Dioxygen controls the nitrosylation reactions of a protein-bound [4Fe4S] cluster

Iron-sulfur clusters are exceptionally tuneable protein cofactors, and as one of their many roles they are involved in biological responses to nitrosative stress. Both iron-sulfur proteins and synthetic model clusters are extremely sensitive to nitrosylation, tending towards rapid multi-step reaction and cluster degradation. Reaction of protein-bound iron-sulfur clusters with nitric oxide can be stopped at partial nitrosylation in vivo, and repair of protein-bound nitrosylated clusters is possible in the cellular environment. We have used a combination of infrared, EPR, and UV-visible spectroscopies to show that a model [4Fe4S] cluster-containing protein, A. ferroxidans high potential iron-sulfur protein (HiPIP), reacts with NO to give a product mixture dominated by Roussin's Black Salt (RBS) and Roussin's Red Ester (RRE) species. We have shown that O₂ plays a critical role in controlling the major product of nitrosylation, with RBS-like products favoured under strictly anaerobic conditions and RRE favoured in the presence of trace O₂. Moreover, addition of trace O₂ to anaerobically nitrosylated samples induces conversion of RBS-like products to RRE. These findings may have implications for mechanisms of iron-sulfur cluster repair following nitrosative stress, suggest a crucial role for trace O₂, and provide an important link between nitrosylation chemistry of iron-sulfur proteins and the well-established reactivity of synthetic iron-sulfur clusters.