They contain nitrogen, which is the fourth most abundant element in the body. Amino acids also contain a carboxyl group, as do fatty acids. The carboxyl group, which is also attached to the alpha carbon atom, consists of a carbon atom, two oxygen atoms and a hydrogen atom. The attachment of both an amino group and a carboxyl group to the alpha carbon give the molecule an asymmetry which bends polarized light. There can be many alpha amino acids, but only twenty are found in human proteins.
Some proteins form structural units in the body, while other proteins are enzymes.
All twenty amino acids are essential to life, but only nine amino acids are dietary essential. The body can form the other eleven amino acids from the dietary essential amino acids. The dietary essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Although histidine was once considered an essential amino acid for infants only, subsequent research has determined that histidine may also be essential for adults. Arginine, ornithine, cysteine, cystine, taurine and tyrosine are classified as non-essential amino acids but may be essential for individuals with certain diseases or nutritional concerns.