Enzymatic N[sbnd]N bond formation: Mechanism for the N-nitroso synthesis catalyzed by non-heme iron SznF enzyme

Abstract

Enzymatic NN bond formation is rare. Herein, the mechanism for the N-nitrosation from Nδ-hydroxy-Nω-hydroxy-Nω-methyl-L-arginine (L-HMA) catalyzed by SznF, a non-heme iron enzyme, has been illustrated, using density functional calculations. The reaction proceeds through the deprotonation of L-HMA hydroxyls, the CO bond formation between the guanidine and the FeOO• radical resulting in an Fe(II)-peroxo bridge, the CN bond dissociation yielding an Fe-bound NO− anion, the heterolytic OO bond cleavage forming an Fe(II)–O2−, the concerted proton transfer leading to an Fe(II)-hydroxide and an N− anion, the NN bond formation generating the N-nitrosourea product. The discovery of Fe(II)–O2− may be resulted from the unique co-coordination of two NO anions to Fe-oxyl. The natures of diverse NO groups during the reaction are assigned. Another mechanism including an O-N bond formation has been ruled out. This work gives deep insights into the enzymatic NN bond formation and advances the understanding of non-heme iron catalysis.