An Immunoglobulin molecule represents the following level of organized protein structure:

D i.e. Quaenary structure Discription of all covalent bonds (mainly peptide and disulfide bonds) linking aminoacid residues in a linear structure refers to primary structure of protein. Secondary structure of protein refers to description of steric relationship (spatial arrangement) between aminoacids located relatively near each other in a selected segment of main polypeptide chain, without regard to the conformation of its side chains or its relationship to other segments. Examples of secondary structures include a-helix, pleated sheet and 13 turns etc. Naturally occurring L-aminoacids can (theoretically) form either right or left handed a-helices, but extended left handed a? helices are theoretically less stable and have not been observed in proteinsQ. Proline and glycine have the least tendency (proclivity) to form a-helicesQ. Large number of charged aminoacids like aspaate (aspaic acid)Q, glutamate (glutamic acid), arginine, lysine or histidine can also disrupt a-helix by forming ionic bonds or electrostatically repelling each other. Secondary structure is stabilized by hydrogen bonds & disulfide bonds; Cooperative hydrogen bonding in repeating secondary structures have an impoant role in guiding the protein folding processQ. However , location of bends (including l turns) in polypeptide chain and the direction and angle of these turns are determined by the numer & location of specific bend producing aminoacid residues, such as Pro, Thr, Ser, and Gly. Interacting segments of polypeptide chains are held in their characteristic teiary (3-dimensional) positions by several kinds of weak interactionsQ (mainly hydrophobic bonds, vanderwall forces, ionic interactions) and sometimes by covalent bonds (such as disulfide cross links) between the segments. Some proteins contain 2 polypeptide chains or subunits which may be identical (homo) or different (hetero). Quaernary structure of protein refers to three dimensional arrangement of these protein subunits. Therefore, homo-or hetero-dimers formed between 2 polypeptide chains is an example of quaternary structureQ. Similarly an immunoglobin molecule represents hetero-tetrameric quaernary structure between 4 polypeptide (2 heavy + 2 light) chains.