Structural features promoting dioxygen production by Dechloromonas aromatica chlorite dismutase

Chlorite dismutase (Cld) is a heme enzyme capable of rapidly and selectively decomposing chlorite (ClO ₂ ^-) to Cl ^- and O ₂. The ability of Cld to promote O ₂ formation from ClO ₂ ^- is unusual. Heme enzymes generally utilize ClO ₂ ^- 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 (NO ₂ ^-) was determined to investigate features responsible for this novel reactivity. The enzyme active site contains a single btype heme coordinated by a proximal histidine residue. Structural analysis identified a glutamate residue hydrogenbonded 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 ClO ₂ ^- 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 O ₂ 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 O ₂ may have a preferential direction for exiting the active site.