Molecular Mechanism of Enzymatic Chlorite Detoxification: Insights from Structural and Kinetic Studies

The
heme enzyme chlorite dismutase (Cld) catalyzes the degradation
of chlorite to chloride and dioxygen. Although structure and steady-state
kinetics of Clds have been elucidated, many questions remain (e.g.,
the mechanism of chlorite cleavage and the pH dependence of the reaction).
Here, we present high-resolution X-ray crystal structures of a dimeric
Cld at pH 6.5 and 8.5, its fluoride and isothiocyanate complexes and
the neutron structure at pH 9.0 together with the pH dependence of
the Fe­(III)/Fe­(II) couple, and the UV–vis and resonance Raman
spectral features. We demonstrate that the distal Arg127 cannot act
as proton acceptor and is fully ionized even at pH 9.0 ruling out
its proposed role in dictating the pH dependence of chlorite degradation.
Stopped-flow studies show that (i) Compound I and hypochlorite do
not recombine and (ii) Compound II is the immediately formed redox
intermediate that dominates during turnover. Homolytic cleavage of
chlorite is proposed.