Disulfide Bonds Determine the Properties of Many Proteins Some natural proteins are rich in disulfide bonds, and their mechanical properties (tensile strength, viscosity, hardness, etc.) are correlated with the degree of disulfide bonding.(a) Glutenin, a wheat protein rich in disulfide bonds, is responsible for the cohesive and elastic character of dough made from wheat flour. Similarly, the hard, tough nature of tortoise shell is due to the extensive disulfide bonding in its ?-keratin. What is the molecular basis for the correlation between disulfide-bond content and mechanical properties of the protein?(b) Most globular proteins are denatured and lose their activity when briefly heated to 65

Most globular proteins are denatured and lose their activity when briefly heated to 65 degree C. However, globular proteins that contain multiple disulfide bonds often must be heated longer at higher temperatures to denature them. One such protein is bovine pancreatic trypsin inhibitor (BPTI), which has 58 amino acid residues in a single chain and contains three disulfide bonds. On cooling a solution of denatured BPTI, the activity of the protein is restored. What is the molecular basis for this property?

Solution

Part A

Disulfide bonds are covalent bonds which are much stronger than the non-covalent forces in protein i.e hydrogen bonds, weak wander vaal forces etc. Being strong in nature, the disulfide bonds causes cross linking between the chains of protein. This cross linking develops strong bonds that enhance the stiffness and hardness of the protein thereby increasing its mechanical strength. Stability increases because the entropy decreases.

Part B

The disulfide bonds with in the BPTI’s three adjacent cysteines, prevents the protein from changing its structure to any stable form and hence prevent it from folding and unfolding completely