In octocorals, a catalase–like allene oxide synthase (AOS) and an 8R-lipoxygenase (LOX) gene are fused together encoding for a single AOS-LOX fusion protein. Although the AOS-LOX pathway is central to the arachidonate metabolism in corals, its biological function in coral homeostasis is unclear. Using an acute incision wound model in the soft coral Capnella imbricata, we here test whether LOX pathway, similar to its role in plants, can contribute to the coral damage response and regeneration. Analysis of metabolites formed from exogenous arachidonate before and after fixed time intervals following wounding indicated a significant increase in AOS-LOX activity in response to mechanical injury. Two AOS-LOX isoforms, AOS-LOXa and AOS-LOXb, were cloned and expressed in bacterial expression system as active fusion proteins. Transcription levels of corresponding genes were measured in normal and stressed coral by qPCR. After wounding, AOS-LOXa was markedly up-regulated in both, the tissue adjacent to the incision and distal parts of a coral colony (with the maximum reached at 1 h and 6 h post wounding, respectively), while AOS-LOXb was stable. According to mRNA expression analysis, combined with detection of eicosanoid product formation for the first time, the AOS-LOX was identified as an early stress response gene which is induced by mechanical injury in coral.

No abstract

Background: Biosynthetic transformation of fatty acid peroxides commonly is catalyzed by cytochromes P450. Results: A catalase-related hemoprotein in the coral Capnella imbricata converts 8R-hydroperoxy-eicosatetraenoic acid in a P450-type reaction to short-chain aldehydes. Conclusion: A catalase-related hydroperoxide lyase is identified in Animalia. Significance: The catalase-related hemoprotein has the catalytic competence of a P450.

The genome of the fungal plant pathogen Fusarium graminearum harbors six catalases, one of which has the sequence characteristics of a fatty acid peroxide-metabolizing catalase. We cloned and expressed this hemoprotein (designated as Fg-cat) along with its immediate neighbor, a 13S-lipoxygenase (cf. Brodhun et al, PloS One, e64919, 2013) that we considered might supply a fatty acid hydroperoxide substrate. Indeed, Fg-cat reacts abruptly with the 13S-hydroperoxide of linoleic acid (13S-HPODE) with an initial rate of 700–1300 s−1. By comparison there was no reaction with 9R- or 9S-HPODEs and extremely weak reaction with 13R-HPODE (~0.5% of the rate with 13S-HPODE). Although we considered Fg-cat as a candidate for the allene oxide synthase of the jasmonate pathway in fungi, the main product formed from 13S-HPODE was identified by UV, MS, and NMR as 9-oxo-10E-12,13-cis-epoxy-octadecenoic acid (with no traces of AOS activity). The corresponding analog is formed from the 13S-hydroperoxide of α-linolenic acid along with novel diepoxy-ketones and two C13 aldehyde derivatives, the reaction mechanisms of which are proposed. In a peroxidase assay monitoring the oxidation of ABTS, Fg-cat exhibited robust activity (kcat 550 s−1) using the 13S-hydroperoxy-C18 fatty acids as the oxidizing co-substrate. There was no detectable peroxidase activity using the corresponding 9S-hydroperoxides, nor with t-butyl hydroperoxide, and very weak activity with H2O2 or cumene hydroperoxide at micromolar concentrations of Fg-cat. Fg-cat and the associated lipoxygenase gene are present together in fungal genera Fusarium, Metarhizium and Fonsecaea and appear to constitute a partnership for oxidations in fungal metabolism or defense.