Editing Openai/6783e979-fa1b-48a4-a59f-bfe4c3d7ceca (section)

===== Post-Translational Modifications (PTMs) of Cysteine =====
# S-sulfenylation: - This modification occurs when cysteine reacts with reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), forming sulfenic acid (–SOH). S-sulfenylation is often a transient modification that can act as a signaling event, modulating the function of enzymes and other proteins. It serves as a precursor to other redox modifications and can be reversed by reducing agents like thioredoxin (Trx).
# S-sulfinylation and S-sulfonylation: - These are more oxidized forms of cysteine and are usually markers of oxidative distress. While sulfinylation (–SO2H) may be reversible in certain cases, sulfonylation (–SO3H) is typically irreversible. These modifications often result from prolonged oxidative stress and can lead to the loss of protein function.
# Disulfide Bridge Formation: - Disulfide bonds (S–S) are covalent links between two cysteine residues, essential for maintaining protein structure and function. Disulfide bonds can be formed and reduced dynamically as part of redox regulation, particularly in the context of oxidative protein folding in the endoplasmic reticulum (ER) and in regulating metabolic enzymes in the chloroplast during light/dark cycles.
# S-glutathionylation: - This involves the addition of a glutathione molecule to a cysteine residue, forming a mixed disulfide bond. S-glutathionylation protects cysteine from irreversible oxidation and serves as a redox switch to regulate protein function during oxidative stress. It is reversible, typically catalyzed by glutaredoxin (Grx) or thioredoxin (Trx), and plays a significant role in protecting metabolic enzymes and regulating cellular redox balance.
# Persulfidation: - Persulfidation involves the addition of a sulfur atom to the thiol group of cysteine, resulting in a persulfide group (Cys–SSH). This modification is part of hydrogen sulfide (H2S) signaling pathways and can protect proteins from oxidative damage. It is reversible and plays a role in various stress responses and metabolic regulations.