SCHEME 3.

TNP-8N₃-ATP photolabeling reaction. When the azide of TNP-8N₃-ATP is exposed to ultraviolet light, it forms a highly reactive but short-lived nitrene that can initiate reactions with neighboring reactive groups, such as the amino group of the Lys⁴⁹² side chain, thereby forming a stable covalent bond between the nucleotide and the protein (20, 45, 46). R₁ represents the TNP-ATP moiety of TNP-8N₃-ATP (see supplemental Fig. S4), and R₂-NH₂ is the Lys⁴⁹² side chain of the Ca²⁺-ATPase.