Disulfide bond is a single covalent bond derived from the coupling of thiol (SH) groups. In proteins they are formed between the thiol groups of cysteine residues.
The linkage is also called as an “S-S-bond” or “disulfide bridge”. Disulfide bonds play an important role in the folding and stability of some proteins usually secreted to the extra cellular medium.
The disulfide bond stabilizes the folded form of a protein in several ways: 1) it holds two portions of the protein together, biasing the protein towards the folded topology. 2) The disulfide bond may form the nucleus of a hydrophobic core of the folded protein, i.e., local hydrophobic residues may condense around the disulfide bond and onto each other through hydrophobic interactions. Light and heavy chains of immunoglobulin’s join together by means of inter disulphide bonds in hinge region.
The number of disulphide bonds in hinge depends upon the length of the hinge region. In addition to inter disulphide bonds, each domine of light and heavy chains contain an intra disulfide bond.
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The total number of these bonds depends upon the number of domains in the polypeptide chains of the immunoglobulin molecule. The cystein residues help in the formation of inter and intra disulphide bonds.
Disulphide bonds present in different parts of immunoglobulins help in stabilization of the structure. Even in the absence of disulphide bonds, under appropriate conditions the heavy and light chain pairs associate spontaneously to form a biologically active immunoglobulin molecule.