Structural features promoting dioxygen production by Dechloromonas aromatica chlorite dismutase

• Published in: Journal of biological inorganic chemistry 2010-08, Vol.15 (6), p.879-888
• Main Authors: Goblirsch, Brandon R, Streit, Bennett R, DuBois, Jennifer L, Wilmot, Carrie M
• Format: Article
• Language: English
• Subjects: Biocatalysis; Biochemistry; Biomedical and Life Sciences; Catalytic Domain; Chlorides - metabolism; Chlorite dismutase; crystal structure; Crystallography, X-Ray; Enzyme Inhibitors - metabolism; Enzyme Inhibitors - pharmacology; heme; Kinetics; Life Sciences; Microbiology; Models, Molecular; Nitrites - metabolism; Nitrites - pharmacology; Original Paper; Oxidoreductases - antagonists & inhibitors; Oxidoreductases - chemistry; Oxidoreductases - metabolism; Oxygen - metabolism; Rhodocyclaceae - enzymology
• ISSN: 0949-8257; 1432-1327
• DOI: 10.1007/s00775-010-0651-0

Chlorite dismutase (Cld) is a heme enzyme capable of rapidly and selectively decomposing chlorite (ClO₂ ⁻) to Cl⁻ and O₂. The ability of Cld to promote O₂ formation from ClO₂ ⁻ is unusual. Heme enzymes generally utilize ClO₂ ⁻ as an oxidant for reactions such as oxygen atom transfer to, or halogenation of, a second substrate. The X-ray crystal structure of Dechloromonas aromatica Cld co-crystallized with the substrate analogue nitrite (NO₂ ⁻) was determined to investigate features responsible for this novel reactivity. The enzyme active site contains a single b-type heme coordinated by a proximal histidine residue. Structural analysis identified a glutamate residue hydrogen-bonded to the heme proximal histidine that may stabilize reactive heme species. A solvent-exposed arginine residue likely gates substrate entry to a tightly confined distal pocket. On the basis of the proposed mechanism of Cld, initial reaction of ClO₂ ⁻ within the distal pocket generates hypochlorite (ClO⁻) and a compound I intermediate. The sterically restrictive distal pocket probably facilitates the rapid rebound of ClO⁻ with compound I forming the Cl⁻ and O₂ products. Common to other heme enzymes, Cld is inactivated after a finite number of turnovers, potentially via the observed formation of an off-pathway tryptophanyl radical species through electron migration to compound I. Three tryptophan residues of Cld have been identified as candidates for this off-pathway radical. Finally, a juxtaposition of hydrophobic residues between the distal pocket and the enzyme surface suggests O₂ may have a preferential direction for exiting the active site.