Compound I formation and reactivity in dimeric chlorite dismutase – Impact of pH and the dynamics of the catalytic arginine

The heme enzyme chlorite dismutase (Cld) catalyzes the degradation of chlorite to chloride and dioxygen. Many questions about the molecular reaction mechanism of this iron protein have remained unanswered, including the electronic nature of the catalytically relevant oxoiron(IV) intermediate and its interaction with the distal, flexible, and catalytically active arginine. Here, we have investigated the dimeric Cld from Cyanothece sp. PCC7425 (CCld) and two variants having the catalytic arginine R127 (i) hydrogen-bonded to glutamine Q74 (wild-type CCld), (ii) arrested in a salt bridge with a glutamate (Q74E), or (iii) being fully flexible (Q74V). Presented stopped-flow spectroscopic studies demonstrate the initial and transient appearance of Compound I in the reaction between CCld and chlorite at pH 5.0 and 7.0 and the dominance of spectral features of an oxoiron(IV) species (418, 528, and 551 nm) during most of the chlorite degradation period at neutral and alkaline pH. Arresting the R127 in a salt bridge delays chlorite decomposition, whereas increased flexibility accelerates the reaction. The dynamics of R127 does not affect the formation of Compound I mediated by hypochlorite but has an influence on Compound I stability, which decreases rapidly with increasing pH. The decrease in activity is accompanied by the formation of protein-based amino acid radicals. Compound I is demonstrated to oxidize iodide, chlorite, and serotonin but not hypochlorite. Serotonin is able to dampen oxidative damage and inactivation of CCld at neutral and alkaline pH. Presented data are discussed with respect to the molecular mechanism of Cld and the pronounced pH dependence of chlorite degradation.