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

Abstract

The second coordination sphere constitutes a distinguishing factor in the active site to modulate 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 between a conserved Tyr197 and the key iron–nitrosyl intermediate of Tm FDP (flavo–diiron protein), which leads to decreased reaction barriers towards N–N formation and N–O cleavage in NO reduction. This finding supports the direct coupling of diiron dinitrosyl as the N–N formation mode in our QM/MM modeling, and reconciles the mechanistic controversy of external reduction between FDPs and synthetic biomimetics of the iron–nitrosyls. This work highlights the application of 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.

NO Reduction to N2O Triggered by a Dinuclear Dinitrosyl Iron Complex via the Associated Pathways of Hyponitrite Formation and NO Disproportionation

“…Nitric oxide (NO) reduction yielding nitrous oxide (N 2 O) triggered by nonheme flavodiiron nitric oxide reductases (FNORs) was reported via the proposed hyponitrite pathway and diferrous dinitrosyl pathway in a biological and biomimetic study. − In this article, we demonstrated that electronically localized dinuclear {Fe­(NO) 2 } 10 –{Fe­(NO) 2 } 9 DNIC 1 resembling FNORs serves as an NO trapper to facilitate NO reduction to nitrous oxide (N 2 O). Through the direct reaction of NO­(g) with complex 1 (stoichiometric molar ratio of n NO/complex 1 ( n = 1, 2, 3, 5, and 10)), the simultaneous formation of trans -hyponitrite-bound complex 2 , nitrite-bound complex 3 , and N 2 O was observed.…”

“…Two scenarios were proposed for ON–NO bond formation: (a) The hyponitrite pathway involving the addition of exogenous ·NO to a proposed mononitrosyl-bridged (κ 2 -N,O-NO) [Fe­(II)–{Fe­(NO)} 7 ] species forms a putative hyponitrito-bound intermediate . (b) Diferrous dinitrosyl pathway: ON–NO coupling occurs via the dinitrosyl diiron [{Fe­(NO)} 7/8 -{Fe­(NO)} 7/8 ] intermediate to release N 2 O, , where Enemark–Feltham notation is adopted …”