Most proteins consist of linear polymers built from series of up to 20 different L @-@ α @-@ amino acids . All proteinogenic amino acids possess common structural features , including an α @-@ carbon to which an amino group , a carboxyl group , and a variable side chain are bonded . Only proline differs from this basic structure as it contain an unusual ring to the N @-@ end amine group , which force the CO – NH amide moiety 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 determine 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 .