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Dimeric chlorite dismutase from the nitrogen‐fixing cyanobacterium C yanothece sp. PCC 7425
Author(s) -
Schaffner Irene,
Hofbauer Stefan,
Krutzler Michael,
Pirker Katharina F.,
Bellei Marzia,
Stadlmayr Gerhard,
Mlynek Georg,
DjinovicCarugo Kristina,
Battistuzzi Gianantonio,
Furtmüller Paul G.,
Daims Holger,
Obinger Christian
Publication year - 2015
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/mmi.12989
Subject(s) - ferric , heme , cyanobacteria , biology , photosystem ii , cyanide , enzyme kinetics , superoxide dismutase , biochemistry , enzyme , chemistry , inorganic chemistry , active site , photosynthesis , bacteria , genetics
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.