Fig.6 UA derivatives biotransformed by fungi
Efficient
synthesis of UA and its derivatives in engineered microbial cell
factories
As mentioned above, traditional extraction methods are increasingly
unfavorable for large-scale production of UA. Furthermore, complex
structure and oxidation position, as well as the optical purity of the
final product all increase the difficulty for its chemical synthesis.
Green biosynthesis of valuable compounds by microbial cell factories may
reduce the limitations of traditional extraction methods from plants as
it is not restricted by the natural climate and is suitable for
large-scale industrial production under high density fermentation. More
importantly, some terpenoids are synthesized efficiently in
microorganisms with various regulatory strategies, such as excavation
and modification of key enzymes 18,64, optimization of
endogenous metabolic pathways 65-67, inhibition of
branching pathways 68-70. Therefore, microbial
engineering also has a wide application prospect in the green synthesis
of UA and its derivatives.
5.1
The biosynthesis pathway of UA
In recent years, synthetic pathway of UA has been well explored in
plants, which is roughly divided into three modules: synthesis of
isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate
(DMAPP), synthesis of α-amyrin and oxidation of α-amyrin. In plant
organelles, IPP and DMAPP are synthesized by methyl
erythritol-4-phosphate (MEP) pathway in plastids and mevalonate (MVA)
pathway in cytoplasm. Afterwards, two IPP molecules are combined with
one molecule of DMAPP end-to-end, and condense to farnesyl diphosphate
(FPP). Two FPP molecules bind together to form the triterpenoid
precursor, squalene followed by oxidiation to form 2,3-oxidosqualene,
which subsequently undergoes cyclization into α-amyrin, the direct
precursor of UA. Eventually, α-amyrin is oxidized at C-28 to form UA
through a three-step oxidation reaction by employing the action of
Cytochrome P450 (CYP450) monooxygenase and its redox chaperone
Cytochrome P450 Reductase (CPR). In addition to the common rate-limiting
enzymes for triterpenoid, α-amyrin synthase (αAS) and CYP450 greatly
affect the synthesis of UA (Fig. 7A ).