cysteinernas

Cysteinernas, in protein context, refers to the cysteine residues that occur throughout proteins. Cysteine is an amino acid with a thiol side chain (-SH) that can participate in a variety of chemical reactions, making these residues especially versatile for structure and function.

The thiol group of cysteine is reactive and can exist as a thiolate anion (S-) at higher pH or in local environments that stabilize it. This reactivity enables the formation of covalent disulfide bonds (S-S) between two cysteine residues, a key post-translational modification. Disulfide bonds help stabilize protein folds and create cross-links within or between polypeptide chains. In many cells, disulfide formation is favored in oxidizing compartments such as the endoplasmic reticulum and is assisted by enzymes like protein disulfide isomerase; cytosolic cysteines are typically kept reduced by the glutathione system and thioredoxin.

Cysteine residues also play essential roles in catalysis and regulation. They are part of the active sites of cysteine proteases and many redox-active enzymes. Beyond catalysis, cysteines coordinate metal ions in zinc finger motifs and bind iron-sulfur clusters in various proteins. They can undergo reversible post-translational modifications, such as S-nitrosylation, S-glutathionylation, and S-palmitoylation, which modulate activity, interactions, and localization.

Overall, cysteine residues contribute to protein folding, stability, catalytic activity, and redox signaling. Their unique chemistry makes them critical for the function of a wide range of biological molecules.