Chemical Biology of Terpenes
With
more than 55,000 known compounds serving myriad functions in all forms
of life, the terpenoids are the largest as well as the most
structurally and stereochemically diverse family of natural products
found on earth.
Terpenoids are found in marine and terrestrial plants, fungi, bacteria
and insects and have many applications in agriculture as antifungals,
herbicides or insecticides, as fragrances in cosmetics, or as
antibiotics, anti-cancer agents, hormones or contraceptive agents in
medicine.
Despite their structural diversity, all terpenoids are biosynthesised
from two chemically related molecules, namely Δ1- and
Δ2-isopentenyldiphosphate, an observation which was first
made by
the German chemist Otto Wallach more than 100 years ago (Nobel
prize 1910) and put on the correct biochemical foundation by Leopold
Ruzicka in Zurich (Nobel prize 1939).
The isopentenyldiphosphates are joined in elongation reactions
to form
the linear precursors of all terpenoids, which are classified according
to the number of isopentenyl-units (C5) they contain as monoterpenes
(C10), sesquiterpenes (C15), diterpenes (C20) and higher molecular
weight terpenoids. The action of terpene synthases then converts these
linear precursors into a large number of cyclic products.
It is these cyclisations that are responsible to a large part for the
diversity seen in terpenoid natural products. Remarkably all these
enzymes share a common three-dimensional fold and hence the complexity
and diversity of the final structures is contrasted with the simplicity
of the generation of these products. In order to decipher the code that
determines the specificity of terpene synthases a much deeper
understanding of individual reaction mechanisms is required.
We have previously used techniques where the substrates were modified
or where individual amino acids within the active site of terpene
synthases were replaced with others and the effects on the reaction
studied. This latter approach is limited by the fact that only 20
different amino acids are normally observed in naturally occurring
proteins.
We now wish to use a new technique that allows the incorporation into
enzymes of non-natural amino acids that can be chosen in such a way as
to obtain answers to specific questions about the reaction mechanisms
and hence further our understanding of this fascinating group of
enzymes. This work should eventually provide us with the knowledge to
generate new terpene-like, but non-natural molecules with many
potential applications in basic science and industry.