Disulfide Bonds in Proteins — Formation, Function, and Importance

What are disulfide bonds?

Disulfide bonds are covalent bonds between the sulfur atoms of two cysteine residues. Written as Cys-S-S-Cys or simply S-S bonds, they are one of the strongest forces stabilizing protein structure. Unlike the weaker noncovalent interactions (hydrogen bonds, hydrophobic interactions), disulfide bonds are true covalent bonds with energies around 60 kcal/mol.

How they form

Disulfide bonds form through oxidation of the thiol (-SH) groups on two cysteine side chains. This requires an oxidizing environment, which is why disulfide bonds are almost exclusively found in extracellular proteins (secreted proteins, membrane protein exteriors) and rarely in the reducing environment inside cells. The endoplasmic reticulum, where secretory proteins fold, has dedicated enzymes (protein disulfide isomerases) that catalyze correct disulfide bond formation.

Structural roles

Disulfide bonds serve several critical functions. They stabilize the folded structure by covalently linking parts of the polypeptide chain that are far apart in the primary sequence. They can connect different polypeptide chains in multi-subunit proteins. In antibodies, for example, disulfide bonds link the heavy and light chains together and connect the two heavy chains in the hinge region.

Insulin provides a classic example: the mature hormone consists of an A chain and B chain held together by two inter-chain disulfide bonds, plus one intra-chain disulfide bond within the A chain.

Counting cysteines

You can predict whether a protein might have disulfide bonds by counting its cysteine residues and knowing its cellular location. An even number of cysteines in an extracellular protein suggests disulfide bond formation. Use our sequence analyzer to check the cysteine content and other compositional properties of any protein.