Fragrant Venezuelaenes A and B with A 5–5–6–7 Tetracyclic Skeleton: Discovery, Biosynthesis, and Mechanisms of Central Catalysts

A cryptic terpenoid
biosynthetic gene cluster ven was characterized from
the model actinomycetic strain Streptomyces
venezuelae ATCC 15439 through a genome mining approach. Silent
gene activation and heterologous expression of ven led to the discovery of two diterpene metabolites venezuelaene A
(1) and B (2, 5-oxo-venezuelaene A), both
of which bear a 5–5–6–7 tetracyclic skeleton
and show fragrance. A class I diterpene synthase VenA with an atypical 115DSFVSD120 motif was revealed to catalyze the
cyclization of geranylgeranyl pyrophosphate (GGPP) generated by the
GGPP synthase VenD, giving rise to 1. The unique cyclization
mechanism was elucidated by 13C-tracer NMR experiments.
The oxidative transformation of 1 into 2 was also characterized to be mediated by the cytochrome P450 enzyme
VenC.