Reversible S-Nitrosylation in an Engineered Azurin

S-nitrosothiols are known as reagents for NO storage and transportation, and as regulators in many physiological processes. While the S-nitrosylation catalyzed by heme proteins is well known, no direct evidence of S-nitrosylation in copper proteins has been reported. Here we report reversible insertion of NO into a copper-thiolate bond in an engineered copper center in Pseudomonas aeruginosa azurin by rational design of the primary coordination sphere and tuning its reduction potential via deleting a hydrogen bond in the secondary coordination sphere. The results not only provide the first direct evidence of S-nitrosylation of Cu(II)-bound cysteine within metalloproteins, but also shed light on the reaction mechanism and structural features responsible for stabilizing the elusive Cu(I)-S(Cys)NO species. The fast, efficient, and reversible S-nitrosylation reaction is used to demonstrate its ability to prevent NO inhibition of cytochrome bo(3) oxidase activity by competing for NO binding with the native enzyme under physiologically relevant conditions.