Unit 4. Part II + III. Proteins - Enzymes
9. 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 α-keratin. What is the
molecular basis for the correlation between disulfide-bond content and mechanical
properties of the protein?
Disulfide bonds are covalent bonds which are much stronger than the non-covalent
forces in protein (hydrogen bonds, weak van der Waal forces etc.). Being strong in
nature, the disulfide bonds have a strong effect on the stabilization of protein
conformation. Disulfide bonds have marked effects on the mechanical strength
(tortoise shell) and stiffness (wheat dough) of the proteins that contain them.
(b) Most globular proteins are denatured and lose their activity when
briefly heated to 65oC. 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?
Disulfide bonds in the BPTI cysteines residues prevent protein from changing its
structure and form. 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.
Heating of globular proteins leads to increased thermal motion of amino acid
residues and disruption of many non-covalent interactions. However, disulfide
bonds are not broken by heating. The presence of these bonds prevents the
polypeptide from becoming completely randomized and can facilitate the refolding
of the protein when the temperature is reduced.
