A disulfide bridge is a covalent bond between two sulfur atoms (S–S) in a protein. It forms between the R groups of two cysteine amino acids, which each contain a thiol group (–SH).When two cysteines are brought close together by the folding of the chain, their –SH groups react. The two hydrogen atoms are removed (an oxidation reaction) and the two sulfur atoms bond together:–SH + HS– → –S–S– + 2H The two cysteines can be far apart in the primary sequence but next to each other once the chain has folded, so disulfide bridges lock parts of the chain together.
Where they are found:
• Within one polypeptide, holding the tertiary structure.
• Between two different polypeptides, holding the quaternary structure. For example, the two chains of insulin are joined by two disulfide bridges, and there is a third within one chain.
• Keratin in hair, nails and horn contains many disulfide bridges, which makes it tough and insoluble.Strength. Because they are covalent, disulfide bridges are much stronger than hydrogen bonds or ionic bonds. They are not broken by moderate heating or pH changes, so proteins rich in them are very stable. They can be broken by reducing agents; hairdressers use this to reshape hair in a perm, then re-form the bridges in new positions.Worked example: a protein contains six cysteines, all paired up. The maximum number of disulfide bridges is 6 ÷ 2 = 3.
Where they are found:• Within one polypeptide, holding the tertiary structure.
• Between two different polypeptides, holding the quaternary structure. For example, the two chains of insulin are joined by two disulfide bridges, and there is a third within one chain.
• Keratin in hair, nails and horn contains many disulfide bridges, which makes it tough and insoluble.Strength. Because they are covalent, disulfide bridges are much stronger than hydrogen bonds or ionic bonds. They are not broken by moderate heating or pH changes, so proteins rich in them are very stable. They can be broken by reducing agents; hairdressers use this to reshape hair in a perm, then re-form the bridges in new positions.Worked example: a protein contains six cysteines, all paired up. The maximum number of disulfide bridges is 6 ÷ 2 = 3.