Leucine

Leucine (abbreviated as Leu or L) is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH(CH3)2. It is an essential amino acid, which means that humans cannot synthesise it. Its codons are UUA, UUG, CUU, CUC, CUA, and CUG. With a hydrocarbon side chain, leucine is classified as a hydrophobic amino acid.

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-ketovalerate 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.