disulfide linkages

Meaning: Two cysteine residues, each with an –SH (thiol) side chain, are oxidized to give R–S–S–R, called cystine when the two cysteines are joined. Each sulfur loses a hydrogen, so the reaction is an oxidation. The bond is covalent, so it is much stronger than hydrogen bonds, ionic interactions or hydrophobic effects. The residues can be far apart in the primary sequence but close in the folded structure. The bond can join two parts of one chain (tertiary structure) or two separate chains (quaternary structure, as in antibodies and insulin).

Equation: 2 R–SH ⇌ R–S–S–R + 2 H⁺ + 2 e⁻ (oxidation to the right, reduction to the left).

On the MCAT: Passages often describe adding a reducing agent such as β-mercaptoethanol or dithiothreitol, sometimes with urea or heat, and ask what happens to a protein. Reduction breaks the S–S bonds, and the protein loses its structure and activity. In SDS-PAGE, a reducing agent splits subunits joined by disulfides, so a band may appear at a lower molecular weight. Disulfides are common in secreted or extracellular proteins, because the cytosol is a reducing environment and the extracellular space is oxidizing.

Common confusion: Disulfides are not peptide bonds and are not part of primary structure, even though they are covalent. Only cysteine forms them. Methionine also contains sulfur, but its sulfur is in a thioether and cannot make this bond.