l-arginine is an amino acid. Amino acids are the basic building blocks of proteins. The best protein sources contain complete amino acid profiles. As a general rule, plant sources do not contain complete amino acid profiles whereas meat poultry, fish, eggs and dairy products do.eucine structure. Lysine structure. There are 22 protein-building amino acids in the human body. Tryptophan is an essential amino acid, and is well-known for its role in producing serotonin.