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 Fe^III-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 N_NO, 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.