Unique intramolecular oxidative rearrangement N-nitrosation mechanism of non-heam iron-containing enzyme SznF

Abstract

Non-heam iron-dependent enzyme SznF catalyzes a critical step of the L-arginine derived guanidine group rearrangement to produce the N-nitrosourea pharmacophore in the process of SZN biosynthesis. The intramolecular oxidative rearrangement process is accomplished in the Fe(II)-containing active site located at the cupin domain of SznF, with which the catalytic mechanism remains elusive. In this work, density functional theory methods have been employed to investigate possible catalytic mechanisms of SznF. The N-nitrosation reaction in SznF was found to follow an energetically favorable pathway which includes six consecutive steps: (1) formation of FeII-superoxo species with dioxgen binding on the iron center; (2) superoxo group attacking on the C{varepsilon} of substrate to form the peroxo-bridge complex; (3) C{varepsilon}-N{omega} bond homolysis to release N{omega}O; (4) peroxo bridge heterolytic cleavage; (5) deprotonation of [Formula] by Fe-O group; (6) the [Formula] couples with the N{omega}O group and generates the N-nitroso product. The reaction proceeds in an unexpected way during which the electrons shuttle among two NO groups of the substrate and the peroxo moiety to promote C{varepsilon}-N{omega} bond homolysis and O-O bond heterolysis sequentially without generating high-valent Fe-O species, which is distinct from any known reactions catalyzed by the iron-containing enzyme. The unusual mechanism of SznF shed light on the area of enzymatic N-nitrosation reactions.