covalent interactions: disulfide bridges
A covalent bond, as you have probably learned in chemistry, involves the sharing of electrons between two atoms.
In proteins, one of the top most covalent bond interactions is the disulfide bond. Interestingly enough cysteine is the only amino acid with side chains that form covalent bonds. Thus cysteine – cysteine interactions, are critical to determine protein structures.
Disulfide bridges are the two sulphurs in cysteine connected together. This formation is critical in stabilizing the tertiary structure of proteins. The keratin found in your hair, for example, is highly stabilized by disulfide bridges. Increasing the number of disulfide bridges in your hair and nails allows them to stay stronger and resist fracture.
One cool application of disulfide bonds is within protein complexes. They play a role in linking insulin’s peptide chains together.
In proteins specifically, disulfide bridges increase their resistance against protein degradation as a result of changes in pH and temperature in the surroundings.
That brings us to the next question – how do disulfide bonds actually make a protein stable?
Disulfide bonds bonds lessen the number of conformations allowed in a protein’s unfolded states. This decreases the protein’s entropy in its unfolded state.
This article puts it very aptly.
“Two amino acids distant from one another in the primary structure of the protein but held in covalent association in three-dimensional space will dramatically lessen the number of conformations accessible to the unfolded protein.”