Redox Control of S-sulfocysteine Formation in Adenosine Phosphosulfate Reductase

Sulfur assimilation fuels bacterial growth by supplying the reduced sulfur required for the biosynthesis of sulfur-containing biomolecules. Adenosine 5'-phosphosulfate reductase (APSR) catalyzes the first reductive step of this pathway, converting adenosine 5'-phosphosulfate (APS) to adenosine monophosphate. This reaction proceeds through nucleophilic attack by catalytic C256, located in the flexible C-terminal tail, on the sulfur atom of APS, forming a thiosulfonate enzyme intermediate. Here, we investigate this reaction in APSR from Pseudomonas aeruginosa, an opportunistic pathogen associated with severe infections, particularly in patients with cystic fibrosis. APSR contains an iron-sulfur [4Fe-4S] cluster, which participates in redox steps of the reaction. Here, we show that the redox state of the iron-sulfur cluster also controls the catalytic step. Multiscale molecular simulations investigate how oxidized and reduced cluster states affect APS binding, active site organization, and the nucleophilic attack step. Molecular dynamics (MD) simulations show that the oxidized [4Fe-4S]2+; cluster stabilizes substrate interactions and the conformation of the C-terminus, facilitating a catalytically productive orientation of C256. The activation barrier of 17.7 {+/-} 1.7 kcal mol-1 from quantum mechanics/molecular mechanics (QM/MM) umbrella sampling MD simulations at the B3LYP-D3(BJ)/6-31G(d) level of theory is in good agreement with the experimental kinetics. The redox state of the iron-sulfur cluster shows its role in modulating the conformation of conserved K144, which is important for transition state stabilization in the nucleophilic attack. These findings illuminate the mechanism of this P. aeruginosa target and provide broader insight into the roles of iron-sulfur clusters in controlling enzyme reactivity.