reduced oxidation state, fewer reaction steps and less reducing power are required for its entry into assimilation pathways. Many microorganisms therefore preferentially use ammonium. Nitrate utilization requires additional reducing equivalents and the participation of cofactors such as molybdenum cofactors, iron–sulfur clusters, or siroheme.

1.2 Major Pathways Through Which Ammonium Enters Organic Nitrogen Metabolism

Ammonium assimilation mainly depends on the GS/GOGAT cycle composed of glutamine synthetase (GS) and glutamate synthase (GOGAT). GS first incorporates ammonium into glutamate to form glutamine. GOGAT then transfers the amide nitrogen of glutamine to 2-oxoglutarate, generating two molecules of glutamate. Glutamate dehydrogenase (GDH) can also catalyze the formation of glutamate from ammonium and 2-oxoglutarate, but its ammonium affinity, reaction direction, and physiological role vary among species and environmental conditions. Under normal growth conditions in most plants, the GS/GOGAT cycle is the principal pathway for ammonium assimilation. In some microorganisms, GDH can perform low-energy-cost glutamate synthesis when ammonium concentrations are high.

Figure 1. Major pathways of nitrate reduction, ammonium assimilation, and organic nitrogen biosynthesis

1.3 Nitrogen-Donor Roles of Glutamine and Glutamate

Glutamine mainly donates amide nitrogen and participates in the synthesis of nucleotides, amino sugars, tryptophan, histidine, asparagine, and other compounds. Glutamate supplies amino groups to different carbon skeletons through aminotransferases and is the direct source of the amino group in most amino acids. Together, glutamine and glutamate form the central node through which inorganic nitrogen enters organic biosynthesis. They also connect nitrogen metabolism with 2-oxoglutarate in the tricarboxylic acid cycle, thereby coordinating carbon and nitrogen metabolism.

2 Mechanism of Ammonium Assimilation Mediated by Glutamine Synthetase

2.1 GS-Catalyzed Reaction

Glutamine synthetase catalyzes the formation of glutamine from glutamate, ammonium, and ATP:

L-Glutamate + NH₄⁺ + ATP → L-Glutamine + ADP + Pi

This ATP-consuming reaction enables high-affinity ammonium fixation at relatively low ammonium concentrations. GS can assimilate both exogenous ammonium and ammonium released during nitrate reduction, amino acid degradation, photorespiration, and nucleic acid metabolism, thereby reducing free ammonium accumulation and its potential toxicity.

2.2 Catalytic Process

GS first uses ATP to phosphorylate the γ-carboxyl group of glutamate, forming the high-energy intermediate γ-glutamyl phosphate. Ammonium then attacks this intermediate in the form of ammonia, replacing the phosphate group and forming glutamine. Divalent metal ions such as Mg²⁺ or Mn²⁺ participate in ATP binding, substrate positioning, and transition-state stabilization. The type and concentration of metal ions therefore affect the reaction rate. The GS active site must simultaneously accommodate glutamate, ATP, and ammonium, and substrate binding involves ordered and spatially coordinated interactions. ATP deficiency, metal-ion imbalance, extreme pH, or excessively high substrate concentrations can all reduce catalytic efficiency.

2.3 Metabolic Regulation of GS

Bacterial GS can be jointly regulated through transcriptional control, covalent modification, and feedback inhibition. In some bacteria, type I GS can undergo adenylylation. Increased adenylylation generally decreases enzyme activity, whereas deadenylylation under nitrogen limitation helps restore GS activity and enhance ammonium assimilation. Plant GS is mainly regulated through gene expression, subcellular localization, protein stability, and phosphorylation. Light, nitrogen-source form, carbon-skeleton availability, senescence, and stress can alter the expression of different GS isoenzymes, thereby coordinating primary nitrogen assimilation, reassimilation of photorespiratory ammonium, and nitrogen remobilization.

3 Glutamate Synthase and the GS/GOGAT Cycle

3.1 GOGAT-Catalyzed Reaction

Glutamate synthase, also known as glutamine:2-oxoglutarate aminotransferase, catalyzes the transfer of the amide nitrogen of glutamine to 2-oxoglutarate:

L-Glutamine + 2-Oxoglutarate + Reducing Equivalents → 2 L-Glutamate

This reaction converts the glutamine generated by GS back into glutamate. One glutamate molecule can continue to serve as a GS substrate, whereas the other enters the biosynthesis of amino acids and other nitrogen-containing compounds. Therefore, GS and GOGAT must function continuously and cooperatively to maintain the cycle.

3.2 Catalytic Characteristics of GOGAT

GOGAT generally contains redox cofactors such as flavins and iron–sulfur clusters. Glutamine is first hydrolyzed at the amidase active site to release ammonia. The ammonia then travels through an internal protein channel to the synthase active site, where it reacts with 2-oxoglutarate to form an imine intermediate. The intermediate subsequently accepts electrons and is reduced to glutamate. According to the electron donor, GOGAT can be classified as ferredoxin-dependent, NADH-dependent, or NADPH-dependent. Although these forms catalyze the same nitrogen-transfer reaction, they differ in subcellular localization, tissue expression, and metabolic context.

3.3 Functions of the GS/GOGAT Cycle

The GS/GOGAT cycle has a high affinity for ammonium and is the principal route of nitrogen assimilation under low-ammonium conditions in plants and many microorganisms. The cycle consumes both ATP and reducing power and therefore has a higher energetic cost than the GDH pathway. However, it can maintain stable supplies of glutamate and glutamine at low ammonium concentrations. It also participates in reassimilation of photorespiratory ammonium, root nitrate utilization, assimilation of products derived from symbiotic nitrogen fixation, and nitrogen redistribution in senescing tissues. Its activity is jointly influenced by 2-oxoglutarate availability, cellular redox status, and the carbon-to-nitrogen ratio.

4 Regulation of Nitrogen Metabolism by Glutamate Dehydrogenase

4.1 GDH-Catalyzed Reaction

Glutamate dehydrogenase catalyzes the reversible reaction between oxidative deamination of glutamate and reductive amination of 2-oxoglutarate:

L-Glutamate + NAD(P)⁺ + H₂O ⇌ 2-Oxoglutarate + NH₄⁺ + NAD(P)H + H⁺

The reaction direction depends on substrate concentrations, coenzyme redox status, pH, and cellular metabolic demand. The reductive amination direction generates glutamate, whereas the oxidative deamination direction releases ammonium and produces 2-oxoglutarate.

4.2 Functions of GDH in Microorganisms

Many microorganisms use NADPH-dependent GDH for glutamate synthesis under high-ammonium conditions. This pathway does not consume ATP and has a lower energy cost than the GS/GOGAT cycle, but it generally has a lower affinity for ammonium and is therefore better suited to ammonium-rich environments. When ammonium concentrations decline, microorganisms can enhance the GS/GOGAT cycle to improve ammonium capture. Switching between the two pathways allows cells to balance energy expenditure against substrate affinity.

4.3 Functions of GDH in Plants

Plant GDH is mainly localized in mitochondria. Under normal conditions, it is generally not the principal pathway for primary ammonium assimilation but more commonly participates in glutamate degradation, replenishment of 2-oxoglutarate, and maintenance of carbon–nitrogen balance. Under carbon starvation, senescence, hypoxia, salt stress, or other adverse conditions, GDH-mediated oxidative deamination of glutamate can replenish carbon skeletons for the tricarboxylic acid cycle. Although plant GDH catalysis is reversible, reductive amination activity measured in vitro does not directly demonstrate that ammonium assimilation is its major in vivo role. Metabolite concentrations, isotope tracing, and net reaction flux must be considered together.

5 Inorganic Nitrogen Conversion Mediated by Nitrate Reductase and Nitrite Reductase

5.1 Reduction of Nitrate by Nitrate Reductase

Nitrate reductase (NR) catalyzes the two-electron reduction of nitrate to nitrite:

NO₃⁻ + 2e⁻ + 2H⁺ → NO₂⁻ + H₂O

Assimilatory NR in plants generally uses NADH or NADPH as the electron donor and contains FAD, heme, and a molybdenum cofactor. Electrons are sequentially transferred through the flavin and heme groups to the molybdenum center, where nitrate reduction is completed. Insufficient molybdenum supply, abnormal cofactor assembly, or inadequate reducing power can limit NR activity.

5.2 Formation of Ammonium by Nitrite Reductase

Nitrite reductase (NiR) catalyzes the six-electron reduction of nitrite to ammonium:

NO₂⁻ + 6e⁻ + 8H⁺ → NH₄⁺ + 2H₂O

Assimilatory NiR generally contains siroheme and an iron–sulfur cluster and can use reduced ferredoxin or NAD(P)H as an electron donor. In plants, NiR is mainly located in plastids or chloroplasts and rapidly converts nitrite, thereby preventing oxidative damage and metabolic toxicity caused by nitrite accumulation.

5.3 Coordination Between Nitrate Assimilation and Ammonium Assimilation

After sequential reduction by NR and NiR, nitrate is converted into ammonium, which is subsequently incorporated into glutamine and glutamate through the GS/GOGAT cycle. Nitrate reduction, ammonium fixation, and carbon-skeleton supply must remain coordinated; otherwise, nitrite or ammonium may accumulate. In plants, nitrate supply, light, sugar signaling, and circadian rhythms can regulate NR expression and activity. NiR and the GS/GOGAT cycle must respond in parallel so that newly generated ammonium can be rapidly fixed.

6 Species Distribution and Isoenzyme Differences of Key Nitrogen Assimilation Enzymes

6.1 Glutamine Synthetase Isoenzymes

Bacteria and archaea may contain different types of GS, including GSI, GSII, and GSIII. GSI commonly forms multimeric complexes and may be regulated by adenylylation. GSII is structurally more similar to eukaryotic GS, whereas GSIII is mainly distributed in certain bacteria and anaerobic microorganisms. Higher plants generally contain cytosolic GS1 and plastidic GS2. GS1 participates in primary nitrogen assimilation in roots, nitrogen remobilization in senescing tissues, and nitrogen transport in vascular tissues. GS2 mainly participates in assimilation of nitrate-reduction products in chloroplasts and reassimilation of ammonium released during photorespiration.

6.2 Glutamate Synthase Isoenzymes

Plants mainly contain ferredoxin-dependent GOGAT and NADH-dependent GOGAT. Ferredoxin-dependent GOGAT functions primarily in chloroplasts of photosynthetic tissues and is linked to reducing power generated by the light reactions. NADH-dependent GOGAT is important in roots, vascular tissues, nonphotosynthetic tissues, and developing grains. Bacteria, fungi, and algae may contain NADPH-dependent, NADH-dependent, or ferredoxin-dependent GOGAT. The form present is related to the organism’s energy metabolism, electron-donor source, and habitat.

6.3 Glutamate Dehydrogenase Isoenzymes

GDH can be classified according to coenzyme preference as NAD⁺-dependent, NADP⁺-dependent, or dual-specificity NAD(P)⁺-dependent. Anabolic GDH generally prefers NADPH, whereas catabolic GDH often prefers NAD⁺, although specific physiological functions cannot be inferred solely from coenzyme preference. Plant GDH usually forms multiple isoenzymes through combinations of different subunits and is mainly localized in mitochondria. Different isoenzymes vary in tissue distribution, stress response, and preferred reaction direction.

6.4 Distribution of Nitrate Reductase and Nitrite Reductase

Assimilatory NR in plants and fungi is generally located in the cytosol, whereas NiR is localized in plant plastids or the microbial cytosol. Prokaryotes may also contain respiratory and dissimilatory nitrate or nitrite reductases. Although these enzymes also participate in inorganic nitrogen conversion, their primary roles are energy metabolism or nitrogen loss rather than classical nitrogen assimilation. Therefore, studies of nitrate reductases must distinguish among assimilatory, respiratory, and dissimilatory enzymes. Physiological function cannot be inferred solely from the ability to convert nitrate or nitrite.

7 Functional Comparison of Glutamine Synthetase, Glutamate Synthase, and Glutamate Dehydrogenase

7.1 Catalytic Functions and Metabolic Positioning

Table 1 Functional Differences Among GS, GOGAT, and GDH

7.2 Metabolic Basis for Pathway Selection

The GS/GOGAT cycle is suited to low-ammonium environments. Although it requires ATP and reducing power, it has a high capacity for ammonium capture. The GDH reductive amination pathway does not directly consume ATP and has an energetic advantage under high-ammonium conditions, but it generally cannot replace the GS/GOGAT cycle for high-affinity ammonium assimilation. In plants, GS, GOGAT, and GDH are not simple parallel alternatives. GS and GOGAT form the principal assimilation cycle, whereas GDH more commonly performs glutamate degradation and carbon-skeleton regulation. Their actual contributions can be determined only by considering in vivo substrate concentrations, reaction direction, and metabolic flux.

8 Activity Assays and Research Applications of Nitrogen Assimilation Enzymes

8.1 Glutamine Synthetase Activity Assays

GS activity can be measured through the biosynthetic reaction or the transferase reaction. Biosynthetic assays measure the formation of glutamine, ADP, or inorganic phosphate and more closely reflect the direction of ammonium assimilation in vivo. Transferase assays use glutamine or glutamate derivatives and hydroxylamine to generate γ-glutamyl hydroxamate, which forms a colored complex with iron ions. Transferase activity is not fully equivalent to the in vivo glutamine-synthesis flux. When comparing different tissues, treatments, or species, extraction conditions, metal ions, substrate concentrations, and reaction directions should be standardized, and the results should be analyzed together with GS protein abundance and glutamine content.

8.2 GOGAT Activity Assays

NADH-GOGAT and NADPH-GOGAT activity can be measured by monitoring the decrease in NADH or NADPH absorbance at 340 nm. Blanks lacking glutamine or 2-oxoglutarate should be included to exclude coenzyme consumption caused by other dehydrogenases in the sample. Ferredoxin-dependent GOGAT requires an appropriate reduced-ferredoxin system, and its activity can also be evaluated by measuring glutamate formation. In complex samples, monitoring absorbance decline alone may lack specificity. High-performance liquid chromatography or mass spectrometry can be used to quantify the glutamate product when necessary.

8.3 GDH Activity Assays

GDH activity is generally measured by monitoring changes in NADH or NADPH absorbance at 340 nm. In the reductive amination direction, 2-oxoglutarate and ammonium are used as substrates and consumption of reduced coenzyme is monitored. In the oxidative deamination direction, glutamate is used as the substrate and formation of NADH or NADPH is monitored. In vitro activity should be measured in both directions, but maximum activity in either direction cannot directly indicate the net in vivo flux. Other NAD(P)H-dependent enzymes, endogenous 2-oxoglutarate, and ammonium in the sample may generate background signals.

8.4 Activity Assays for Nitrate Reductase and Nitrite Reductase

NR activity is generally evaluated by measuring the nitrite generated during the reaction. Nitrite can react with sulfonamide reagents and N-(1-naphthyl)ethylenediamine to form a colored azo compound. Assays should include controls lacking substrate, coenzyme, or active enzyme. Continued nitrite consumption by NiR in the sample should also be prevented because it may lead to underestimation of NR activity. NiR activity can be measured by monitoring nitrite depletion or ammonium formation. Because nitrite is chemically reactive, sample pigments, reducing substances, and nonenzymatic decomposition may affect the result. Reaction blanks and standard curves should therefore be included.

8.5 Stable-Isotope and Metabolomic Studies

¹⁵N-labeled nitrate, ammonium salts, glutamine, or glutamate can be used to trace the movement of nitrogen from inorganic nitrogen sources into amino acids, nucleotides, and proteins. Combined with mass spectrometry, isotope tracing can distinguish nitrogen uptake, primary assimilation, reassimilation, and intertissue transport and can evaluate the contributions of different enzymes to nitrogen metabolic flux. Enzyme-activity assays reflect potential catalytic capacity under defined conditions, transcriptomic and proteomic analyses reflect enzyme expression, and metabolomic and isotope-tracing approaches more closely reflect actual metabolic flux. Integrating multiple types of data can prevent overinterpretation of an entire nitrogen assimilation pathway based only on gene expression or a single enzyme-activity measurement.

8.6 Nitrogen Nutrition and Stress Research

Nitrogen assimilation enzymes can be used to evaluate plant responses to nitrogen fertilizers, nitrogen-use efficiency, root nitrogen uptake, leaf senescence, and grain nitrogen accumulation. They can also be used to study microbial adaptation to different nitrogen sources. Drought, salt stress, low temperature, hypoxia, and heavy metals may alter GS, GOGAT, GDH, NR, and NiR activity by affecting the supply of carbon skeletons, ATP, and reducing power. Experimental results should be interpreted together with tissue type, developmental stage, nitrogen-source form, and sampling time. For example, decreased NR activity in leaves may result from insufficient nitrate supply, changes in light conditions, or protein inactivation and cannot independently be interpreted as a decline in overall nitrogen assimilation capacity.

9 Products Related to Key Nitrogen Assimilation Enzymes, Regulators, and Activity Assays

Nitrogen assimilation is jointly completed through nitrate reduction, ammonium fixation, and glutamate regeneration. The GS/GOGAT cycle performs high-affinity ammonium assimilation, GDH participates in glutamate degradation and ammonium utilization under specific conditions, and NR and NiR connect inorganic nitrogen uptake with organic nitrogen biosynthesis. Combining enzyme-activity assays, metabolite quantification, and ¹⁵N tracing enables more accurate analysis of nitrogen-metabolism regulation across different species and environmental conditions.

For more related articles, please see below:

[1] Nitrate Reduction and Nitrogen Cycle Partitioning: Denitrification, DNRA, and Assimilatory Utilization Mechanisms