A lipoxygenase with linoleate diol synthase activity from Nostoc sp. PCC 7120
Résumé
The dioxygenation of polyunsaturated fatty acids in plants is mainly catalyzed by members of the lipoxygenase enzyme family. Lipoxygenase products may be further metabolized and are known as signalling substances in plant development and in responses to wounding and pathogen attack. In contrast to the situation in eukaryotes, information on the relevance of lipidperoxide metabolism in prokaryotic organisms is scarce. Therefore, we aimed to analyze lipoxygenases and oxylipin patterns of cyanobacterial origin. A search of the genomic sequence of the cyanobacterium, Nostoc sp. PCC 7120, suggested an open reading frame encoding a putative lipoxygenase named NspLOX that harboured an amino-terminal extension. Individual analysis of recombinant carboxy-terminal domain revealed enzymatic activity as a linoleate (9R)-lipoxygenase. Analysis of the full-length NspLOX-protein however, revealed linoleate diol synthase activity, generating (10E,12E)-9,14-dihydroxy-10,12-octadecadienoic acid as the main product from linoleic acid and (10E,12E,14E)-9,16-dihydroxy-10,12,14-octadecatrienoic acid as the main product from α-linolenic acid substrates, respectively, with α-linolenic acid as preferred substrate. The enzyme exhibited a broad pH optimum between pH 7 and 10. Soluble extracts of Nostoc sp. contain more 9-lipoxygenase-derived hydroperoxides in sonified than in non-sonified cells, but products of full-length NspLOX were not detectable under the conditions used. As no other lipoxygenase-like sequence was identified in the genome of Nostoc sp. PCC 7120, the results presented suggest that (9R)-lipoxygenase-derived oxylipins may represent the endogenous products of NspLOX. Based on the biochemical data of NspLOX we suggest that this bifunctional enzyme may represent a more ancient way to control the intracellular amount of oxylipins in this cyanobacterium.
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