Structure and reactivity of chlorite dismutase nitrosyls

Ferric nitrosyl ({FeNO}6) and ferrous nitrosyl ({FeNO}7) complexes of the chlorite dismutases (Cld) from Klebsiella pneumoniae and Dechloromonas aromatica have been characterized using UV–visible absorbance and Soret-excited resonance Raman spectroscopy. Both of these Clds form kinetically stable {FeNO}6 complexes and they occupy a unique region of ν(Fe–NO)/ν(N–O) correlation space for proximal histidine liganded heme proteins, characteristic of weak Fe–NO and N–O bonds. This location is attributed to admixed FeIII–NO character of the {FeNO}6 ground state. Cld {FeNO}6 complexes undergo slow reductive nitrosylation to yield {FeNO}7 complexes. The effects of proximal and distal environment on reductive nitroylsation rates for these dimeric and pentameric Clds are reported. The ν(Fe–NO) and ν(N–O) frequencies for Cld {FeNO}7 complexes reveal both six-coordinate (6c) and five-coordinate (5c) nitrosyl hemes. These 6c and 5c forms are in a pH dependent equilibrium. The 6c and 5c {FeNO}7 Cld frequencies provided positions of both Clds on their respective ν(Fe–NO) vs ν(N–O) correlation lines. The 6c {FeNO}7 complexes fall below (along the ν(Fe–NO) axis) the correlation line that reports hydrogen-bond donation to NNO, which is consistent with a relatively weak Fe–NO bond. Kinetic and spectroscopic evidence is consistent with the 5c {FeNO}7 Clds having NO coordinated on the proximal side of the heme, analogous to 5c {FeNO}7 hemes in proteins known to have NO sensing functions. Synopsis: A new class of ferric heme protein nitrosyls is reported. Their location in FeNO vibrational correlation space is heretofore unknown in heme proteins and attributed to admixed FeIII–NO character of the ground state. Reductive nitrosylation of these complexes is slow and yields pentacoordinate ferrous heme nitrosyls having proximal NO ligands. [Display omitted] •NO adducts of chlorite dismutase (Cld) have been studied by resonance Raman spectroscopy.•A new class of ferric heme nitrosyls, {FeNO}6, in Cld is reported.•{FeNO}6 hemes in Clds occupy a new region of FeNO vibrational correlation space.•Reductive nitrosylation of{FeNO}6 Clds to yield ferrous heme nitrosyls {FeNO}7 is slow.•Pentacoordinate {FeNO}7 Clds have proximally coordinated NO ligands.