 Most proteins consist of linear polymers built from series of up to 20 different L @-@ α @-@ amino acids . All <unk> amino acids possess common structural features , including an α @-@ carbon to which an amino group , a <unk> group , and a variable side chain are bonded . Only <unk> differs from this basic structure as it contains an unusual ring to the N @-@ end amine group , which forces the CO – <unk> <unk> <unk> into a fixed conformation . The side chains of the standard amino acids , detailed in the list of standard amino acids , have a great variety of chemical structures and properties ; it is the combined effect of all of the amino acid side chains in a protein that ultimately determines its three @-@ dimensional structure and its chemical reactivity . The amino acids in a polypeptide chain are linked by peptide bonds . Once linked in the protein chain , an individual amino acid is called a residue , and the linked series of carbon , nitrogen , and oxygen atoms are known as the main chain or protein backbone . 
