Impact of subunit and oligomeric structure on the thermal and conformational stability of chlorite dismutases

Chlorite dismutases (Cld) are unique heme b containing oxidoreductases that convert chlorite to chloride and dioxygen. Recent phylogenetic and structural analyses demonstrated that these metalloproteins significantly differ in oligomeric and subunit structure. Here we have analyzed two representatives of two phylogenetically separated lineages, namely pentameric Cld from Candidatus "Nitrospira defluvii" and dimeric Cld from Nitrobacter winogradskyi having a similar enzymatic activity at room temperature. By application of a broad set of techniques including differential scanning calorimetry, electronic circular dichroism, UV-vis and fluorescence spectroscopy the temperature-mediated and chemical unfolding of both recombinant proteins were analyzed. Significant differences in thermal and conformational stability are reported. The pentameric enzyme is very stable between pH 3 and 10 (T(m)=92°C at pH 7.0) and active at high temperatures thus being an interesting candidate for bioremediation of chlorite. By contrast the dimeric protein starts to unfold already at 53°C. The observed unfolding pathways are discussed with respect to the known subunit structure and subunit interaction.