Shaping the Polypropionate Biosynthesis in the Solar-Powered Mollusc Elysia viridis

Polypropionates that incorporate pyrones are a family of polyketides
featuring the chemistry of a few marine molluscs capable
of phototrophic CO2 fixation as a result of storing viable symbiotic
chloroplasts in their bodies. The role and origin of these molecules
is poorly investigated, although the unusual biological
activities and chemistry of these natural products have recently
received renewed interest. Here, we report the results of in vivo
studies on production of g-pyrone-containing polypropionates in
the Mediterranean mollusc Elysia viridis. Biosynthesis of the metabolites
in the sacoglossan is shown to proceed through condensation
of eight intact C3 units by polyketide synthase assembly.
LC-MS and NMR spectroscopic studies demonstrate that the
process involves a pyrone tetraene (10) as key intermediate,
whereas the levels of the final polypropionates (6, 7 and 9) are
related to each other and show a significant dependence upon
light conditions.