Active Sites of O2-Evolving Chlorite Dismutases Probed by Halides and Hydroxides and New Iron-Ligand Vibrational Correlations.

Abstract

O2-evolving chlorite dismutases (Clds) fall into two subfamilies, which efficiently convert ClO2- to O2 and Cl-. The Cld from Dechloromonas aromatica (DaCld) represents the chlorite-decomposing homopentameric enzymes found in perchlorate- and chlorate-respiring bacteria. The Cld from the Gram-negative human pathogen Klebsiella pneumoniae (KpCld) is representative of the second subfamily, comprising homodimeric enzymes having truncated N-termini. Here steric and nonbonding properties of the DaCld and KpCld active sites have been probed via kinetic, thermodynamic, and spectroscopic behaviors of their fluorides, chlorides, and hydroxides. Cooperative binding of Cl- to KpCld drives formation of a hexacoordinate, high-spin aqua heme, whereas DaCld remains pentacoordinate and high-spin under analogous conditions. Fluoride coordinates to the heme iron in KpCld and DaCld, exhibiting ν(FeIII-F) bands at 385 and 390 cm-1, respectively. Correlation of these frequencies with their CT1 energies reveals strong H-bond donation to the F- ligand, indicating that atoms directly coordinated to heme iron are accessible to distal H-bond donation. New vibrational frequency correlations between either ν(FeIII-F) or ν(FeIII-OH) and ν(FeII-His) of Clds and other heme proteins are reported. These correlations orthogonalize proximal and distal effects on the bonding between iron and exogenous π-donor ligands. The axial Fe-X vibrations and the relationships between them illuminate both similarities and differences in the H-bonding and electrostatic properties of the distal and proximal heme environments in pentameric and dimeric Clds. Moreover, they provide general insight into the structural basis of reactivity toward substrates in heme-dependent enzymes and their mechanistic intermediates, especially those containing the ferryl moiety.