Intermolecular disulfide bond to modulate protein function as a redox-sensing switch.

Recently, redox-regulated biological reactions have been elucidated. In the regulation of these reactions, redox-sensing molecular switches function as unique biological machineries that modulate the functional proteins present in enzymes, transcriptional factors, sensor proteins, and transcriptional factor modulators. The redox-sensing cysteine residues and the disulfide bond formed between these cysteine residues serve as redox-sensing molecular switches; these switches sense cellular oxidizing factors such as oxygen, reactive oxygen species, and cellular reducing factors such as thioredoxin (Trx), glutathione (GSH), and their family molecules. Depending on the redox status, the switch directly modulates the protein function via the "locking and unlocking" of the critically functional residue or indirectly modulates the protein function via "protein conformational changes," which affects the functioning of a distantly located critical residue in an allostery-like fashion or a topology change. Redox-sensing switches can be classified into two types-intramolecular (intrasubunit) and intermolecular (intersubunit) ones. Further, depending on the sensing specificity to reducing factors, the switch subtype is classified into Trx, GSH, or their family molecules-specific type. This review focused on the intermolecular redox-sensing switches found in various proteins.