Disulfide bridges in proteins involve which residues?

• A. Cysteine residues
• B. Methionine residues
• C. Proline residues
• D. Glycine residues

Answer: (A)

Disulfide bridges form when two cysteine residues come together and their thiol groups (-SH) are oxidized to create a covalent -S-S- bond. This covalent link helps lock folded structures in place, providing stability to proteins, especially in oxidizing environments like the endoplasmic reticulum or secreted proteins. Other common amino acids don’t have the reactive thiol group needed for disulfide bonds: methionine has sulfur in a thioether linkage that isn’t capable of forming disulfides, and proline and glycine lack sulfur-containing side chains altogether.