Disulfide bond

A covalent bond between the sulfur atoms of two cysteine residues. It can link parts of one protein or connect separate protein chains, stabilizing their structure.

Cysteine: A sulfur-containing amino acid whose side chain bears a thiol group. Its thiol groups provide the two sulfur atoms joined in a disulfide bond.
Protein folding: The process by which a protein chain adopts a functional three-dimensional structure. Disulfide bonds can stabilize a folded structure by linking distant chain regions.
Insulin: A peptide hormone that helps regulate blood glucose by promoting its uptake and storage. Its two chains are connected by disulfide bonds, and another bond stabilizes one chain.
Hydrogen bond: An attractive interaction involving a hydrogen atom bonded to an electronegative atom and another electronegative atom. Hydrogen bonds stabilize protein structures without forming the covalent sulfur–sulfur link found here.
Thiol: A functional group consisting of a sulfur atom bonded to a hydrogen atom. Two cysteine thiols form a disulfide when oxidized.
Protein primary structure: The linear sequence of amino acids in a protein chain. Cysteine positions in the sequence constrain which disulfide pairings are possible.
Antibody: An immune-system protein that binds a specific molecular target. Disulfide bonds stabilize antibody domains and connect heavy and light chains.
Ionic bond: An electrostatic attraction between oppositely charged ions or groups. Ionic interactions can stabilize proteins but do not join cysteine sulfur atoms covalently.
Oxidation–reduction reaction: A chemical reaction in which electrons are transferred between reactants. Disulfide formation is an oxidation, while bond cleavage is a reduction.
Covalent bond: A chemical bond formed when atoms share one or more pairs of electrons. A disulfide is a covalent link whose formation and cleavage depend on redox chemistry.

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