Structure and Nitrite Reductase Activity of the Di-iron Protein ScdA in <i>Staphylococcus aureus</i>

Pathogenic <i>Staphylococcus aureus</i> endures bursts of host-derived reactive nitrogen species, yet the molecular defenses that enable this resilience have remained unclear. We now show that the previously enigmatic di-iron enzyme ScdA functions as a nitrite reductase, converting nitrite to nitric oxide (NO), and we elucidate the structural elements that support this activity. Using an integrative toolkit─X-ray crystallography, solution NMR, AlphaFold modeling, and pulsed EPR/DEER─we solved the full-length homodimeric structure of ScdA and identified a robust di-iron center that forms stable iron-nitrosyl intermediates. Targeted mutagenesis reveals that redox-active cysteines and dimerization state tune catalytic output, whereas steady-state kinetics confirm efficient nitrite-to-NO turnover. In vivo, ScdA overexpression in <i>Escherichia coli</i> suppresses growth under nitrite-rich conditions, highlighting the cytotoxic potency of the NO it generates. By coupling structure to function, our work clarifies <i>S. aureus</i> strategies for managing nitrosylative stress and points to ScdA as a potential vulnerability in antibiotic-resistant pathogens.