Publication Type:Magazine Article
Source:J. Mol. Biol. (2009)
DOI Name (links to online publication)10.1016/j.jmb.2009.01.036
Keywords:heme-based oxygen production; chloro-oxyanions; remediation; Azospira oryzae strain GR-1; EPR
Chlorite dismutase (Cld) is a key enzyme of perchlorate and chlorate respiration. This heme-based protein reduces the toxic compound chlorite into the innocuous chloride anion in a very efficient way while producing molecular oxygen. A sequence comparison between Cld homologues shows a highly conserved family. The crystal structure of Azospira oryzae strain GR-1 Cld is reported to 2.1 angstrom resolution. The structure reveals a hexameric organization of the Cld, while each monomer exhibits a ferredoxin-like fold. The six subunits are organized in a ring structure with a maximal diameter of 9 nm and an inner diameter of 2 nm. The heme active-site pocket is solvent accessible both from the inside and the outside of the ring. Moreover, a second anion binding site that could accommodate the assumed reaction intermediate ClO- for further transformation has been identified near the active site.
The environment of the heme cofactor was investigated with electron paramagnetic resonance spectroscopy. Apart from the high-spin ferric signal of the five-coordinate resting-state enzyme, two low-spin signals were found corresponding to six-coordinate species. The current crystal structure confirms and complements a recently proposed catalytic mechanism that proceeds via a ferryl species and a ClO- anion. Our structural data exclude cooperativity between the iron centers. (C) 2009 Elsevier Ltd. All rights reserved.