Glutamine (Gln) Amino Acid - Creative Peptides (2025)

What is glutamine?

Glutamine is the predominant amino acid in plasma, and its significance for good cellular growth in culture has been recognized since the 1950s. It demonstrates exceptionally swift cellular turnover rates and functions as a crucial metabolic precursor in the creation of nucleotides, glucose, and amino sugars, as well as in glutathione homeostasis, protein synthesis, and as a source of oxidative energy. Glutamine is more lipophilic and less dipolar than asparagine. Glutamine and glutamate, along with proline, histidine, arginine, and ornithine, account for 25% of dietary amino acid consumption and form the "glutamate family" of amino acids, which are metabolized into glutamate.

Metabolic products derived from glutamine. (Tapiero H., et al., 2002)

Glutamine structure

Glutamine is a carboxylic acid that has five carbon atoms, a molecular weight of 146.15 kDa, and the following elemental composition: carbon (41.09%), hydrogen (6.90%), oxygen (32.84%), and nitrogen (19.17%). Although it is not considered a necessary amino acid from a dietary standpoint, glutamine is considered neutral in terms of its physiological pH. The ability of glutamine to transport nitrogen and carry NH3 is made possible by its two amino groups, the α-amino group and the easily-hydrolyzable side-chain amide group. Around 5–6% of bound amino acids are glutamine, which is another example of a proteinogenic amino acid.

Glutamine synthesis and hydrolysis. (Cruzat V., et al., 2018)

Glutamine pKa

Unlike carboxyl and amino groups, the amide group on the side chain of glutamine (the NH2 group of the side chain) does not ionize under typical physiological circumstances and so does not have a conventional pKa value. In normal pH ranges (around 7.4 in the human body), the side chain remains neutral. Glutamate is an amino acid that is neutral with regard to its physiological pH. The pKa values of its α-Carboxyl group (COOH) are approximately 2.18, whereas the pKa of the α-Amine group (NH2) is approximately 9.13.

Protolytic forms of glutamine. (Kiersikowska E., et al., 2016)

Glutamine solubility

Glutamine exhibits excellent solubility in water. Its solubility is roughly 1200 mg/mL at 25°C. Glutamine readily dissolves in water, rendering it appropriate for numerous biological and biochemical applications, such as its incorporation in cell culture conditions and as a nutritional supplement. At a pressure of 98.8 kPa and a temperature range of 283.15–323.15 K, the static gravimetric method was utilized to investigate the solubility of L-glutamine in certain monosolvents. The experimental results indicated that an increase in temperature correlates with a progressive rise in the solubility of L-glutamine in solvents. The solubility hierarchy of L-glutamine in pure solvents at 303.15 K is as follows: Water > n-Hexane > Ethanol > n-Butanol > Isobutanol > Ethyl Acetate > sec-Butanol > 2-Butanone > n-Pentanol > n-Propanol > Acetonitrile > Methanol. The PXRD spectra indicated that there was no alteration in crystal shape throughout the entire experimental procedure. The results of the solvent effect analysis indicate that the solvation behavior of L-glutamine is affected by numerous factors.

Mole fraction solubility of L-glutamine in monosolvent systems. (Liu D., et al., 2023)

Fluorescent glutamine analog

To directly observe glutamine in living cells derived from brain tissues, one study created a fluorescent rhodamine-tagged glutamine molecule (RhGln) to use as a probe. The tiny, stable, highly fluorescent, water-soluble, and non-hydrolyzable glutamine analog RhGln was produced in a 5-step process. Just like theanine, the fluorescent tag was attached to the amide side chain of glutamine so that it cannot be hydrolyzed to glutamate by glutaminase. This allowed for the imaging of glutamine alone, apart from any of its metabolites. The positively charged rhodamine group is flat. Density functional theory (DFT) studies show that the ideal separation between it and glutamine preserves the latter's biological activities. The absorption and emission maxima of RhGln were found to be 580 and 601 nm, respectively, according to further characterization. A modest bathochromic shift of the absorption and emission spectra and a fluorescence emission quantum yield of 0.6 were seen in RhGln when contrasted with the unconjugated rhodamine molecule (Rh101).

Characterization of fluorescent rhodamine-tagged glutamine (RhGln). (Cheung G., et al., 2022)

Glutamine amino acids at Creative Peptides

What does glutamine do?

In mammalian cells, glutamine serves as a crucial connection between carbohydrate and protein carbon metabolism, significantly contributing to the proliferation of fibroblasts, lymphocytes, and enterocytes. It enhances nitrogen equilibrium and maintains glutamine levels in skeletal muscle. When plasma glutamine levels are inadequate to meet demand, glutamine is synthesized from skeletal muscle and liver. Reduced glutamine availability for macrophages and lymphocytes was associated with diminished plasma glutamine and citrulline concentrations. Post-trauma, diminished arginine levels can be reinstated to physiological levels with glutamine supplementation, although physiological glutamine levels (650 µmol/l) are only partially restored. Glutamine is a crucial substrate for ammoniagenesis in the gastrointestinal tract and the kidneys, owing to its significant role in regulating acid-base homeostasis. It quickly decomposes to produce ammonia and glutamate or, by intramolecular catalysis, to pyroglutamate. The transamination and deamidation of glutamine facilitate ammonia transport among different tissues. Deamidation of glutamine through glutaminase yields glutamate, a precursor of gamma-aminobutyric acid, an inhibitor of neurotransmission. The transference of amide nitrogen from glutamine through the amido transferase process is crucial to the biosynthesis of purines and pyrimidines, as well as the synthesis of hexosamines. Glutamine is converted to α-ketoglutarate by glutamate, which is a crucial element of the citric acid cycle. It is a constituent of the antioxidant glutathione and polyglutamated folic acid. The cyclization of glutamate yields proline, an amino acid essential for collagen and connective tissue formation. Excessive glutamine in a protein holds pathological significance, as several neurodegenerative diseases have been linked to a CAG expansion that results in the proliferation of glutamine repeats in the affected proteins (the CAA and CAG codons facilitate the incorporation of glutamine from its transfer RNA, utilizing its anti-codon triplet, into the genetically specified location of the polypeptide chain). This results in aberrant protein conformation and neurodegenerative disorders. Proteins with repeats exceeding 41 glutamine residues generate hazardous neuronal nuclear aggregates in the afflicted cells. Despite being categorized as a nonessential amino acid, glutamine becomes conditionally essential with acute trauma, significant surgery, sepsis, bone marrow transplantation, and intensive chemotherapy and radiotherapy, when its consumption surpasses its production.

Glutamine metabolism

The majority of tissues have the ability to produce glutamine from α-ketoglutarate and glutamate using the cytosolic enzymes glutamate aminotransferase and glutamine synthetase. Reversible coupling between exergonic adenosine triphosphate (ATP) cleavage to adenosine diphosphate and inorganic phosphate and endergonic ammonia addition to glutamate forms glutamine. You can get the carbon atom of α-ketoglutarate that will turn into glutamine from glucose or other amino acids that have been released from proteins. Proteolysis can also lead to the formation of free glutamine.

The first stage in the breakdown of glutamine is for an enzyme in the mitochondrial matrix called phosphate-dependent glutaminase to convert it to glutamate. This enzyme is closely associated with the glutamine transport pathway into the mitochondrial matrix. Depending on the location, the glutamate can be transaminated by the cytosol and mitochondrial matrix enzymes glutamate-oxaloacetate-transaminase and glutamate-alanine-transaminase, or, less crucially, deaminated by the mitochondrial matrix enzyme glutamate dehydrogenase. An intermediate of the tricarboxylic acid cycle, α-ketoglutarate is produced by both processes. One of two systems, the electrogenic glutamate/aspartate antiporter or the electroneutral glutamate/hydroxyl antiporter, is responsible for transporting glutamate into the mitochondria. The enzyme glutamate semialdehyde dehydrogenase can convert glutamate to glutamate semialdehyde in certain tissues, such as the jejunum, where proline and arginine are subsequently formed. The majority of tissues convert glutamine to carbon dioxide (CO2), but some, like the liver and kidneys (described later), and some, like the muscle and liver, have the enzyme machinery to convert it to glucose and glycogen.

The metabolic mechanism for the conversion of glutamine to glucose in hepatocytes and renal tubular cells entails deamination to glutamate, transamination to α-ketoglutarate, and subsequent conversion to oxaloacetate, an intermediate of the Krebs cycle, which then enters the gluconeogenic pathway. Carbons 1 through 3 or 2 through 4 of glutamine are immediately integrated into glucose, while the remaining carbons are released as CO2 throughout the Krebs cycle. The phosphoenolpyruvate carboxykinase step can, theoretically, re-enter the gluconeogenic pathway at the pyruvate carboxylase step. In contrast to gluconeogenesis from alternative substrates (lactate, alanine, pyruvate), glutamine gluconeogenesis is distinctive as it constitutes an exergonic reaction, yielding a net of 8 mol ATP per mol of glucose synthesized, contingent upon the aerobic oxidation of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH). This occurs because reducing equivalents are produced in the processes of glutamate dehydrogenase, α-ketoglutarate dehydrogenase, malate dehydrogenase, and succinate dehydrogenase within the Krebs cycle, which are not implicated in gluconeogenesis from alternative substrates.

Biochemistry of glutamine metabolism. (Stumvoll M., et al., 1999)

Release and uptake of glutamine by various organs in humans. (Stumvoll M., et al., 1999)

Glutamine sources

Glutamine can be found in foods that are high in protein, such as meat, eggs, dairy, cabbage, spinach, parsley, beans, beets, carrots, parsley, wheat, papaya, celery, kale, and fermented foods like miso. It can also be found in vegetables and vegetable juices.

Around 70 to 80 g of glutamine is dispersed throughout the body of a healthy 70 kg person. Estimates of glutamine endogenous production range from 40 to 80 g/day based on isotopic and pharmacokinetic methods. Approximately 20% of the total pool of free amino acids in the blood is glutamine, which is present in plasma samples taken from fasting individuals at concentrations ranging from 500 to 800 μM/L. Glutamate makes up around 40% to 60% of the total amino acid pool in tissues including skeletal muscles and the liver, which is significantly higher than in plasma. Glutamate is the most prevalent amino acid in the body because its concentration in plasma and tissues is ten to one hundred times higher than that of any other amino acid.

A lot of glutamine can be absorbed from food or the circulation, and both the small and large intestines can process it. When comparing glucose with glutamine, the former is statistically more important for the digestive tract. Take glutamine carbon metabolism in enterocytes as an example. There are two primary pathways: (i) generating delta1-pyrroline-5-carboxylate, and (ii) converting to alpha-ketoglutarate via the Krebs cycle. The first route uses around 10% of the intestinal amino acid concentration to produce proline, ornithine, and citrulline from glutamine carbon. Proteins in the body also contain an additional 10-15% glutamine, with the vast majority of that amount (around 75%) being used as fuel in the Krebs cycle.

The availability and metabolism of glutamine in the body are closely linked to the skeletal muscle tissue. Despite the comparatively modest GS enzyme activity per muscle tissue-unit mass, skeletal muscles are quantitatively the most relevant site for glutamine stock, synthesis, and release. Since they are among the most numerous tissues in the human body, skeletal muscles are crucial to glutamine metabolism. Half to two thirds of the free amino acids in skeletal muscle come from glutamine, which is present within the muscle itself. The concentration of glutamine in skeletal muscle is 30 times higher than that in human plasma, and it accounts for around 80% of the body's glutamine. Different types of muscle fibers have different quantities of free amino acids. Research into rat skeletal muscle revealed that type 1 fibers, which are responsible for slow-twitch contractions, have three times the glutamine concentration of type 2 fibers, which are responsible for fast-twitch contractions. The high levels of glutamine synthesis in slow-twitch muscle fibers are a result of the high levels of the glutamine synthase enzyme and the availability of ATP.

Glutamine function

In mammals, glucose-derived carbon and free ammonium can be used to manufacture glutamine, an NEAA, from scratch. Therefore, it is unnecessary to acquire glutamine through food. The fact that glutamine remains one of the amino acids found in human plasma in the highest concentrations (0.5~0.8 mM) is in line with its adaptable role as a substrate for biosynthesis. To begin with, the synthesis of nucleotides and other NEAAs requires nitrogen and carbon, which glutamine supplies. Notably, at various points in the biosynthetic process, the carbon and nitrogen atoms can originate either directly from glutamine or via metabolites produced from glutamine. As an illustration, two nitrogen atoms are obtained from the γ position of two molecules of glutamine when inosine monophosphate (IMP), a precursor to purine, is synthesized. Aspartate is the mechanism by which the third nitrogen is obtained. Nevertheless, the nitrogen atom in aspartate does in fact come from glutamate by way of transamination. On the other hand, glutamate can be made from glutamine by a series of events that eliminate the nitrogen atom from the γ position of glutamine. Therefore, the α position of glutamine is the primary source for the third nitrogen in IMP. Likewise, glutamine supplies both the γ and α nitrogen atoms while pyrimidine's precursor, uridine monophosphate (UMP), is being synthesized. Nevertheless, three carbon atoms from aspartate are also incorporated with the α nitrogen from aspartate in order to construct the orotate ring. Glutamate is the primary source of carbon atoms for UMP because it is replaced by the tricarboxylic acid cycle (TCA) cycle, which is where aspartate gets its carbon backbone.

Additionally, glutamine has the ability to propel the ATP-producing TCA cycle. Cancer cells depend on elevated amounts of external glutamine for various reasons, one of which is that glutamine can be utilized to power the TCA cycle via α-ketoglutarate, enabling its additional oxidation. Research has demonstrated that a decrease in glutamine levels lowers the NADH/NAD+ ratio, which in turn hinders the uptake of oxygen and the synthesis of ATP. Continuous oxidation of glutamine-derived α-ketoglutarate through the TCA cycle not only replenishes the intermediates for biosynthesis, but also supplies energy.

Thirdly, glutamine helps reduce oxidative stress by promoting the synthesis of glutathione and NADPH. There are at least two ways in which glutamine aids in the production of glutathione, the principal antioxidant in cells. Glutamate, cysteine, and glycine are the building blocks of glutathione, hence glutamine-derived glutamate is an immediate building block for glutathione. Aside from that, the xCT transporter primarily imports cystine into cells for reduction, which means glutamate exporting is sacrificed. Since they are unable to store intracellular cysteine for glutathione synthesis and anti-oxidative defense, xCT-positive triple negative breast cancer cells are similarly vulnerable to glutamine deprivation. In addition, glutamine can help maintain redox balance by producing NADPH. On the other hand, glutamine is not a substrate in this case. Instead, it's a process that starts with glutamine catabolism and ends with pyruvate, malate, and oxaloacetate, all of which are converted to aspartate in the cytosol. This process is governed by the TCA cycle. Malic enzyme 1 (ME1) transmits electrons to NADP+ to produce NADPH, the last step in these processes. The NRF2 pathway is activated to reduce oxidative stress in KRAS-driven lung tumors with KEAP1 mutations, which are further rendered vulnerable due to glutamine-dependent redox control.

Glutamine catabolism and its cellular functions. (Jiang J., et al., 2019)

The pivotal role of glutamine in the cellular metabolism. (Dos Santos K., et al., 2024)

Glutamine application

(1) Promote vascular smooth muscle cell proliferation

Cells that produce a lot of protein and nucleic acid use a lot of energy, and glutamine is a big supply of carbon and nitrogen for them. Glutamate makes up over 50% of the tricarboxylic/critic acid (TCA) metabolites and is the most prevalent amino acid in plasma. In order to make l-arginine, which is a building block for both ATP and proteins, you need glutamine and glutamate. It has been extensively studied and established how important l-glutamine and its metabolite are. Another potential energy source that rapidly multiplying VSMCs have is an increased requirement for glutamine. It is also known that glutamine induces a phenotypic shift in VSMCs, moving them from a dormant contractile state to a synthetic, proliferating one. Intracellular glutamine levels are increased and VSMC-related thrombogenicity is determined by the introduction of exogenous glutamine. To be sure, intracellular glutamine levels are useful markers for PCI result prediction. Glutaminolysis regulates many enzymes that turn glutamine into metabolites of the TCA cycle. Increased glutaminolysis does help with vascular cell migration, proliferation, and collagen production [86]. Inhibiting glutamine transport in mice reduces vascular smooth muscle cell (VSMC) proliferation, migration, and ligation-induced neointima formation; this suggests that controlling glutamine levels could be an innovative way to avoid restenosis within stents.

Amino acids metabolism during VSMC reprogramming. (Sarkar A., et al., 2024)

(2) Immune regulation

Patients with life-threatening illnesses, such as sepsis, burns, or injuries, may have a drop in plasma glutamine concentration due to the high activity of their immune cells. Mammals primarily obtain glutamine from their skeletal muscles. In order to meet the needs of various cells and organs, including lymphoid organs and leukocytes, this tissue produces, stores, and eventually releases this amino acid. Inadequate glutamine generation in skeletal muscle or excessive consumption by using cells, or both, can lead to a drop in plasma/bloodstream glutamine levels. Reportedly, reduced immune activity in several clinical situations is associated with decreased plasma glutamine availability. Actually, glutamine deficiency stimulates cell death, decreases lymphocyte proliferation, and hinders cytokine synthesis and surface activation protein expression. Experimental animals are better able to withstand a bacterial infection when glutamine is added to their food. Glutamine administered intravenously has been found to help individuals recuperating from radiation therapy, surgery, bone marrow transplant, or injuries. Glutamate prevents infections in both animals and people when given to them before they start, which may be because it avoids amino acid deficiencies.

Regarding the method of action, glutamine modulates the expression of many genes involved in cellular metabolism, signal transduction proteins, cellular defense, and repair regulators, while also activating intracellular signaling pathways. The action of glutamine also entails the activation of signaling pathways by phosphorylation, including NF-κB and MAPKs. Consequently, the role of glutamine extends beyond only serving as a metabolic substrate or a precursor for protein synthesis. This amino acid is a crucial regulator of leukocyte function, influencing either gene expression or the activation of signaling pathways. Heat shock protein contributes to tissue protection following stress or injury, as its deficiency results in heightened cellular death. Glutamine stimulates the expression of heat shock proteins and diminishes the expression of inflammatory cytokines. The influence of glutamine on the activation of heat shock proteins may correlate with the advantageous outcomes of glutamine supplementation, including reduced duration of hospitalization and ventilator dependency in critically ill patients.

(3) Cause cellular ferroptosis

Glutamine metabolism was found to be aberrant in all 25 patients who had fluorine 18-(2S,4R)-4-fluoroglutamine positron emission tomography for various clinically aggressive cancers (lung, breast, colon, or lymphoma). Although they had minimal impact on luminal breast cancer cells, pharmacological inhibitors mediated by ASCT2 considerably decreased Glutamine absorption by triple-negative basal-like breast cancer cells. Anticancer medications may be able to target ASCT2. Antitumor strategies that include ASCT2 inhibitors or a mix of these drugs with others show promise. This area of leukemia, however, requires additional research. There may be direct or indirect effects of glutamine on leukemia diagnosis and treatment due to its possible close relationship to leukemogenesis and progression.

Glutamine induces ferroptosis. (Wang Z., et al., 2024)

(4) Control chondrocyte identity and function

The development of long bones and the healing of fractures depend on chondrocytes operating properly. Despite being anabolic, these cells may live and work in an avascular setting, which suggests they have unique metabolic needs that are currently unknown. There is a feedforward relationship between chondrocyte identity and function and glutamine metabolism. An increase in glutamine intake and levels of the rate-controlling enzyme in this pathway, glutaminase 1 (GLS1), are both stimulated by the master chondrogenic transcription factor SOX9. There is a tripartite mechanism via which GLS1 activity is crucial for chondrocyte function and characteristics. In the first place, histone acetylation is dependent on glutamate dehydrogenase, which glutamine regulates epigenetically by controlling chondrogenic gene expression. Additionally, the production of aspartate and subsequent chondrocyte proliferation and matrix formation are both facilitated by transaminase. Third, chondrocytes can survive in the avascular growth plate because glutamine-derived glutathione production prevents the buildup of reactive oxygen species. As a whole, glutamine regulates cartilage fitness throughout bone growth through metabolic pathways.

Glutamine regulates chondrocyte identity and function. (Stegen S., et al., 2020)

(5) Regulate lysosomal function

Glutamine ranks high among the amino acids found in cells. The enzymes glutaminases 1 and 2 (GLS1/2) in the mitochondria break down glutamine into glutamate, the building block of many other metabolites. The ammonium that is produced during glutaminolysis that is controlled by GLS1/2 controls the pH of the lysosomes and, consequently, their destruction. The capacity of lysosomes to degrade lipidated microtubule-associated proteins 1A/1B light chain 3B (LC3-II), various autophagic receptors, endocytosed DQ-BSA, and other proteins was enhanced in primary human skin fibroblasts BJ cells and mouse embryonic fibroblasts after a one-hour total amino acid deprivation. Similar to total amino acid fasting, lysosomal breakdown was accelerated when glutamine was removed from the culture medium but no other amino acids were affected. Regular culture conditions containing glutamine raised lysosomal pH by >0.5 pH unit, while lysosomal acidification occurred upon glutamine removal. Lysosomal pH was found to be lowered in the presence of glutamine when GLS1/2 knockdown, GLS1 antagonist, or ammonium scavengers were administered. At the beginning of amino acid starvation, adding glutamine or NH4Cl stopped the increase in lysosomal degradation and limited the extension of mTORC1 action. Our research indicates that glutamine controls lysosomal breakdown to match cellular activity needs by adjusting lysosomal pH through ammonium production. To ensure that a temporary reduction in the availability of amino acids does not drastically alter normal anabolism, the glutamine-dependent mechanism enables a more efficient utilization of endocytosed proteins and internal stores during the early stages of amino acid deprivation to prolong mTORC1 activation.

Glutamine alkalizes lysosomes. (Xiong J., et al., 2022)

References

1. Tapiero H., et al., II. Glutamine and glutamate, Biomedicine & pharmacotherapy, 2002, 56(9): 446-457.
2. Kiersikowska E., et al., Hydrolytic and redox transformations of chromium (III) bis-oxalato complexes with glutaminic acid and glutamine: a kinetic, UV–Vis and EPR, study, Transition Metal Chemistry, 2016, 41: 435-445.
3. Cruzat V., et al., Glutamine: metabolism and immune function, supplementation and clinical translation, Nutrients, 2018, 10(11): 1564.
4. Liu D., et al., Determination of L-Glutamine Solubility in 12 Pure Solvent Systems from 283.15 to 323.15 K, Journal of Chemical & Engineering Data, 2023, 68(6): 1419-1427.
5. Cheung G., et al., Physiological synaptic activity and recognition memory require astroglial glutamine, Nature communications, 2022, 13(1): 753.
6. Stumvoll M., et al., Role of glutamine in human carbohydrate metabolism in kidney and other tissues, Kidney international, 1999, 55(3): 778-792.
7. Jiang J., et al., Starve cancer cells of glutamine: break the spell or make a hungry monster?, Cancers, 2019, 11(6): 804.
8. Dos Santos K., et al., Glutamine: A key player in human metabolism as revealed by hyperpolarized magnetic resonance, Progress in Nuclear Magnetic Resonance Spectroscopy, 2024.
9. Sarkar A., et al., Gamut of glycolytic enzymes in vascular smooth muscle cell proliferation: Implications for vascular proliferative diseases, Biochimica et Biophysica Acta (BBA)-Molecular Basis of Disease, 2024: 167021.
10. Wang Z., et al., Glutamine and leukemia research: progress and clinical prospects, Discover Oncology, 2024, 15(1): 391.
11. Stegen S., et al., Glutamine metabolism controls chondrocyte identity and function, Developmental cell, 2020, 53(5): 530-544. e8.
12. Xiong J., et al., Glutamine produces ammonium to tune lysosomal pH and regulate lysosomal function, Cells, 2022, 12(1): 80.