Rieske business

An N-bound dinitrosyl iron complex has been formed from the reaction of an iron–sulfur complex with nitric oxide

Credit: © 2009 ACS

Iron–sulfur clusters are common cofactors in biological systems and are involved in a range of functions, including electron transfer, substrate binding and activation. These clusters are usually ligated by the cysteine residues of a protein, but there are a few examples of ligation by other amino acid residues, such as histidine, as found in the Rieske proteins. Little is known about the reactivity of Rieske proteins with nitric oxide (NO), which has a variety of roles in the body, including acting as a vasodilator and a neurotransmitter.

Now, Stephen Lippard and co-workers at Massachusetts Institute of Technology have shown¹ that a synthetic analogue of a Rieske cluster reacts with NO to form dinitrosyl iron complexes. As a model for the Rieske cluster, the iron atoms in the Fe₂S₂ cluster were ligated with chelating thiolate and indole ligands as mimics for cysteine and histidine. The displacement of the thiolate ligand by NO resulted in the first example of an N-bound dinitrosyl iron complex prepared directly from the reaction of an iron–sulfur complex with NO.

The reaction of this synthetic cluster demonstrates that naturally occurring Rieske clusters may be disassembled by NO. This is similar to the findings for cysteine-bound clusters and suggests that the Fe₂S₂ core has an intrinsic reactivity towards NO.