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

Irene Schaffner,Gerhard Stadlmayr,Holger Daims,Paul G Furtmüller,Marzia Bellei,Kristina Djinovic‐Carugo,Christian Obinger,Katharina F Pirker,Michael Krutzler,Georg Mlynek,Gianantonio Battistuzzi,Stefan Hofbauer

doi:10.1111/mmi.12989

Abstract

SummaryIt 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’, Cld). 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 Clds have a truncated N‐terminus and are dimeric (i.e. clade 2) proteins. As model protein, Cld from C yanothece sp. PCC7425 (CCld) 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, 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 7.0. Cyanide mediates the formation of a low‐spin complex with k on = (1.6 ± 0.1) × 105 M−1 s−1 and k off = 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.

Highlights

Cyanobacteria are the only known bacteria capable of oxygenic photosynthesis and are important model organisms for studies of the bioenergetics, evolution, and ecology of photosynthesis and aerobic respiration
It is demonstrated that cyanobacteria contain heme b oxidoreductases that can convert chlorite to chloride and molecular oxygen
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

Summary

Introduction

Cyanobacteria are the only known bacteria capable of oxygenic (plant-type) photosynthesis and are important model organisms for studies of the bioenergetics, evolution, and ecology of photosynthesis and aerobic respiration. They have accommodated both a photosynthetic electron transport chain and a respiratory transport chain within a single prokaryotic cell (Jones and Myers, 1963; Peschek et al, 2004; Paumann et al, 2005). It catalyses the decomposition of chlorite (ClO2−) into chloride (Cl−) and molecular oxygen (O2) (van Ginkel et al, 1996).

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Discussion

Conclusion