Flavodiiron nitric oxide reductases: Recent developments in the mechanistic study and model chemistry for the catalytic reduction of NO.

Inducible NO synthase in mammals helps to produce up to micromolar concentration of nitric oxide (NO) which acts as a key immune defense agent to kill invading pathogens. In order to counter the toxic effects of NO, the pathogens have expressed flavodiiron nitric oxide reductases (FNORs). The FNORs reduce the toxic NO into much less toxic N2O and thus help the pathogens to survive under nitrosative stress. As a consequence, these pathogens proliferate in the human body and cause harmful infections. An appreciable amount of research work has been performed to discover the true mechanism of the FNORs. Different mechanisms involving both mononitrosyl and dinitrosyl diiron complexes as key intermediates are proposed. Evidences for the involvement of new intermediates and more and more experimental evidences for existing ones in the proposed catalytic cycle of FNORs are coming up. These interesting biochemical events have recently boosted the biomimetic chemistry of the FNOR activity as well. This article discusses the importance and the currently understood mechanistic aspects of FNORs. Structural and functional models for the active site of FNORs are discussed along with their success and limitations. Possible future prospects of the modeling chemistry are also suggested.