Cys25

Cys25 is a notation used to refer to the cysteine residue at position 25 in a protein sequence. The numbering follows the N-terminus, with the first amino acid assigned as position 1. When discussed in the literature, Cys25 may indicate a specific protein where the 25th residue is cysteine, or a domain where a conserved cysteine occupies that position. The functional significance of Cys25 is determined by its context within the protein’s structure and function.

Cysteine residues feature a thiol side chain that is relatively reactive. The pKa of the cysteine thiol is around 8.3, so at physiological pH a fraction can exist as a reactive thiolate. This reactivity enables roles in catalysis, redox chemistry, and chemical modification. Cys residues frequently form disulfide bonds under oxidizing conditions, which can stabilize folded structures, particularly in secreted or extracellular proteins; in the reducing environment of the cytosol, many disulfides remain reduced.

Possible roles for Cys25 include acting as a catalytic residue in cysteine-dependent enzymes, participating in metal binding motifs, or serving as a redox sensor or switch. Post-translational modifications at cysteine, such as S-nitrosylation, S-palmitoylation, and S-glutathionylation, can regulate activity, localization, or interactions with other molecules.

The significance of Cys25 is highly context-specific. Its importance may arise from participation in catalytic mechanisms, formation of structural disulfides, or serving as a modification site. Experimental approaches such as site-directed mutagenesis, mass spectrometry to detect modifications or oxidation states, and structural methods like X-ray crystallography or NMR help elucidate its role. Comparative sequence analysis can reveal conservation of Cys at position 25 across related proteins. See also cysteine, disulfide bond, redox regulation, S-nitrosylation.