Dimeric chlorite dismutase from the nitrogen‐fixing cyanobacterium C yanothece sp. PCC 7425

Summary It is demonstrated that cyanobacteria (both azotrophic and non‐azotrophic) contain heme b oxidoreductases that can convert chlorite to chloride and molecular oxygen (incorrectly denominated chlorite ‘dismutase’, C ld). Beside the water‐splitting manganese complex of photosystem II , this metalloenzyme is the second known enzyme that catalyses the formation of a covalent oxygen–oxygen bond. All cyanobacterial C lds have a truncated N ‐terminus and are dimeric (i.e. clade 2) proteins. As model protein, C ld from C yanothece sp. PCC 7425 ( CC ld) was recombinantly produced in E scherichia coli and shown to efficiently degrade chlorite with an activity optimum at pH 5.0 [ k cat 1144 ± 23.8 s −1 , K M 162 ± 10.0 μM, catalytic efficiency (7.1 ± 0.6) × 10 6  M −1  s −1 ]. The resting ferric high‐spin axially symmetric heme enzyme has a standard reduction potential of the F e( III )/ F e( II ) couple of −126 ± 1.9 mV at pH 7.0. Cyanide mediates the formation of a low‐spin complex with k on  = (1.6 ± 0.1) × 10 5  M −1  s −1 and k off  = 1.4 ± 2.9 s −1 ( K D  ∼ 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 C lds. We ask for the physiological substrate and putative function of these O 2 ‐producing proteins in (nitrogen‐fixing) cyanobacteria.