Fig. 4 UA derivatives modified on ring A
4.2Directed transformation of
UA by microorganisms
Biosynthesis of valuable products have advantages over chemical synthsis
in terms of milder reaction conditions and presence of abundant enzymes
along with high stereo-, regio- and chemo- selectivities53. Therefore, microbial transformation is
increasingly adopted as an alternative way to modify the structure of
natural and synthetic compounds 54,55. In particular,
it is easy to achieve the glycosylation of pentacyclic triterpenes that
is difficult during chemical synthesis 56. Generally,
UA biotransformation is carried out by dissolving the appropriate
quantity of UA in strain suspended in fresh sterile broth under sterile
conditions. After co-cultivation with strains, the transformed products
of UA are eventually separated through suction filtration and
extraction.
In recent years, various UA derivatives have been synthesized through
the transformation of numerous microbial strains. It is reported thatNocardia sp. NRRL 5646 converts UA into oleanolic acid methyl
ester (Fig.5A ), in which two intermediates are formed by
participation in the ”reverse biosynthesis” involving methyl migration
from C-19 to C-20 57. A single component is produced
by this biotransformation followed by hydrolysis to yield the
corresponding triterpenoid acid. At the same time, UA has been found to
be transformed into different derivatives by the same strain (Fig.
5B ). Researchers speculate that 3β-hydroxysteroid dehydrogenase and
3-ketosteroid-Δ1-dehydrogenase are involved in the transformation of
steroid 58.