Origin of Nitric Oxide Reduction Activity in Flavo‐Diiron NO Reductase: Key Roles of Second Coordination Sphere

Second coordination sphere constitutes a distinguishing factor in active site to modulate the enzymatic reactivity. To unravel the origin of NO‐to‐N2O reduction activity of non‐heme diiron enzymes, herein we report a strong second coordination sphere interaction from a conserved Tyr197 residue for the key iron‐nitrosyl intermediate of Tm FDP (flavo‐diiron protein). With its assistance, the reaction barriers of N‐N formation and N‐O cleavage in NO reduction are significantly reduced. Direct coupling of diiron dinitrosyl is supported as the N‐N formation mode in our QM/MM (combined quantum mechanical/molecular mechanical) modelings, which is in line with recent experiments. Furthermore, the revealed second coordination sphere effect reconciles the mechanistic controversy of external reduction between FDPs and synthetic biomimetics of the iron‐nitrosyls. This work highlights the QM/MM 57Fe Mössbauer modeling in elucidating the structural features of not only first, but also second coordination spheres of the key transient species involved in NO/O2 activation by non‐heme diiron enzymes.

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