The amino acid leucine is incorporated into the nascent polypeptide chain during protein biosynthesis in response to six codons—UUA, UUG, CUU, CUA, CUC, and CUG—and represents approximately 9.0% of the residues of the proteins that have been characterized. The leucyl residue incorporated has a mass of 113.16 Da, a van der Waals volume of 124 A3, and an accessible surface area of 180 A2. Leu residues are infrequently changed during divergent evolution; they are interchanged in homologous proteins most frequently with valine, isoleucine, methionine, and phenylalanine residues.
The Leu side chain is nonpolar with no functional or reactive groups:
Leu is one of the most hydrophobic amino acid residues, and 45% of the residues in native protein structures are fully buried. Leu is one of the residues that favors most the alpha-helical conformation in model peptides. It also occurs frequently in a-helices in folded proteins, somewhat more frequently than in b-sheet type of secondary structure.
Leucine residues are very frequently involved in interactions in coiled coils, and they have given their name to the so-called leucine zipper.
