Abstract
Sterols are biologically important molecules that serve as membrane fluidity regulators and precursors of signaling molecules, either endogenous or involved in biotic interactions. There is currently no model of their biosynthesis pathways in brown algae. Here, we benefit from the availability of genome data and gas chromatography-mass spectrometry (GC-MS) sterol profiling using a database of internal standards to build such a model. We expand the set of identified sterols in 11 species of red, brown, and green macroalgae and integrate these new data with genomic data. Our analyses suggest that some metabolic reactions may be conserved despite the loss of canonical eukaryotic enzymes, like the sterol side-chain reductase (SSR). Our findings are consistent with the principle of metabolic pathway drift through enzymatic replacement and show that cholesterol synthesis from cycloartenol may be a widespread but variable pathway among chlorophyllian eukaryotes. Among the factors contributing to this variability, one could be the recruitment of cholesterol biosynthetic intermediates to make signaling molecules, such as the mozukulins. These compounds were found in some brown algae belonging to Ectocarpales, and we here provide a first mozukulin biosynthetic model. Our results demonstrate that integrative approaches can already be used to infer experimentally testable models, which will be useful to further investigate the biological roles of those newly identified algal pathways.
Highlights
Sterols are biologically important molecules in eukaryotes, which function as membrane fluidity regulators and precursors of signaling molecules, either endogenous or involved in biotic interactions
Cycloartenol was found in all algae except C. crispus, as previously noticed (Belcour et al, 2020), and L. digitata, where we could interpret this absence as it being below the detection limit
The three red algal species synthesize mainly sterols with 27 carbons, which correlate with the loss of the canonical sterol methyltransferases, transferring additional methyl groups in other lineages (Belcour et al, 2020; Gallo et al, 2020)
Summary
Sterols are biologically important molecules in eukaryotes, which function as membrane fluidity regulators and precursors of signaling molecules, either endogenous or involved in biotic interactions. Among C27 derivatives, specialized metabolites have been identified, such as a molecule bearing structural similarity to a brassinosteroid biosynthesis precursors in the fucale Cytoseira myrica (Hamdy et al, 2009), or the mozukulins from another ectocarpale species, the mozuku Cladosiphon okamuranus (Cheng et al, 2016). Some of these specialized metabolites could be important signaling or defense molecules, but so far there is little knowledge about their potential biological roles (Markov et al, 2018)
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