Valine (symbol ValorV)[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-polaraliphatic 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).
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]
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]
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]
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.[14] 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.[15] 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.[16] The valine catabolite 3-hydroxyisobutyrate promotes insulin resistance in mice by stimulating fatty acid uptake into muscle and lipid accumulation.[17] In mice, a BCAA-restricted diet decreased fasting blood glucose levels and improved body composition.[18]
Dietary valine is essential for hematopoietic stem cell (HSC) self-renewal, as demonstrated by experiments in mice.[19] 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.
^Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2000). Principles of Biochemistry (3rd ed.). New York: W. H. Freeman. ISBN1-57259-153-6..
^Mathews CK (2000). Biochemistry. Van Holde, K. E., Ahern, Kevin G. (3rd ed.). San Francisco, Calif.: Benjamin Cummings. p. 776. ISBN978-0-8053-3066-3. OCLC42290721.
^Xiao F, Yu J, Guo Y, Deng J, Li K, Du Y, et al. (June 2014). "Effects of individual branched-chain amino acids deprivation on insulin sensitivity and glucose metabolism in mice". Metabolism. 63 (6): 841–50. doi:10.1016/j.metabol.2014.03.006. PMID24684822.
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