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Valine
Valine
Valine
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Valine
Skeletal formula of neutral valine
Names
IUPAC name
Valine
Systematic IUPAC name
2-Amino-3-methylbutanoic acid
Other names
2-Aminoisovaleric acid
Valic acid
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.000.703 Edit this at Wikidata
EC Number
  • L: 200-773-6
KEGG
UNII
  • InChI=1S/C5H11NO2/c1-3(2)4(6)5(7)8/h3-4H,6H2,1-2H3,(H,7,8)/t4-/m0/s1 checkY
    Key: KZSNJWFQEVHDMF-BYPYZUCNSA-N checkY
  • D/L: Key: KZSNJWFQEVHDMF-UHFFFAOYSA-N
  • D: Key: KZSNJWFQEVHDMF-SCSAIBSYSA-N
  • L: CC(C)[C@@H](C(=O)O)N
  • L Zwitterion: CC(C)[C@@H](C(=O)[O-])[NH3+]
Properties[3]
C5H11NO2
Molar mass 117.148 g·mol−1
Density 1.316 g/cm3
Melting point 298 °C (568 °F; 571 K) (decomposition)
soluble, 85 g/L [1]
Acidity (pKa) 2.32 (carboxyl), 9.62 (amino)[2]
−74.3·10−6 cm3/mol
Supplementary data page
Valine (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Valine ball and stick model spinning

Valine (symbol Val or V)[4] is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and a side chain isopropyl group, making it a non-polar aliphatic amino acid. Valine is essential in humans, meaning the body cannot synthesize it; it must be obtained from dietary sources which are foods that contain proteins, such as meats, dairy products, soy products, beans and legumes. It is encoded by all codons starting with GU (GUU, GUC, GUA, and GUG).

History and etymology

[edit]

Valine was first isolated from casein in 1901 by Hermann Emil Fischer.[5] The name valine comes from its structural similarity to valeric acid, which in turn is named after the plant valerian due to the presence of the acid in the roots of the plant.[6][7]

Nomenclature

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According to IUPAC, carbon atoms forming valine are numbered sequentially starting from 1 denoting the carboxyl carbon, whereas 4 and 4' denote the two terminal methyl carbons.[8]

Metabolism

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Source and biosynthesis

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Valine, like other branched-chain amino acids, is synthesized by bacteria and plants, but not by animals.[9] It is therefore an essential amino acid in animals, and needs to be present in the diet. Adult humans require about 24 mg/kg body weight daily.[10] It is synthesized in plants and bacteria via several steps starting from pyruvic acid. The initial part of the pathway also leads to leucine. The intermediate α-ketoisovalerate undergoes reductive amination with glutamate. Enzymes involved in this biosynthesis include:[11]

  1. Acetolactate synthase (also known as acetohydroxy acid synthase)
  2. Acetohydroxy acid isomeroreductase
  3. Dihydroxyacid dehydratase
  4. Valine aminotransferase

Degradation

[edit]

Like other branched-chain amino acids, the catabolism of valine starts with the removal of the amino group by transamination, giving alpha-ketoisovalerate, an alpha-keto acid, which is converted to isobutyryl-CoA through oxidative decarboxylation by the branched-chain α-ketoacid dehydrogenase complex.[12] This is further oxidised and rearranged to succinyl-CoA, which can enter the citric acid cycle and provide direct fuel in muscle tissue.[13][14]

Synthesis

[edit]

Racemic valine can be synthesized by bromination of isovaleric acid followed by amination of the α-bromo derivative.[15]

HO2CCH2CH(CH3)2 + Br2 → HO2CCHBrCH(CH3)2 + HBr
HO2CCHBrCH(CH3)2 + 2 NH3 → HO2CCH(NH2)CH(CH3)2 + NH4Br

Medical significance

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Metabolic diseases

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The degradation of valine is impaired in the following metabolic diseases:[citation needed]

Insulin resistance

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Lower levels of serum valine, like other branched-chain amino acids, are associated with weight loss and decreased insulin resistance: higher levels of valine are observed in the blood of diabetic mice, rats, and humans.[16] Mice fed a BCAA-deprived diet for one day had improved insulin sensitivity, and feeding of a valine-deprived diet for one week significantly decreases blood glucose levels.[17] In diet-induced obese and insulin resistant mice, a diet with decreased levels of valine and the other branched-chain amino acids resulted in a rapid reversal of the adiposity and an improvement in glucose-level control.[18] The valine catabolite 3-hydroxyisobutyrate promotes insulin resistance in mice by stimulating fatty acid uptake into muscle and lipid accumulation.[19] In mice, a BCAA-restricted diet decreased fasting blood glucose levels and improved body composition.[20]

Hematopoietic stem cells

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Dietary valine is essential for hematopoietic stem cell (HSC) self-renewal, as demonstrated by experiments in mice.[21] Dietary valine restriction selectively depletes long-term repopulating HSC in mouse bone marrow. Successful stem cell transplantation was achieved in mice without irradiation after 3 weeks on a valine restricted diet. Long-term survival of the transplanted mice was achieved when valine was returned to the diet gradually over a 2-week period to avoid refeeding syndrome.

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Valine

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from Grokipedia
Valine is an essential α-amino acid with the chemical formula C₅H₁₁NO₂ and a molecular weight of 117.15 g/mol, featuring a nonpolar, aliphatic side chain of an isopropyl group attached to the β-carbon, making it hydrophobic and typically buried in the interior of proteins. As one of the nine essential amino acids that humans cannot synthesize de novo, valine must be obtained from dietary sources such as meat, dairy, grains, and legumes to support protein synthesis, growth, and nitrogen balance. In biological systems, valine serves as a building block for proteins, promotes muscle growth and tissue repair, and plays a key role in energy metabolism through its catabolism to succinyl-CoA, contributing to the tricarboxylic acid cycle and ATP production via NADH and FADH₂ generation. Belonging to the branched-chain amino acids (BCAAs) alongside leucine and isoleucine, valine exhibits stimulant activity and is implicated in various physiological processes, including mitochondrial function enhancement and protection against oxidative stress, though its dysregulation is associated with disorders like maple syrup urine disease and hypervalinemia. Physically, L-valine appears as a white crystalline solid with a melting point of 293–315 °C and good solubility in water (58.5 mg/mL at 25 °C), and it is utilized in nutritional supplements, flavoring agents, and as a precursor in penicillin biosynthesis.

Chemical Properties

Molecular Structure

Valine is an α-amino acid characterized by the molecular formula C₅H₁₁NO₂. Its molecular structure features a central chiral α-carbon atom bonded to an amino group (–NH₂), a group (–COOH), a , and an isopropyl (–CH(CH₃)₂), which imparts hydrophobic properties to the molecule. This arrangement is typical of proteinogenic , with the side chain distinguishing valine as a non-polar, aliphatic residue. The α-carbon in valine is asymmetric due to the four different substituents, resulting in with two enantiomers: L-valine and D-valine. In biological systems, only the L-enantiomer is incorporated into proteins, corresponding to the (2S)-2-amino-3-methylbutanoic acid configuration. L-Valine displays a positive specific of +28.9° (measured in 6 N HCl at 20°C). Valine's is a , specifically the isopropyl moiety, which sets it apart from other . This contrasts with the longer isobutyl of and the sec-butyl of , positioning valine as the most compact member of the family while sharing their overall hydrophobic and β-sheet-promoting characteristics. The ionizable functional groups of valine include the α-carboxyl group, with a pKₐ of 2.32, and the α-amino group, with a pKₐ of 9.62; the isopropyl lacks ionizable protons and remains neutral across physiological ranges. These pKₐ values influence valine's zwitterionic form at neutral , where the carboxyl group is deprotonated (–COO⁻) and the amino group is protonated (–NH₃⁺).

Physical and Chemical Characteristics

Valine appears as a white crystalline powder at . Its molecular weight is 117.15 g/mol. The compound has a of 295–300 °C, at which it decomposes. Valine exhibits moderate in water, with a value of 58.5 g/L at 25 °C, influenced by its non-polar hydrophobic isopropyl side chain that limits overall hydrophilicity compared to more polar . Chemically, valine, as an α-amino acid, forms a at physiological pH (around 7.4), where the group is deprotonated (–COO⁻) and the amino group is protonated (–NH₃⁺), conferring and in aqueous environments. This zwitterionic structure contributes to its stability under mildly acidic or basic conditions, though prolonged exposure to strong acids or bases can lead to or shifts. The hydrophobic reduces reactivity with polar solvents but allows participation in formation via the α-amino and carboxyl groups. Spectroscopic analysis reveals characteristic signatures for valine. In ¹H NMR (500 MHz, D₂O, pH 7), the isopropyl group's methyl protons appear as doublets at approximately 0.97 and 1.04 ppm, the methine proton at 2.27 ppm, and the α-proton at 3.60 ppm, confirming the branched aliphatic structure. Infrared (IR) spectroscopy shows key absorptions for the α-amino acid backbone, including N–H stretches at 3300–3500 cm⁻¹, C=O stretch of the carboxyl at around 1710 cm⁻¹, and C–H deformations for the isopropyl group near 1380–1460 cm⁻¹. The (pI) of valine is 5.96, calculated as the average of its pKₐ values (2.32 for the carboxyl group and 9.62 for the amino group), at which the net charge is zero. This pI value is relevant for techniques like , where valine migrates minimally at 5.96 but toward the or at higher or lower , respectively, due to charge alterations.

History and Nomenclature

Discovery and Etymology

Valine was first isolated in 1856 by the Austrian-German chemist Eugen Freiherr von Gorup-Besanez from protein hydrolysates derived from pancreatic tissue. This early extraction marked an initial step in identifying valine as a component of biological proteins, though its full characterization as an occurred later. In 1901, German chemist Emil Fischer isolated valine from the milk protein casein through acid hydrolysis and established its chemical structure, confirming it as a distinct α-amino acid essential to protein composition. Fischer's work built on prior isolations and provided the definitive structural elucidation that integrated valine into the growing catalog of known amino acids. The name "valine" originates from "valeric acid," reflecting the isopropyl side chain's close relation to isovaleric acid, a compound first obtained from the roots of the valerian plant (Valeriana officinalis). This etymological link highlights the historical connection between amino acid nomenclature and naturally occurring organic acids identified in plant sources. During the 1930s, American biochemist William C. Rose conducted pivotal experiments that recognized valine as an , demonstrating its necessity for growth and nitrogen balance in animal models and later in humans. Rose's studies quantified minimum dietary requirements and underscored valine's irreplaceable role in protein synthesis, influencing modern understanding of .

Naming Conventions

Valine is systematically named 2-amino-3-methylbutanoic acid under IUPAC nomenclature for organic compounds, with the L-enantiomer specified as (2S)-2-amino-3-methylbutanoic acid. In biochemical and protein sequence contexts, valine is denoted by the three-letter code Val or the one-letter code V, as established by the IUPAC-IUBMB recommendations for symbolism. Valine is classified as a non-polar, aliphatic, (BCAA) within biochemical taxonomy, sharing this category with and due to its hydrophobic isopropyl side chain. The incorporation of valine into proteins is specified by the genetic codons GUU, GUC, GUA, and GUG in the standard . Derivatives of valine follow IUPAC conventions for amino acid modifications, where peptides are named by connecting residue abbreviations with hyphens (e.g., L-valyl-L-valine for the ) or using full systematic names like (2S)-2-[(2S)-2-amino-3-methylbutanamido]-3-methylbutanoic acid. Esters are named as alkyl valinates, such as methyl L-valinate, indicating the esterification of the group. Isotopically labeled variants, used in metabolic and NMR studies, employ nuclide symbols in brackets, as in [²H₈]-L-valine for the fully deuterated form where all eight hydrogen atoms are replaced by .

Biosynthesis and Sources

Microbial and Plant Biosynthesis

In microorganisms, valine is produced via the (BCAA) biosynthesis pathway, a conserved process that also yields and . In bacteria such as , the pathway for valine initiates with the condensation of two pyruvate molecules to form 2-acetolactate, catalyzed by acetohydroxy acid synthase (AHAS). Subsequent steps involve ketol-acid reductoisomerase (KARI), which reduces and isomerizes 2-acetolactate to 2,3-dihydroxyisovalerate; dihydroxy-acid dehydratase (DHAD), which dehydrates this intermediate to 2-ketoisovalerate; and aminotransferase (BCAT), which transfers an amino group from glutamate to yield L-valine. The enzymes are encoded by genes in the ilv family, with AHAS existing as three isozymes (encoded by ilvBN, ilvGM, or ilvIH), KARI by ilvC, DHAD by ilvD, and BCAT by ilvE. In E. coli, the ilvGMEDA coordinates expression of several of these genes, while ilvBN and ilvC are in separate transcriptional units. occurs primarily through feedback inhibition, where valine specifically inhibits AHAS s I (ilvBN) and III (ilvIH), preventing overaccumulation, whereas II (ilvGM) is insensitive to valine but responsive to . Additionally, transcriptional attenuation in the ilvGMEDA is triggered by elevated levels of BCAAs, fine-tuning pathway flux. In higher , valine biosynthesis follows a parallel enzymatic pathway to that in , utilizing the same core intermediates and enzymes (AHAS, KARI, DHAD, and BCAT) starting from pyruvate. The pathway is predominantly localized in chloroplasts, where light-derived ATP and reducing power support the energy-intensive reactions. However, certain enzymes like BCAT exhibit cytosolic isoforms, which may activate as a compensatory mechanism during abiotic stresses such as oxidative damage or nutrient limitation, ensuring continued production outside the .

Dietary Sources and Human Acquisition

Valine is classified as an for humans, meaning it cannot be synthesized de novo in the body due to the absence of key enzymes such as acetohydroxy acid synthase (AHAS), which catalyzes the initial step in biosynthesis. As a result, valine must be acquired entirely through dietary intake to meet physiological needs. Dietary sources of valine are primarily protein-rich foods, with animal products generally providing higher concentrations than plant-based options. Meats such as and are particularly abundant, containing approximately 1.7 g of valine per 100 g of cooked lean tissue. products like low-fat offer around 0.5 g per 100 g, while s such as provide about 0.6 g per 100 g, and like navy beans contribute roughly 0.5 g per 100 g. These sources collectively account for the majority of valine intake in typical diets, with , , and products serving as top contributors. The recommended dietary allowance (RDA) for valine in adults is 26 mg per kg of body weight per day, as established by the (WHO) to support protein synthesis and maintenance. This equates to approximately 1.8–2.0 g daily for a 70-kg individual, easily achievable through balanced consumption of the aforementioned food groups. Once ingested, valine is absorbed in the primarily through sodium-dependent neutral transporters, such as B⁰AT1 (encoded by SLC6A19), which facilitates the uptake of neutral including valine across the apical membrane of epithelial cells. of valine is influenced by the overall of the source, often assessed using metrics like the Protein Digestibility-Corrected (PDCAAS) or (DIAAS), which account for digestibility and composition; animal proteins typically exhibit higher scores (e.g., >100 for PDCAAS in ) compared to many plant proteins. In , valine is commonly supplemented as part of (BCAA) formulations to support muscle recovery and reduce during exercise, with typical daily doses ranging from 2 to 5 g, often in a 2:1:1 ratio with and . Such supplementation is generally safe at these levels for healthy individuals engaging in intense physical activity.

Metabolism

Anabolic Pathways

In protein synthesis, valine is incorporated via charging of its cognate tRNA by (ValRS), a class-Ia that recognizes specific structural features of tRNA^Val, including the anticodon loop and acceptor stem, to ensure accurate aminoacylation. This enzyme catalyzes the ATP-dependent attachment of L-valine to the 3'-end of tRNA^Val, forming valyl-tRNA^Val, which is then delivered to the during . At the , valine is added to the growing polypeptide in response to its four codons—GUU, GUC, GUA, and GUG—enabling its precise positioning based on mRNA sequence. Valine participates in reversible cycles, primarily catalyzed by branched-chain aminotransferase (BCAT) isozymes, which convert it to α-ketoisovalerate (KIV) by transferring its α-amino group to α-ketoglutarate, producing glutamate. This reaction is reversible, allowing KIV to be reaminated back to , which facilitates inter-tissue shuttling of branched-chain (BCAAs) and maintains pools for anabolic reuse, particularly in muscle and liver. Such cycling supports 's redistribution without net loss, complementing dietary intake as the essential external source. Beyond general incorporation, valine's isopropyl imparts hydrophobicity, positioning it within the buried cores of proteins like , where it stabilizes tetrameric structure through hydrophobic interactions in the β-chain pockets. In enzymes, this property similarly contributes to architecture by forming clusters with other aliphatic residues, enhancing stability and substrate binding in hydrophobic environments. Daily protein turnover in humans recycles amino acids through degradation and resynthesis, with endogenous recycling meeting approximately 80% of total amino acid requirements, thereby minimizing reliance on exogenous supply for anabolic maintenance.

Catabolic Degradation

The catabolism of valine initiates with a transamination reaction in which the amino group is transferred to α-ketoglutarate, producing α-ketoisovalerate and glutamate; this reversible step is catalyzed by branched-chain aminotransferases (BCATs), primarily the mitochondrial BCAT2 isoform in most tissues, with BCAT1 predominant in the brain. The subsequent irreversible oxidative decarboxylation of α-ketoisovalerate to isobutyryl-CoA occurs via the branched-chain α-keto acid dehydrogenase (BCKDH) complex, a multienzyme system requiring thiamine pyrophosphate, lipoamide, coenzyme A, FAD, and NAD+ as cofactors, and generating CO₂, NADH, and isobutyryl-CoA. From isobutyryl-CoA, valine degradation proceeds through a series of transformations unique to this . Isobutyryl-CoA is dehydrogenated to methacrylyl-CoA by isobutyryl-CoA (encoded by ACAD8), followed by hydration to 3-hydroxyisobutyryl-CoA via short-chain enoyl-CoA hydratase (ECHS1). by 3-hydroxyisobutyryl-CoA (HIBCH) yields 3-hydroxyisobutyrate, which is then oxidized to methylmalonic semialdehyde by 3-hydroxyisobutyrate (HIBADH) using NAD+ as a cofactor. Finally, methylmalonic semialdehyde (MMSDH) converts methylmalonic semialdehyde to propionyl-CoA, completing the segment that yields a three-carbon unit from valine's five-carbon skeleton, with two carbons released as CO₂ earlier in the BCKDH step. Propionyl-CoA enters the tricarboxylic acid (TCA) cycle via carboxylation to (2R)-methylmalonyl-CoA, catalyzed by the biotin-dependent propionyl-CoA carboxylase (PCCA/PCCB heterodimer) using ATP and CO₂. Racemization to (2S)-methylmalonyl-CoA is facilitated by methylmalonyl-CoA epimerase (MCEE), followed by rearrangement to succinyl-CoA by vitamin B12-dependent methylmalonyl-CoA mutase (MUT), an α-helix barrel enzyme that inverts the configuration at the C2 carbon. Succinyl-CoA then integrates into the TCA cycle, enabling energy production through subsequent oxidation. The BCKDH complex, the primary regulatory point in valine catabolism, is controlled by reversible phosphorylation: BCKDH kinase (BCKDK) phosphorylates the E1α subunit (BCKDHA) at serine 293 to inhibit activity, while 2Cm (PP2Cm) dephosphorylates it for activation, with regulation influenced by nutritional status, hormones like insulin, and branched-chain levels. Additionally, BCKDH is allosterically inhibited by its products NADH and branched-chain acyl-CoAs, as well as by high ATP/NADH ratios, ensuring catabolic flux aligns with cellular energy demands.

Chemical Synthesis

Laboratory Methods

One of the classical laboratory methods for synthesizing valine involves the Strecker synthesis, which produces the racemic DL-valine from , , and , followed by acid of the resulting α-aminonitrile intermediate. In this procedure, reacts with to form an , which then undergoes with to yield 2-amino-2-cyano-3-methylbutane; subsequent with cleaves the nitrile group to afford DL-valine , which is neutralized to the free . This method, first adapted for valine in detailed studies during the mid-20th century, remains a straightforward bench-scale approach for preparing racemic valine in settings. For the preparation of enantiomerically pure L-valine, asymmetric synthesis via enantioselective of dehydrovaline derivatives is a widely employed technique, utilizing rhodium-based chiral catalysts. Dehydrovaline, typically as an N-acyl-α,β-unsaturated such as (E)-N-acetyl-3-methylbut-2-enoate, is hydrogenated under mild conditions (e.g., 1-5 H₂, ) in the presence of a rhodium(I) complex coordinated with chiral ligands like DIOP or phosphoramidites, achieving high enantioselectivity through preferential binding of the substrate's si or re face to the catalyst. Seminal work in the 1980s demonstrated this approach with rhodium-DiCAMP catalysts, yielding up to 72% enantiomeric excess for dehydrovaline derivatives, while modern variants with monodentate phosphoramidites have improved selectivities to over 95% . In laboratory applications, particularly for incorporating valine into peptides, strategies are essential to prevent unwanted side reactions during coupling. The amino group of valine is commonly protected with tert-butoxycarbonyl (Boc) via reaction with in the presence of a base like , or with benzyloxycarbonyl (Cbz) using under aqueous alkaline conditions, both affording high-yield protected derivatives suitable for solid-phase or solution-phase . These protections are selectively removed post-coupling—Boc with and Cbz via hydrogenolysis—enabling efficient assembly of valine-containing sequences. Typical laboratory-scale syntheses of valine via these routes achieve overall yields of 70-90%, depending on optimization of reaction conditions and scale, with racemic Strecker methods often reaching 83% from the starting material. Purification is routinely accomplished by recrystallization from or for the free , yielding products of >98% purity, or by reverse-phase (HPLC) for enantiopure forms to ensure separation from diastereomeric impurities.

Industrial Production

The predominant method for industrial production of L-valine is microbial fermentation, leveraging genetically engineered strains of Corynebacterium glutamicum to achieve overproduction through deregulation of the ilv biosynthetic pathway. This pathway, involving enzymes such as acetohydroxyacid synthase (encoded by ilvBN) and ketol-acid reductoisomerase (encoded by ilvC), is optimized by strategies like feedback inhibition relief, deletion of competing pathways (e.g., aceE for ), and enhancement of NADPH supply to boost flux toward L-valine. Industrial strains typically attain titers of up to 50 g/L in fed-batch fermentations, with glucose as the primary carbon source under controlled aerobic or oxygen-limited conditions. Although less common due to higher costs and complexity, chemical synthesis routes provide an alternative for L-valine production, involving multi-step processes starting from precursors like acetone or derivatives. These typically include cyanohydrin formation (Strecker synthesis) to generate racemic valine, followed by steps and techniques—such as enzymatic or preferential —to isolate the L-enantiomer with high enantiomeric purity (>99%). Such methods are scalable but account for a minority of output compared to , primarily used when high-purity product is required without biological contaminants. Post-fermentation or synthesis, L-valine is purified via ion-exchange using strong cation-exchange resins to separate it from impurities, followed by concentration and from aqueous solutions to yield crystalline product with purity exceeding 98.5%. Global production of L-valine reached approximately 2,650 tons as of 2024, driven by demand in . The food and segment dominates the market, with approximately 70% of output directed toward animal feed additives to balance profiles in diets, enhancing growth efficiency, while the remainder supports human nutraceuticals for and medical supplements.

Physiological Roles

Role in Protein Synthesis

Valine is incorporated into polypeptides during protein synthesis via its four codons: GUU, GUA, GUC, and GUG. In the human proteome, valine accounts for approximately 6.8% of all residues, reflecting its prevalence in diverse protein sequences. This frequency underscores valine's role as a common building block in eukaryotic proteins, contributing to overall structural integrity without dominating the composition. The beta-branched structure of valine's isopropyl side chain imparts a conformational preference for beta-sheet secondary structures, where it promotes strand packing through steric constraints that disfavor alpha-helices. This propensity enhances the stability of beta-sheet motifs in folded proteins. Furthermore, valine's hydrophobic nature allows it to stabilize protein interiors; for instance, in , valine residues line the hydrophobic pocket that accommodates the ring of the NAD+ coenzyme, facilitating substrate binding near the . Valine positions within conserved protein domains, particularly in hydrophobic cores, display invariance across , as substitutions would disrupt core packing and functional stability. This conservation highlights valine's essential contribution to maintaining ancestral protein architectures over phylogenetic timescales. Mistranslation errors during occur at rates of about 1 in 1,000 to 10,000 codons, potentially leading to valine-leucine swaps due to the similar hydrophobic properties of these branched-chain . Such substitutions typically exert minimal impact on and function, as both residues support comparable nonpolar interactions.

Functions in Muscle and Energy Metabolism

Valine, as one of the branched-chain amino acids (BCAAs), contributes to signaling pathways that regulate muscle protein synthesis through synergy with leucine in activating the mammalian target of rapamycin (mTOR) pathway. This activation promotes the phosphorylation of key downstream targets like S6K1 and 4E-BP1, enhancing translational efficiency and anabolic responses in skeletal muscle cells during nutrient availability or mechanical loading. Although leucine exerts the dominant effect, valine's presence in BCAA mixtures amplifies mTORC1 signaling, supporting overall muscle maintenance and growth without directly binding the pathway's primary sensors. In energy metabolism, valine serves as a substrate for during physical activity, undergoing and oxidative to form propionyl-CoA, which is carboxylated to methylmalonyl-CoA and ultimately . This intermediate enters the tricarboxylic acid (TCA) cycle, providing carbons for ATP production and enabling to sustain blood glucose levels when stores deplete. Branched-chain , including valine, contribute to muscle energy demands during prolonged endurance exercise, particularly when availability is limited. Valine supports nitrogen homeostasis by participating in ammonia scavenging mechanisms in and the , where its with α-ketoglutarate generates branched-chain α-keto acids and glutamate, facilitating the conversion of toxic to non-toxic via . This process is crucial in muscle to buffer exercise-induced accumulation from nucleotide and in the to mitigate hyperammonemia's neurotoxic effects, such as those seen in . During physiological stress like or , valine elevates in to supply and gluconeogenic amid increased protein breakdown, helping preserve lean mass while adapting to balance. This heightened breakdown aligns with typical dietary or supplemental BCAA ratios of 2:1:1 (::valine), which optimize metabolic flux and minimize imbalances in catabolic rates among the three .

Medical and Nutritional Significance

Essential Amino Acid Status and Deficiency

Valine is classified as one of the nine required by humans, as the body cannot synthesize it endogenously and it must be obtained through dietary sources. This plays a in protein synthesis and metabolic functions, with daily requirements established based on age, body weight, and physiological needs. For infants (birth to 6 months), the adequate (AI) is set at 87 mg/kg body weight per day to support rapid growth and development. In adults over 19 years, the estimated average requirement (EAR) is 19 mg/kg per day, translating to a recommended dietary allowance (RDA) of 24 mg/kg per day to meet the needs of nearly all healthy individuals. Deficiency of valine, though rare in isolation due to its presence in many protein-rich foods, can occur in the context of overall protein or imbalanced diets low in branched-chain (BCAAs). Although isolated valine deficiency is extremely rare in humans, in rats have demonstrated neurological impairments such as motor incoordination and damage to structures like the red nuclei upon valine deprivation. In humans, valine deficiency typically arises alongside shortages of other essential in severe , leading to symptoms such as growth retardation, , skin lesions, , and resembling those in . Individuals at higher risk for valine inadequacy include vegans relying on plant-based proteins without supplementation, as some vegan diets may provide lower bioavailable BCAAs unless diverse sources like and grains are emphasized. The elderly are also vulnerable due to reduced , lower protein , and diminished absorption , potentially exacerbating muscle wasting (). Athletes with intense training demands face elevated requirements for valine to support muscle repair and energy production, and inadequate can impair recovery and . Valine status is commonly assessed through plasma amino acid analysis, with normal concentrations ranging from 120 to 300 μmol/L in healthy adults. Levels below this range may indicate deficiency, particularly in at-risk populations. Valine supplementation is sometimes used therapeutically, such as in mixtures for managing in or to support muscle preservation in .

Associations with Metabolic Disorders

Valine, as one of the branched-chain amino acids (BCAAs), plays a significant role in certain metabolic disorders due to its accumulation or dysregulation in catabolic pathways. (MSUD) is an autosomal recessive inborn error of caused by deficiencies in the branched-chain α-ketoacid dehydrogenase (BCKDH) complex, leading to impaired catabolism of valine, , and , and subsequent toxic accumulation of these BCAAs and their ketoacids in plasma, , and tissues. This accumulation, particularly of valine-derived metabolites, disrupts neurological function and . Symptoms manifest acutely in the neonatal period with , characterized by poor feeding, , , and a distinctive in , progressing to , seizures, coma, and potential death if untreated; chronic effects include developmental delays and intellectual impairment. MSUD variants include the classic form, with 0-2% residual BCKDH activity and rapid neonatal onset, and the intermittent form, with 5-20% activity that triggers symptoms during catabolic stress like infections, though patients are otherwise. Hypervalinemia represents a rare isolated defect in valine metabolism, resulting from mutations in the BCAT2 gene encoding the mitochondrial , which catalyzes the initial step converting valine to its α-ketoacid. These mutations reduce activity, causing selective elevation of plasma valine levels while sparing and metabolism to a lesser extent, often accompanied by hyperleucine-isoleucinemia. The disorder is extremely rare, with an estimated incidence below 1 in 1,000,000 live births, presenting in infancy with , , developmental delays, and white matter lesions on imaging, though some cases respond to supplementation to enhance residual function. In the context of lifestyle-related metabolic disorders, elevated circulating valine levels serve as a for and (T2D), with multiple post-2010 cohort studies linking higher baseline valine concentrations to increased T2D risk over follow-up periods of 3-12 years. Mechanistically, excess valine contributes to by promoting overactivation of the (mTOR) pathway, particularly , which induces serine of insulin receptor substrate-1 (IRS-1), thereby inhibiting insulin signaling and exacerbating glucose dysregulation in obese individuals. Recent research from the highlights valine's potential hematopoietic effects, where supplementation enhances proliferation and maintenance of hematopoietic s (HSCs) in bone marrow niches, as valine supports HSC self-renewal and retention through metabolic reprogramming. This mechanism suggests therapeutic promise for valine supplementation in treating anemias, such as those associated with chemotherapy-induced myelosuppression or congenital disorders, by improving mobilization and without excessive differentiation. Emerging evidence points to the gut 's understudied role in modulating valine metabolism during , where dysbiotic microbial communities alter BCAA , leading to elevated systemic valine that promotes and via gut barrier disruption. For instance, high-valine diets reshape composition, increasing pro-inflammatory taxa and metabolites that aggravate hepatic accumulation and , representing a novel therapeutic target yet to be fully explored in human trials.

References

  1. https://pubchem.ncbi.nlm.nih.gov/compound/L-Valine
  2. https://www.ncbi.nlm.nih.gov/books/NBK557845/
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