Valine Amino Acids: Properties, Function, Benefits, and Sources

What is valine?

Valine (Val) is an essential amino acid and a type of branched-chain amino acid. Due to the involvement of branched-chain amino acids in various metabolic pathways, there has been a surge of interests in valine nutrition and its role in animal physiology. In pigs, the interactions between valine and other branched-chain amino acids or aromatic amino acids are complex. Valine cannot be de novo synthesized by animals, and it must to be obtained through protein degradation from diet, such as grains and fish meal. Unlike D-valine, which forms bacterial cell walls, L-valine is more widely used to synthesize proteins in the body. In the swine industry, L-valine is commonly used as a white crystalline or crystalline powder.

Valine structure

A natural aliphatic α-amino acid with a lateral isopropyl chain, valine has a melting point ranging from 295 to 300°C and the molecular formula is NH₂CH(R)COOH. This compound possesses the standard backbone of an amino acid, which includes an α-amino group (-NH₂) and an α-carboxyl group (-COOH) connected to the α-carbon. In proteins, valine is normally located inside the folds to evade water, and its branched side chain is an integral part of its structural and functional roles. The conformational behavior of valine, similar to that of glycine (R=H) and alanine (R=CH₃), should mimic the stabilizing effects of the cis-carboxylic functional group and the possible establishment of three intramolecular hydrogen bonds. The first configuration is held together by a bifurcated hydrogen bond from the amine to the carbonyl, and the second one shows an intramolecular hydrogen bond from the hydroxy group's hydrogen atom to the nitrogen atom's lone pair of electrons (N⋅⋅⋅H-O). A third possible form III shows that the amine group and the oxygen atom of the hydroxy group create an intramolecular hydrogen bond (N-H⋅⋅⋅O-H). Rotational spectroscopy has revealed two possible configurations in glycine and alanine, the most stable of which being configuration I.

Intramolecular hydrogen bonds and a-amino acid back bone configurations. (Lesarri A., et al., 2004)

Structure and pathways. (Wang C., et al., 2023)

Valine pka and charge

The two primary pKa values for valine are roughly 2.27 for the α-carboxyl group and about 9.52 for the α-amino group. In its zwitterionic form, valine has a protonated amino group and a deprotonated carboxyl group at physiological pH (~7.4). Due to its lack of ionizability, valine's side chain maintains a neutral pH at physiological conditions. These pKa values impact valine's function in protein structure and function by determining its behavior in various situations.

Valine is in its zwitterionic form at physiological pH (~7.4), which means it carries a positive charge on the α-amino group and a negative charge on the α-carboxyl cluster. The nonpolar isopropyl group that makes up valine's side chain isn't responsible for the charge. Although valine contains ionizable groups, its neutral overall charge is a result of the cancellation of positive and negative charges on the amino and carboxyl groups.

Comparison of the pKa and pKb values. (Sebastiani F., et al., 2021)

Valine synthesis

The process of L-valine synthesis begins with pyruvate and requires the activity of four enzymes: acetohydroxyacid synthase (AHAS), acetohydroxyacid isomeroreductase (AHAIR), dihydroxyacid dehydratase (DHAD), and transaminase B (TA). Bacillus subtilis, Escherichia coli, and Clostridium glutamicum all have different ways of producing and controlling L-valine. An essential enzyme in the production of L-valine, AHAS initiates the synthesis of three branched-chain amino acids in Candida glutamicum. The enzyme AHAS, which was encoded by the ilvB and ilvN genes, hydrolyzed 2-acetolactate from two molecules of pyruvate. In order to construct the catalytic domain and the structural domain, this enzyme uses four different subunits: two big ones and two smaller ones. In addition to the cofactors magnesium and flavin adenine dinucleotide (FAD), the level of environmental oxidation controls the activity of AHAS. Mg2+ and NADPH are required for the activity of the AHAIR encoded by ilvC, which catalyzes the isomerization reduction of 2-acetolactate to produce dihydroxyisovalerate. One dehydrated dihydroxyisovalerate to 2-ketoisovalerate using DHAD, which was encoded by ilvD. Lastly, in C. glutamicum, the TA decoded by ilvE and AvtA, encoded by avtA, catalyze the conversion of 2-ketoisovalerate to L-valine. The operon ilvBNC controls the transcription of three genes: ilvB, ilvN, and ilvC. These genes are started to produce mRNA of varying lengths by independent promoters. Due to its position at the very end of the operon ilvBNC, the ilvC gene had the highest expression efficiency and thrice higher transcription rate than the other two genes. To export L-valine, C. glutamicum uses BrnFE, a two-component osmotic enzyme. The BrnFE gene is controlled by the global regulatory factor Lrp. To enhance brnFE expression, Lrp is triggered. While BrnQ is responsible for importing L-valine into the cell, BrnFE is responsible for transporting it to the exterior of the cell.

It seems that E. coli has a more intricate system for regulating L-valine synthesis compared to C. glutamicum. Three distinct AHAS isoenzymes in E. coli, each with its own unique set of biochemical and regulatory characteristics, set it apart from C. glutamicum. The three AHAS genes—ilvBN for AHAS I, ilvIH for AHAS II, and ilvIH for AHAS III—are quite similar to those in C. glutamicum. The 172 amino acids encoded by ilvN in C. glutamicum are 39% identical to the 163 amino acids encoded by ilvH in E. coli. An ACT domain is located at its N-terminal end. Small subunits of AHAS I (encoded by ilvN) and AHAS III (encoded by ilvH) include sites that limit L-valine feedback, whereas AHAS II (encoded by ilvGM) is resistant to L-valine. At the transcriptional level, the ilvGMEDA operon (ilvE encodes transaminase, ilvD encodes dihydroxyacid dehydratase, ilvA encodes L-threonine dehydratase) is regulated by the attenuation control of the ilvBN operon by means of three BCAAs: L-valine, L-leucine, and L-isoleucine. The export of L-valine, L-leucine, and L-isoleucine is regulated by YgaZH in E. coli, just like BrnFE in C.glutamicum. Transcription of the global regulatory factor Lrp, which is also involved in L-valine synthesis, activates YgaZH. By preventing Lrp binding, L-leucine suppresses ilvIH operon expression, while it stimulates the expression of AHAS III (ecoded by ilvIH) isozyme. Lrp also suppresses the expression of the ilvGMEDA operon. Lrp blocks the transport of LivJ protein in E. coli, and BrnQ is also involved in the import of BCAAs.

Overview of biosynthetic pathway of L-valine. (Gao H., et al., 2021)

Valine function

Supplementing food or feed, using it in medicine, and using it as a precursor in biochemical synthesis are all reasons for L-valine's commercial importance. One of the limiting amino acids in animal feed for pigs and poultry, L-valine can increase the lactation function of breeding animals. Enhancing the moisturizing function and collagen synthesis of cosmetics is possible with the addition of L-valine. Because of its importance in pharmacological nutrients for patients with chronic liver illness and its high tolerance for the production and breakdown of muscle protein, L-valine finds extensive application in the pharmaceutical sector as an ingredient in third-generation amino acid infusions. In addition, L-valine increases NO expression via activating the PI3K/Akt1 signaling pathway and inhibiting arginase activity. Hence, L-valine is a chemical component of antibiotics and anti-viral medications (monensin, cervimycin, and valanimycin) and can improve macrophage phagocytosis of drug-resistant microbes. Demand for amino acids is on the rise around the world, and experts predict that manufacturing of these building blocks will bring in $25.6 billion by 2022. Of all the amino acid markets, animal feed amino acids (BCAAs) will be the most lucrative, with a projected revenue of $10.4 billion by 2022. Recent years have seen tremendous growth in the global L-valine market, with a CAGR of 65% from 2016–2019 and an anticipated CAGR of 24% from 2020–2023. Among these, feed grade L-valine has shown the most remarkable acceleration, with a CAGR of 48% from 2015–2020. With this in mind, studies aiming at the efficient manufacturing of amino acids for animal feed may undergo some changes in the near future. Improvements in L-valine production are thus strongly encouraged. Conventional L-valine manufacturing has revolved around the hydrolysis process using subcritical water technology. This process makes use of inexpensive feedstock solid waste, primarily comprising proteins made of various amino acids, such as skin, feathers, viscera, blood, bones, and residual meat from poultry processing. The manufacturing of L-valine has also made use of chemical synthesis and enzymatic techniques. But these tactics are less appealing and won't last because of how complicated the processes are and how inefficient the production is. One of the most promising processes for producing L-valine on an industrial scale is the direct fermentation of hexoses, particularly glucose. Due to the novel genetic engineering technologies used to optimize yield, titer, and productivity, the fermentation process that involves microbes is gaining more and more attention. In addition, a lot of work has gone into making sure that metabolic flux is directed to the L-valine pathway by deactivating competitive pathways and overexpressing the rate-limiting enzymes and transporters. The most common types of bacteria that produce L-valine are Corynebacterium glutamicum subsp. flavum, Bacillus subtilis, Bacillus licheniformis, Saccharomyces cerevisiae, Escherichia coli, and Bacillus subtilis. The interdependence of pathways in designed strains makes it challenging to produce high synthesis of L-valine. Unanticipated metabolic demands can cause low energy and put a host in an unstable physiological state, which can hinder the optimization of L-valine biosynthesis pathways in a single host. Unanticipated metabolic load hinders L-valine biosynthesis pathway in a single host body, which in turn may lead to insufficient energy and an unstable host state of physiology.

The roles of valine in swine nutrition and whole-body homeostasis. (Wang C., et al., 2023)

Valine amino acids at Creative Peptides

What is valine used for?

(1) Improve exercise performance

One study used a swimming exercise test to see how acute supplementation with valine, leucine, and isoleucine affected indicators associated to metabolism in rats. We analyzed valine and leucine's effects on post-workout spontaneous activity as a behavioral study. Leucine reduced blood glucose levels while isoleucine had no influence on exercise-induced decline of liver glycogen and blood glucose, but acute valine supplementation before exercise greatly reduced these effects. After exercise, taking valine or leucine reduced plasma corticosterone levels considerably, whereas isoleucine had little influence. Behavioral investigation showed that leucine had no influence on post-workout spontaneous activity, whereas valine had a substantial effect. The results show that acute valine supplementation in rats is helpful in reducing exercise exhaustion by enhancing spontaneous activity after exercise and sustaining liver glycogen and blood glucose levels, but not in leucine or isoleucine.

Effect of valine or leucine on spontaneous activity after exercise. (Tsuda Y., et al., 2018)

(2) Regulate mouse teste function

All animals require the three branched-chain amino acids—leucine, isoleucine, and valine—for proper growth and metabolic function. Still little is known about valine's impact on male reproduction or the chemical mechanism by which it does so. Researchers in one study found that while adding l-valine to mice's water for three weeks (at concentrations of 0.30 or 0.45 percent) had no effect on their testicular or body weights, it did change the shape of the germ cells and sertoli cells within the seminiferous tubules and increased the distance between them. The levels of Caspase3/9 mRNA and CASPASE9 protein were dramatically elevated by l-valine administration (0.45%), leading to the induction of apoptosis in the testis of mice. In addition, a number of genes involved in autophagy, DNA 5 mC methylation, RNA m6A methylation, Ythdf1/2/3, and Igf2bp1/2 were all markedly downregulated. While ATG5 and TET2 showed no change, the protein abundances of ALKBH5, FTO, and YTHDF3 were all markedly decreased. Among the 537 differentially expressed genes (DEGs) discovered by testis transcriptome sequencing, 26 were up-regulated and 511 were down-regulated. These genes are involved in various significant signaling pathways. In line with the findings from RNA-seq, RT-qPCR confirmed a significant decrease in 8 out of 9 differentially expressed genes (DEGs): Cd36, Scd1, Insl3, Anxa5, Lcn2, Hsd17b3, Cyp11a1, Cyp17a1, and Agt. The combined effects of l-valine therapy on many signaling cascades, including autophagy and RNA methylation, might lead to apoptosis and the eventual destruction of the mouse testis's structural integrity.

Effects of l-valine treatment on mouse growth, testis development and structure. (Wu Z W., et al., 2024)

(3) Increase colostral protein synthesis and colostral protein synthesis

Supplementation of valine during late pregnancy significantly increased content of protein (P < 0.01), fat (P = 0.02) and solids-non-fat (P = 0.04) in colostrum. valine supplementation increased protein synthesis and cell proliferation in porcine mammary epithelial cells (PMEC). Furthermore, these changes were associated with elevated phosphorylation levels of mammalian target of rapamycin (mTOR), and ribosomal protein S6 kinase (S6) and eukaryotic initiation factor 4E-binding protein-1 (4EBP1) in valine-supplemented cells, which could be effectively blocked by the antagonists of mTOR.

Effects of valine on protein expression of mTOR signaling pathway. (Che L., et al., 2019)

(4) Ameliorate liver fibrosis

Patients with cirrhosis have been found to have higher survival rates when treated with BCAAs. One looked at how L-valine, a BCAA, affected rat liver fibrosis. Male Wistar rats were administered 2.0 mL/kg of intraperitoneal carbon tetrachloride (CCl₄) twice weekly for 12 weeks in order to promote liver fibrosis. For seven days, the rats (ranging from seven to fifteen per group) were given either 1.688 g/kg/day of L-valine intravenously or a 10% amino acid solution that supplied the same quantity of nitrogen. The valine group had significantly higher blood platelet counts (13.2 ± 38.3 vs. 106.3 ± 14.5 × 104/μL, p = 0.04) and bone marrow megakaryocyte counts (18.0 ± 2.1 vs. 13.5 ± 2.2 per field, p < 0.01) compared to the control group seven days following the last treatment. Thrombopoietin, an essential regulator of thrombopoiesis, had a noticeably greater mRNA level in the valine group's liver compared to the control group. This finding is highly important. In addition, the valine group showed a marked decrease in hepatic fibrosis, and 10 days following the last administration, the livers of valine group participants had significantly lower mRNA levels of factors related to liver fibrosis, including procollagen α1(III), transforming growth factor-β1, and connective tissue growth factor. The results show that rats exposed to CCl4 have liver fibrosis, but after receiving L-valine treatment, their thrombopoiesis is restored. Liver cirrhosis patients may benefit from using L-valine supplements.

Effect of L-valine on the CCl4-induced liver fibrosis. (Nakanishi C., et al., 2010)

(5) Improve mitochondrial function

The effects of the branched-chain amino acids leucine and isoleucine on mitochondrial activity and oxidative stress have been extensively investigated. Nevertheless, the exact function of valine in controlling mitochondrial activity and oxidative stress has not been determined. In vitro effects of valine on mitochondrial activity and oxidative stress were the subject of one investigation. There was an upregulation of genes related to mitochondrial biogenesis and dynamics in response to valine. It enhances electron transport chain complex I, complex II, and complex IV mitochondrial activity. Researchers found that valine decreased oxidative stress by reducing 4-hydroxynonenal protein production and mitochondrial reactive oxygen species, according to flow cytometry tests. The XFe96 Analyzer was used to examine valine's functional involvement in protecting against oxidative damage. In the face of oxidative stress, valine maintained oxidative phosphorylation and increased the rate of ATP production. Finally, the importance of valine in reducing mitochondrial/cellular damage caused by oxidative stress is further shown by our results.

Valine improves mitochondrial function and protects against oxidative stress. (Sharma S., et al., 2024)

FAQ

1. Is valine acidic or basic?

Valine is categorized as a neutral amino acid due to its lack of pronounced acidic or basic characteristics. At physiological pH (~7.4), the α-carboxyl group is deprotonated to yield –COO^–, while the α-amino group is protonated to produce –NH₃⁺, resulting in a zwitterionic configuration. The side chain, an isopropyl group, is non-ionizable and hydrophobic, contributing to valine's overall neutrality. Consequently, valine possesses no net charge and is classified as neither acidic nor basic inside biological systems.

2. Is valine hydrophobic or hydrophilic?

Valine has a hydrophobic property. The reason behind this is that the isopropyl group (-CH(CH₃)₂), a branched aliphatic structure that makes up its side chain, is nonpolar and has poor water interaction. The side chain of valine is hydrophobic, hence it is normally located inside proteins, far from any water. By reducing the number of times the protein interacts with water, this feature aids in structural stabilization. Thus, valine is characterized as a hydrophobic, nonpolar amino acid.

3. Is valine polar or nonpolar?

Valine is a nonpolar amino acid. The side chain comprises an isopropyl group, characterized as hydrophobic and nonpolar. Valine's side chain lacks charged or strongly electronegative elements, such as oxygen or nitrogen, which prevents it from establishing strong connections with water molecules. The nonpolarity of valine often situates it within the inner of proteins, where it evades the aqueous environment. Consequently, valine is categorized as a nonpolar amino acid.

References

1. Lesarri A., et al., The shape of neutral valine, Angewandte Chemie International Edition, 2004, 43(5): 605-610.
2. Wang C., et al., The biological functions and metabolic pathways of valine in swine, Journal of Animal Science and Biotechnology, 2023, 14(1): 135.
3. Sebastiani F., et al., Probing local electrostatics of glycine in aqueous solution by THz spectroscopy, Angewandte Chemie International Edition, 2021, 60(7): 3768-3772.
4. Gao H., et al., Engineering of microbial cells for L-valine production: challenges and opportunities, Microbial Cell Factories, 2021, 20: 1-16.
5. Tsuda Y., et al., Acute supplementation of valine reduces fatigue during swimming exercise in rats, Bioscience, Biotechnology, and Biochemistry, 2018, 82(5): 856-861.
6. Wu Z W., et al., L-valine supplementation disturbs vital molecular pathways and induces apoptosis in mouse testes, Theriogenology, 2024, 215: 31-42.
7. Che L., et al., Valine supplementation during late pregnancy in gilts increases colostral protein synthesis through stimulating mTOR signaling pathway in mammary cells, Amino Acids, 2019, 51(10): 1547-1559.
8. Che L., et al., Mammary tissue proteomics in a pig model indicates that dietary valine supplementation increases milk fat content via increased de novo synthesis of fatty acid, Food Science & Nutrition, 2021, 9(11): 6213-6223.
9. Nakanishi C., et al., Treatment with L-valine ameliorates liver fibrosis and restores thrombopoiesis in rats exposed to carbon tetrachloride, The Tohoku journal of experimental medicine, 2010, 221(2): 151-159.
10. Sharma S., et al., Valine improves mitochondrial function and protects against oxidative stress, Bioscience, Biotechnology, and Biochemistry, 2024, 88(2): 168-176.