Extracellular disulfide bonds in mucus and epithelial surface proteins

Extracellular disulfide bonds are covalent linkages between cysteine residues that provide critical structural stability to secreted and membrane-bound proteins. In the respiratory and gastrointestinal tracts, these bonds are the primary force linking mucin monomers (such as MUC5AC and MUC5B) into the massive, gel-forming polymers that constitute mucus. In pathological states like asthma, cystic fibrosis, and COPD, oxidative stress and inflammation promote excessive disulfide cross-linking, transforming mucus into a thick, elastic sludge that obstructs airways and impairs mucociliary clearance. Beyond their structural role in mucus, labile extracellular disulfides on epithelial surface proteins, such as integrins and the epithelial sodium channel (ENaC), function as redox-sensitive switches that modulate protein activity and signaling in response to the extracellular environment. Therapeutic strategies targeting these bonds utilize reducing agents, known as mucolytics, which break the disulfide linkages via thiol-disulfide exchange to depolymerize mucus and restore its fluid properties for easier clearance.

Other names

Mucin disulfide bondsExtracellular protein disulfidesAllosteric disulfide bondsRedox-labile disulfidesThiol-disulfide switches

Reduction of intermolecular and intramolecular disulfide bonds via thiol-disulfide exchange, leading to the depolymerization of mucin glycoproteins and modulation of surface protein conformations.