The heme enzyme chlorite dismutase (Cld) degrades chlorite to chloride and dioxygen. Although the structure and steady-state kinetics of pentameric Clds have been elucidated, many questions remain, such as 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 cannot recombine and (ii) Compound II is the immediately formed redox intermediate that dominates during reaction. Homolytic cleavage of chlorite is proposed
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