Dioxygenase-cytochrome P450 (DOX-CYP) fusion enzymes are present in diverse human and plant pathogenic fungi. They oxygenate fatty acids to lipid mediators which have regulatory functions in fungal development and toxin production. These enzymes catalyze the formation of fatty acid hydroperoxides which are subsequently converted by the P450 activities or reduced to the corresponding alcohols. The N-terminal DOX domains show catalytic and structural homology to mammalian cyclooxygenases, which belong to the most thoroughly studied human enzymes.
7,8-Linoleate diol synthase (LDS) of the plant pathogenic fungus Gaeumannomyces graminis was the first characterized member of the DOX-CYP fusion enzyme family. It catalyzes the conversion of linoleic acid to 8R-hydroperoxylinoleic acid (HPODE) and subsequently to 7S,8S-dihydroxylinoleic acid by its DOX and P450 domains, respectively. By now, several enzymes with homology to 7,8-LDS have been identified in important fungi, e.g., psi factor-producing oxygenase (ppo)A, ppoB, and ppoC, of Aspergillus nidulans and A. fumigatus.
By cloning and recombinant expression, ppoA of A. fumigatus was identified as 5,8-LDS. Partial expression of the 8R-DOX domains of 5,8-LDS of A. fumigatus and 7,8-LDS of G. graminis yielded active protein which demonstrates that the DOX activities of LDS are independent of their P450 domains. The latter domains were shown to contain a conserved motif with catalytically important amide residues. As judged by site-directed mutagenesis studies, 5,8- and 7,8-LDS seem to facilitate heterolytic cleavage of the oxygen-oxygen bond of 8R-HPODE by aid of a glutamine and an asparagine residue, respectively.
Cloning and expression of putative DOX-CYP fusion proteins of A. terreus and Fusarium oxysporum led to the discovery of novel enzyme activities, e.g., linoleate 9S-DOX and two allene oxide synthases (AOS), specific for 9R- and 9S-HPODE, respectively. The fungal AOS are present in the P450 domains of two DOX-CYP fusion enzymes and show higher sequence homology to LDS than to plant AOS and constitute therefore a novel class of AOS.
In summary, this thesis describes the discovery of novel fatty acid oxygenases of human and plant pathogenic fungi and the characterization of their reaction mechanisms.
Uppsala: Acta Universitatis Upsaliensis, 2013. , 67 p.
Fusion protein, Linoleate diol synthase, Allene oxide synthase, Cyclooxygenase, Oxygenase, HPLC, Mass spectrometry, Hydroperoxide isomerase, Aspergillus, Fusarium oxysporum