<P> Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation, electrophoresis, and chromatography; the advent of genetic engineering has made possible a number of methods to facilitate purification . Methods commonly used to study protein structure and function include immunohistochemistry, site - directed mutagenesis, X-ray crystallography, nuclear magnetic resonance and mass spectrometry . </P> <P> 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 contains an unusual ring to the N - end amine group, which forces 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 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 . </P> <P> The peptide bond has two resonance forms that contribute some double - bond character and inhibit rotation around its axis, so that the alpha carbons are roughly coplanar . The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone . The end with a free amino group is known as the N - terminus or amino terminus, whereas the end of the protein with a free carboxyl group is known as the C - terminus or carboxy terminus (the sequence of the protein is written from N - terminus to C - terminus, from left to right). </P> <P> The words protein, polypeptide, and peptide are a little ambiguous and can overlap in meaning . Protein is generally used to refer to the complete biological molecule in a stable conformation, whereas peptide is generally reserved for a short amino acid oligomers often lacking a stable three - dimensional structure . However, the boundary between the two is not well defined and usually lies near 20--30 residues . Polypeptide can refer to any single linear chain of amino acids, usually regardless of length, but often implies an absence of a defined conformation . </P>

What is the main function of a protien