Leucine, like its cousins isoleucine and valine, is a hydrophobic amino acid that is found as a structural element on the interior of proteins and enzymes. There appears to be no other significant metabolic role for these amino acids, but they are essential and because they are not synthesized by mammalian tissues, must be taken in the diet. Leucine ties glycine for the position of second most common amino acid found in proteins with a concentration of 7.5 percent on a molar basis compared to the other amino acids.