Used as an affinity labeling reagent to study ATP-binding sites in enzymes and proteins. It selectively modifies lysine residues near nucleotide-binding regions, allowing researchers to identify and map active sites. Commonly applied in biochemical research to probe enzyme mechanisms, particularly in kinases and ATPases. Its fluorosulfonyl group reacts covalently with nucleophilic amino acids, providing a stable label for detection or isolation. Often employed in conjunction with radiolabeled or fluorescent tags for enhanced sensitivity in binding studies.