Protein Structure

Proteins are described at four levels of structure.Primary structure is the sequence of amino acids in the polypeptide chain, held together by peptide bonds. It is set by the gene coding for the protein. Changing even one amino acid can change the whole shape of the protein.Secondary structure is the local coiling or folding of the chain into an α-helix or a β-pleated sheet, held by hydrogen bonds between the C=O and N–H groups of the backbone.Tertiary structure is the further folding of the whole chain into a precise three-dimensional shape. It is held by bonds between R groups: hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions (non-polar R groups cluster together away from water). The tertiary structure gives each protein its specific shape, such as the active site of an enzyme.Quaternary structure exists only in proteins made of more than one polypeptide chain. It is the way the chains are arranged together, held by the same types of bond. Haemoglobin has four polypeptide chains; insulin has two.Because the primary structure decides where the R groups end up, it decides all the higher levels. This is why a mutation in DNA can produce a protein that does not work.Heat and extreme pH break the weaker bonds (hydrogen and ionic) of the secondary, tertiary and quaternary structure, but not the peptide bonds, so a denatured protein keeps its primary structure but loses its shape.