Towards the Nitric Oxide Reductase Mechanism of Flavodiiron Proteins

Abstract

Flavo‐diiron proteins (FDPs) catalyze reductive scavenging of molecular oxygen and nitric oxide in air‐sensitive microorganisms. The flavin cofactor in close proximity to the diiron site distinguishes FDPs from other non‐heme diiron proteins. FDPs from various sources catalyze dioxygen reduction or nitric oxide reduction (NOR) to varying degrees, and questions arise as to how the active site optimizes levels of the two activities and about what structural features affect the catalytic mechanism of FDPs. Published studies have identified diferrous mononitrosyl and diferrous dinitrosyl complexes for the FDP from Thermotoga maritima (TmFDP). An NOR mechanism consistent with these results consists of reduction of a diferrous‐dintrosyl adduct by the proximal reduced flavin prior to N‐N bond formation and release of N2O. In attempts to determine the reasons for the varying NOR activities in FDPs, we are conducting analogous studies on the FDP from Moorella thermoacetica (MtFDP). The reported turnover number for NOR activity in MtFDP is ~1000‐fold greater than that of TmFDP. Rapid kinetics studies are in progress to identify NO complexes in the higher‐activity MtFDP.