<P> The primary structure of a protein refers to the sequence of amino acids in the polypeptide chain . The primary structure is held together by peptide bonds that are made during the process of protein biosynthesis . The two ends of the polypeptide chain are referred to as the carboxyl terminus (C - terminus) and the amino terminus (N - terminus) based on the nature of the free group on each extremity . Counting of residues always starts at the N - terminal end (NH - group), which is the end where the amino group is not involved in a peptide bond . The primary structure of a protein is determined by the gene corresponding to the protein . A specific sequence of nucleotides in DNA is transcribed into mRNA, which is read by the ribosome in a process called translation . The sequence of amino acids in insulin was discovered by Frederick Sanger, establishing that proteins have defining amino acid sequences . The sequence of a protein is unique to that protein, and defines the structure and function of the protein . The sequence of a protein can be determined by methods such as Edman degradation or tandem mass spectrometry . Often, however, it is read directly from the sequence of the gene using the genetic code . It is strictly recommended to use the words "amino acid residues" when discussing proteins because when a peptide bond is formed, a water molecule is lost, and therefore proteins are made up of amino acid residues . Post-translational modification such as disulfide bond formation, phosphorylations and glycosylations are usually also considered a part of the primary structure, and cannot be read from the gene . For example, insulin is composed of 51 amino acids in 2 chains . One chain has 31 amino acids, and the other has 20 amino acids . </P> <P> Secondary structure refers to highly regular local sub-structures on the actual polypeptide backbone chain . Two main types of secondary structure, the α - helix and the β - strand or β - sheets, were suggested in 1951 by Linus Pauling and coworkers . These secondary structures are defined by patterns of hydrogen bonds between the main - chain peptide groups . They have a regular geometry, being constrained to specific values of the dihedral angles ψ and φ on the Ramachandran plot . Both the α - helix and the β - sheet represent a way of saturating all the hydrogen bond donors and acceptors in the peptide backbone . Some parts of the protein are ordered but do not form any regular structures . They should not be confused with random coil, an unfolded polypeptide chain lacking any fixed three - dimensional structure . Several sequential secondary structures may form a "supersecondary unit". </P> <P> Tertiary structure refers to the three - dimensional structure of monomeric and multimeric protein molecules . The α - helixes and β - pleated - sheets are folded into a compact globular structure . The folding is driven by the non-specific hydrophobic interactions, the burial of hydrophobic residues from water, but the structure is stable only when the parts of a protein domain are locked into place by specific tertiary interactions, such as salt bridges, hydrogen bonds, and the tight packing of side chains and disulfide bonds . The disulfide bonds are extremely rare in cytosolic proteins, since the cytosol (intracellular fluid) is generally a reducing environment . </P> <P> Quaternary structure is the three - dimensional structure consisting of the aggregation of two or more individual polypeptide chains (subunits) that operate as a single functional unit (multimer). The resulting multimer is stabilized by the same non-covalent interactions and disulfide bonds as in tertiary structure . There are many possible quaternary structure organisations . Complexes of two or more polypeptides (i.e. multiple subunits) are called multimers . Specifically it would be called a dimer if it contains two subunits, a trimer if it contains three subunits, a tetramer if it contains four subunits, and a pentamer if it contains five subunits . The subunits are frequently related to one another by symmetry operations, such as a 2-fold axis in a dimer . Multimers made up of identical subunits are referred to with a prefix of "homo -" (e.g. a homotetramer) and those made up of different subunits are referred to with a prefix of "hetero -", for example, a heterotetramer, such as the two alpha and two beta chains of hemoglobin . </P>

The normal 3 dimensional state of a protein is called its