Biosynthesis of the fungal metabolite, piliformic acid (2-hexylidene-3-methylsuccinic acid)
Abstract
The biosynthesis of piliformic acid, a secondary
metabolite of various xylariaceous fungi, has been studied in
Poronia piliformis and Xylaria mali. The
metabolite can be retro-biosynthetically cleaved to generate a
C8 and a C3 moiety. The study reveals that
the C8 unit is derived directly from octanoate and
that the octanoate in turn originates from a fatty acid synthase
(FAS) rather than from a polyketide synthase (PKS). This
conclusion is drawn after assaying the stereochemical course of
the enoyl reductase involved in the synthesis of the octanoate
unit. The C3 unit is efficiently labelled by succinate
and the citric acid cycle intermediate oxaloacetate is implicated
as a key biosynthetic precursor. The location of deuterium after
isotopic labelling with sodium
[2H15]octanoate reveals a 1,3-hydrogen
shift, indicative of a double-bond isomerisation, operating at
a late stage in the biosynthesis. A hypothesis for piliformic
acid biosynthesis is presented and discussed in the context of
structurally related fungal and lichen metabolites.
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