Dimeric chlorite dismutase from the nitrogen-fixing cyanobacterium Cyanothece sp. PCC7425

It is demonstrated that cyanobacteria (both azotrophic and non-azotrophic) may 34 contain heme b oxidoreductases that can convert chlorite to chloride and molecular oxygen (incorrectly denominated chlorite “dismutase”, Cld). Beside the water-splitting manganese complex of photosystem II this metalloenzyme is the second known enzyme that catalyzes the formation of a covalent oxygen-oxygen bond. All cyanobacterial Clds have a truncated N-terminus and are dimeric (i.e. clade 2) proteins. As model protein, Cld from Cyanothece sp. PCC7425 (CCld) was recombinantly produced in E. coli and shown to efficiently degrade chlorite with an activity optimum at pH 5 (kcat 1144 ± 23.8 s-1, KM 162 ± 10.0 μM, catalytic efficiency (7.1 ± 0.6) × 106 M-1 s-1). The resting ferric high-spin axially symmetric heme enzyme has a standard reduction potential of the Fe(III)/Fe(II) couple of -126 ± 1.9 mV at pH 7. Cyanide mediates the formation of a low-spin complex with kon = (1.6 ± 0.1) × 105 M-1 s-1 and koff = 1.4 ± 2.9 s-1 (KD ~ 8.6 μM). Both, thermal and chemical unfolding follows a non-two state unfolding pathway with the first transition being related to the release of the prosthetic group. The obtained data are discussed with respect to known structure-function relationships of Clds. We ask for the physiological substrate and putative function of these O2-producing proteins in (nitrogen-fixing) cyanobacteria.