Leucine

Leucine (abbreviated as Leu or L; encoded by the six codons UUA, UUG, CUU, CUC, CUA, and CUG) is an α-amino acid used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+
3 form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and an isobutyl side chain, classifying it as a nonpolar (at physiological pH) amino acid. It is essential in humans—meaning the body cannot synthesize it and thus must obtain from the diet.