Occurrence and biochemistry of hydroperoxidases in oxygenic phototrophic prokaryotes (cyanobacteria)

Blue-green algae (cyanobacteria) have evolved as the most primitive, oxygenic, plant-type photosynthetic organisms. They were the first which produced molecular oxygen as a byproduct of photosynthetic activity. Also today they live in habitats with potentially damaging photooxidative conditions due to high irradiation and oxygen concentrations. Therefore, the cells must have evolved protective mechanisms to cope with reactive oxygen species produced by incomplete reduction of molecular oxygen via electron transport processes to prevent damage of biologically important macromolecules. Hydrogen peroxide and organic peroxides can be removed by enzymes called hydroperoxidases which on the one hand disproportionate it (catalases and catalase-peroxidases) and on the other hand use electron donors to reduce it to water or the corresponding alcohols. Until now the sequenced or partially sequenced genomes of six cyanobacteria are available in databases. Based on similarity searches and multiple sequence alignments, several cyanobacterial hydroperoxidases can be detected. All the cyanobacteria possess peroxiredoxins which use thioredoxin or other reduced thiols to get rid of hydrogen peroxide and lipid peroxides. Nearly all cyanobacteria contain an NADPH-dependent glutathione peroxidase-like protein which uses NADPH to reduce unsaturated fatty acid hydroperoxides. The best analyzed cyanobacterial antioxidative enzyme is the hemoprotein catalase-peroxidase which has a high catalase activity but concerning the sequence it is a typical peroxidase. Two species seem to encode a manganese-containing catalase and Nostoc punctiforme could use a monofunctional catalase. There are as well additional peroxidases encoded in cyanobacteria whose physiological relevance is unknown. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.