Leucine

Leucine (abbreviated as Leu or L) is a branched-chain α-amino acid with the chemical formula HO2CCH(NH2)CH2CH(CH3)2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are UUA, UUG, CUU, CUC, CUA, and CUG. With a hydrocarbon side chain, leucine is classified as a hydrophobic amino acid. It has an isobutyl R group. Leucine is a major component of the sub units in ferritin, astacin and other 'buffer' proteins.

Biosynthesis
As an essential amino acid, leucine is not synthesized in animals, hence it must be ingested, usually as a component of proteins. It is synthesized in plants and microorganisms via several steps starting from pyruvic acid. The initial part of the pathway also leads to valine. The intermediate α-ketoisovalerate is converted to α-isopropylmalate and then β-isopropylmalate, which is dehydrogenated to α-ketoisocaproate, which in the final step undergoes reductive amination. Enzymes involved in a typical leucine biosynthesis include
 * Acetolactate synthase,
 * Acetohydroxy acid isomeroreductase,
 * Dihydroxyacid dehydratase,
 * α-Isopropylmalate synthase,
 * α-Isopropylmalate isomerase,
 * Leucine aminotransferase.

Biology
Leucine is utilized in the liver, adipose tissue, and muscle tissue. In adipose and muscle tissue, leucine is used in the formation of sterols, and the combined usage of leucine in these two tissues is seven times greater than its use in the liver.

Leucine is the only dietary amino acid that has the capacity to stimulate muscle protein synthesis. As a dietary supplement, leucine has been found to slow the degradation of muscle tissue by increasing the synthesis of muscle proteins in aged rats. While once seen as an important part of the three branch chained amino acids in sports supplements, leucine has since earned more attention on its own as a catalyst for muscle growth and muscular insurance. Supplement companies once marketed the "ideal" 2:1:1 ratio of leucine, iso-leucine and valine; but with furthered evidence that leucine is the most important amino acid for muscle building, it has become much more popular as the primary ingredient in dietary supplements.

Leucine potently activates the mammalian target of rapamycin kinase that regulates cell growth. Infusion of leucine into the rat brain has been shown to decrease food intake and body weight via activation of the mTOR pathway.

Leucine toxicity, as seen in decompensated Maple Syrup Urine Disease (MSUD), causes delirium and neurologic compromise, and can be life-threatening.

In yeast genetics, mutants with a defective gene for leucine synthesis (leu2) are transformed with a plasmid that contains a working leucine synthesis gene (LEU2) and grown on minimal media. Leucine synthesis then becomes a useful selectable marker.

Chemical properties
Leucine is a branched-chain amino acid (BCAA) since it possesses an aliphatic side-chain that is non-linear.

Racemic leucine had been subjected to circularly polarized synchrotron radiation in order to better understand the origin of biomolecular asymmetry. An enantiomeric enhancement of 2.6 % had been induced, indicating a possible photochemical origin of biomolecules' homochirality.

Food additive
As a food additive, L-Leucine has E number E641 and is classified as a flavor enhancer.