Defining and refining the cysteine redoxome through sulfur chemical biology.

Cysteine is one of the rarest amino acids yet exerts a profound influence on biology through the exceptional chemistry of its thiol group. Tunable acidity, high nucleophilicity and access to multiple oxidation states position cysteine as both a dominant cellular redox buffer and a privileged regulatory site. Chemoproteomics has revealed a vast, dynamic cysteine redoxome in which oxidative post-translational modifications act as sensors, switches and buffers across metabolism, signaling and stress responses, respectively. This study advances the following three frameworks: (1) separating intrinsic reactivity from redox sensitivity and regulatory function; (2) using probe chemistry to capture metastable intermediates with site-level precision; and (3) integrating ratiometric measurements with occupancy, exposure and flux to decode redox dynamics. Case studies show how ratiometric chemoproteomics resolves distinct oxoform kinetics, links enzymatic repair to function and exposes the cysteine redoxome as a dynamic regulatory layer and frontier for therapeutic targeting.