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Alanine (Ala) is an amino acid frequently present in proteins. It is categorized as a non-essential amino acid as the human body can produce it from pyruvate, a byproduct of glucose metabolism. Alanine is integral to numerous biological activities, encompassing energy production and metabolism. Alanine exists in two forms: L-alanine and D-alanine. The L-form is predominantly present in proteins, whereas the D-form is more frequently located in bacterial cell walls.
Primary amino acid substrates for gluconeogenesis and ureagenesis in the liver include alanine and glutamine. After being released into the systemic circulation, alanine is mostly taken up by the liver and to a lesser degree by the kidney. It is created in peripheral tissues by transamination activities with glutamate and other amino acids. In this case, alanine can undergo deamination to produce pyruvate and an amino group; the latter can then be utilized in transamination reactions, ureagenesis, or lost in urine. Glycolysis in the periphery, particularly in the muscles, might begin with alanine that has been converted to glucose in the kidneys or liver. When metabolic stress or critical disease increases the release of endogenous alanine from peripheral organs, this so-called glucose-alanine cycle may become more important. This is also one way that alanine transports nitrogen. To make glutamine more soluble and stable in nutritional solutions, glutamine dipeptides commonly include alanine as the second amino acid. Despite the fact that alanine has never been investigated as a possible source of the therapeutic effects of the dipeptide alanine-glutamine, which are often associated with glutamine, no clinical advantages have been associated with alanine supplementation. It is important to note that alanine is the most powerful inducer of hepatic ureagenesis, which results in the breakdown of alanine, in this context.
Branched-chain amino acid synthesis. (Reitzer L., 2009)
Alanine, a type of α-amino acid, possesses a carbon atom known as the alpha-carbon, which is connected to both an amino group (-NH2) and a carboxyl group (-COOH). A methyl group (-CH3) is found on the side chain of alanine. L-alanine is characterized by the L-configuration of the amino acid, adhering to the usual Fischer projection convention and the (S) configuration at the chiral center, applicable to common L-amino acids. D-alanine is the enantiomer of L-alanine; in the Fischer projection, the amino group is positioned on the right and the carboxyl group is located at the top of the central carbon.
β-alanine is distinct from L-α-alanine in that its amino group is bonded to the carbon, and it is not a proteinogenic amino acid. It possesses a greater density and a unique hydrogen bonding configuration that enhances its stability. In its chemical configuration, it may function as a neurotransmitter and contributes to muscle carnosine production. Because of their crucial involvement in protein synthesis and other processes, proteinogenic amino acids have been the primary focus of most amino acid investigations. Toxins against both vertebrates and invertebrates, nitrogen storage, anti-herbivory, anti-microbial, response to abiotic stresses, plant hormones, and more than 250 non-proteinogenic amino acids are all produced by plants. These compounds are in addition to the 20 ubiquitous amino acids utilized in protein synthesis. The non-proteinogenic acid β-alanine is mostly recognized from research conducted on humans. Pantothenate, a component of β-Alanine, is present in all living things, including plants, and is an essential component of the carbon shuttle Coenzyme A and acyl carrier protein. This review primarily aims to cover the processes of β-alanine production, metabolism, and its function in plants. β-Alanine, whose IUPAC name is 3-aminopropanoic acid, is an amino acid that does not produce proteins because its amino group is located at the β-position, opposite the carboxylate group. There is no stereo center in β-alanine, unlike L-alanine, a proteinogenic amino acid. Pantothenate (Vitamin B5) contains β-Alanine, which is a building block of Coenzyme A (CoA) and an acyl-carrier protein that transports carbon into the cell. A component of carnosine, a dipeptide found mostly in muscle and brain tissue, β-alanine is widely used as a supplement to enhance strength in humans. For instance, most scholarly studies on β-alanine focus on its use as an exercise supplement.
The chemical structures of L-alanine and β-alanine. (Parthasarathy A., et al., 2019)
The structures of L- and D-alanine. (Wilson C C., et al., 2005)
By moving one proton from the carboxylic acid to the amine group, alanine stabilizes in a zwitterion state when solvated in water. There are two chiral forms of this amino acid, and both are non-essential, non-reactive, and non-polar. The different alanine enantiomers interact with each other through hydrogen bonds and van der Waals interactions in the solid phase, resulting in a variety of molecular crystals. With their acentric unit cells and weak conjugate bonds, alanine chiral molecular crystals have nonlinear optical properties that could be used in optical computation and signal processing, ultrafast electro-optical switches, ultra-short pulsed lasers, and laser amplification, among other possible uses. Their transparency range extends into the visible and ultraviolet regions. The pKa of the α-amino group in alanine is around 9.7 and pKa of the α-carboxyl group in alanine is around 2.35. Alanine has a simple methyl group as its side chain, and this side chain does not have a dissociable group, so it does not contribute to the overall pKa values of alanine.
Alanine is a water-soluble amino acid. At atmospheric pressure and temperatures ranging from 283.15 to 323.15 K, the gravimetrical method was used to determine the solubility of l-alanine in five binary systems: water + methanol, water + ethanol, water + 1-propanol, water + 2-propanol, and water + acetone, as well as in twelve neat solvents: water, 1-butanol, methanol, 2-butanol, 2-methyl-1-propanol, 1-pentanol, 1-propanol, ethanol, 2-propanol, ethyl acetate, acetonitrile, and acetone. According to the results of the experiments, the solubility of all the solvents studied rose with rising temperatures and water mole ratios, but fell with rising mole fractions of five distinct organic solvents in the binary solvent systems. In the pure solvents that were chosen, the solubility of l-alanine was ranked as follows: water, 1-butanol, methanol, 2-butanol, 2-methyl-1-propanol, 1-pentanol, 2-propanol, ethyl acetate, acetonitrile, and acetone. The Apelblat-Jouyban-Acree, Jouyban-Acree, and modified Apelblat models were used to mathematically represent the solubility. The experimental data agrees well with all of the models.
The calculated pKa compare with the experimental data. (Shahadha K M., et al., 2020)
Solubility of l-Alanine. (An M., et al., 2020)
l-Alanine serves as the precursor for d-alanine, a principal constituent of the cell wall. L-Alanine binds to Lrp, a global regulator, influencing its activity. l-Alanine auxotrophs have yet to be isolated from E. coli, perhaps due to the presence of numerous enzymes capable of catalyzing alanine synthesis. The primary pathway of synthesis is unequivocally the transamination of pyruvate. Two alanine-synthesizing transaminases, AvtA and AlaB, have been identified. They vary in their amino acid donors: valine for AvtA and glutamate for AlaB. Two labeling investigations indicate a pyruvate-independent mechanism; however, no specific enzyme has been identified that could facilitate this reaction or pathway. d-Alanine is produced from l-alanine through a constitutive racemase.
L- and D-Alanine serve as sources of nitrogen or carbon. l-Alanine is racemized to d-alanine, which is subsequently metabolized to pyruvate and ammonia by the nonspecific membrane-bound d-amino acid dehydrogenase. The expression of dehydrogenase necessitates l-alanine, cAMP, its binding protein, and Lrp.
Alanine amino acids at Creative Peptides
Out of the twenty amino acids that occur naturally, alanine is the only one that can form a stable α helix in water and has the strongest tendency to form helices. As a residue, why is alanine ideal for forming helices? The fact that alanine's helix-coil transition has the lowest entropic penalty is one explanation. Just like glycine, the peptide group in an alanine helix is the one that is most exposed to the solvent, which is another explanation. The idea is that β-branched residues in a helix will make the backbone peptide group less solvent accessible, leading to an increase in the dehydration penalty while forming a helix. The fact that the ΔH for the helix-coil transition is less for β-branched residues (Thr, Val, Ile) lends credence to this idea. Properly defining the hydrogen bond would help settle the debate over whether it stabilizes protein folding, according to this view: Proteins with hydrogen bonds that are exposed to the solvent, such as those in an alanine helix, are stabilized by 3.8 kJ/mol upon establishment of these bonds. The peptide group will be buried, reducing this margin. It is still debatable whether or not a hidden hydrogen bond helps keep proteins stable.
The distribution of the effect of charge-to-alanine. (Wong K B., et al., 2012)
As an α-amino acid, alanine comes in two different enantiomers: L-alanine and D-alanine. Both varieties taste sweet and pleasant when consumed in solution. The racemic mixture of alanine has a mildly suppressive influence on the acidity of citric acid solutions and a moderately boosting effect on the sweetness of sucrose, according to anecdotal evidence. The FDA has approved Alanine as a flavoring agent and it is listed on the GRAS list. According to reports, DL-alanine can be used as a food ingredient to improve the flavor of sweets in pickling combinations without posing any risks.
The voltage outputs produced by alanine are comparable to those of inorganic ceramics due to its small dielectric constant, which enables it to produce a piezoelectric voltage constant. In addition, the ability to cultivate amino acid microcrystals on flexible substrates opens up exciting possibilities for applications such as motion detection and energy harvesting within living organisms. Both experimental evidence and density-functional theory calculations have shown that DL-alanine crystal films are piezoelectric, with a maximum open-circuit voltage of 0.8 V produced by hand compression. Similar results have been seen with quartz crystals. On the flip side, amino acid crystals' biodegradability is a plus because it makes them more environmentally friendly and improves their biocompatibility (they don't induce immunogenic reactions). This makes them ideal for use in transient implanted sensors and energy harvesting biodevices. The pyroelectric and spontaneous polarization characteristics, as well as the longitudinal piezoelectricity of orthorhombic amino acid films, make them ideal for the production of high-performance transducers.
One of the most abundant amino acids in proteins is alanine, a glucogenic amino acid. It is possible to convert other amino acids to alanine as well, especially BCAAs like valine, leucine, and isoleucine. For instance, the breakdown of muscle proteins during fasting results in the release of alanine into the bloodstream, which is then absorbed by the liver. Because it is a glucogenic amino acid, alanine is easily transformed into glutamate and pyruvate in the liver by the catalytic action of glutamate-pyruvate transaminase (GPT), which is also called alanine transaminase (ALT). The gluconeogenic pathway is responsible for converting pyruvate to glucose. Once produced, glucose in the liver can enter the bloodstream through glucose transporter 2 (GLUT2), where it can be absorbed by muscles and utilized for energy. The alanine amino group is first delivered to the liver, where it is transformed into urea through the urea cycle and subsequently excreted.
The alanine cycle. (Litwack G., 2021)
The nonessential amino acid l-alanine, also known as α-alanine, plays an important role in the production of proteins in humans. It is well-known that l-alanine may be converted into glucose by the liver, making it an ideal energy source for preworkout and intense exercise. Also, because α-alanine builds up, it inhibits pyruvate kinase, which in turn lowers glycolysis pyruvate synthesis and stops the liver from digesting glucose. Ingesting l-alanine from sports and energy drinks (SED) avoids hypoglycemia, however it may cause problems for people with diabetes or other metabolic illnesses since it raises blood sugar levels. Combinations of amino acids, such as arginine, alanine, and phenylalanine, have been the subject of numerous research, in contrast to studies examining individual l-alanine supplementation in liquid or solid forms. In the pre-exercise period, this combination increased adrenaline and glucagon production, which in turn shifted the energy source from carbohydrate to fat burning. l-Alanine may have gastrointestinal side effects, but only at very large dosages.
Pyruvate is produced by transamination or deamination of alanine, and via the incomplete Krebs cycle in LAB, it can be converted to lactic acid or other intermediates and amino acids. Cheese ripens to produce aldehydes and ketones like acetaldehyde, acetone, 2,3-butanediol, and diacetyl from an initial substrate, alanine, which may produce flavor component precursors like pyruvate and oxaloacetate.
It has been found that l-alanine (Ala) and l-arginine (Arg) control the physiology of pancreatic β-cells and inhibit the buildup of body fat in obesity caused by dietary changes. Adipose tissue phosphorylation of the adipogenic enzyme, ACC, was reduced in adult monosodium glutamate (MSG)-mice, which was related with increased adiposity. Insulin secretion was found to be decreased and IRβ expression was reduced in adipose tissue and skeletal muscle in MSG mice with glucose intolerance. Both Ala and Arg animals showed reduced perigonad fat depots, but only Ala treatment reduced retroperitoneal fat pads. Ala was the only one that increased glucose tolerance and insulin secretion, although Arg and Ala both improved fed-state glycemia, IRβ, and pAS160 levels. Increased AMPK phosphorylation and pACC levels in fat depots were indicators of an upregulation of adipostatic signals in MAla mice. Activation of the AMPK/ACC system may have suppressed lipogenesis, since supplementation with Ala resulted in more noticeable metabolic benefits than Arg.
β-Alanine is often utilized by competitive athletes as a supplement to enhance their performance on the field. A number of recent reviews have evaluated its effectiveness, and there is additional evidence that suggests that tactical athletes may benefit from using β-alanine supplements. It does not seem that β-Alanine, an amino acid that is not proteogenic, has any ergogenic potential on its own. After consumption, it becomes carnosine by combining with histidine in various organs and skeletal muscle. As a result of its combination with histidine, β-alanine becomes a powerful intracellular pH buffer since its imidazole ring's pKa is raised to 6.83. Muscle carnosine synthesis is believed to be capped by β-Alanine. Therefore, taking β-alanine as a supplement mainly aims to raise the amount of carnosine in skeletal muscle, which in turn improves the capacity of intracellular buffering and allows for a higher tolerance to prolonged anaerobic exercise.
Potential effects of β-alanine supplementation on brain function when exposed to stress. (Hoffman J R., et al., 2018)
Stimulating T cells is a metabolically intensive process. Environmental nutrients, including as glucose and the amino acids glutamine, leucine, serine, and arginine, are essential for T cells to emerge from quiescence. In order for T cells to activate, the expression of transporters for these nutrients must be carefully controlled. A single transamination is all that is needed to produce alanine from pyruvate, in contrast to the necessary or multi-step biosynthesis processes that are required for the other amino acids. The extracellular alanine is still necessary for successful naive T cell activation and memory T cell restimulation escape from quiescence. Impairments in metabolism and function are brought about by alanine deficiency. This susceptibility arises, mechanistically speaking, since activated T cells generate alanine transporters, yet low expression of alanine aminotransferase—the enzyme necessary for interconverting pyruvate and alanine—results in its absence. Alanine is not catabolized but rather stimulates protein synthesis, according to stable isotope tracking. Therefore, T cell protein synthesis and proper activation are dependent on exogenous alanine.
Extracellular alanine is necessary for T cell activation. (Ron-Harel N., et al., 2019)
References
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