Licorice root is incorporated into a number of traditional medicines. Photo by Kiminori Toyooka, RIKEN Plant Science Center, 2011, American Society of Plant Biologists |
Plants of the genus Glycyrrhiza
are best known as key ingredients in the popular treat licorice, but
they also have a valuable place in the medicine cabinet. These plants
employ a complex assembly line of enzymes to produce a molecule called
glycyrrhizin, a potent sweetener that also acts as a highly effective
anti-inflammatory and antiviral agent.
The
process of glycyrrhizin biosynthesis is incompletely understood, but
research from a team led by Kazuki Saito and Toshiyuki Muranaka at the
RIKEN Plant Science Center in Yokohama helps to fill some of the gaps.
According to Saito, these efforts depended on close collaboration
between multiple research teams. Members of the ‘All-Japan Licorice
Research Consortium’, pooled their research resources, which was the
strong basis for the success of this project, according to Saito.
The
researchers were particularly interested in enzymes known as cytochrome
P450 mono-oxygenases. For a previous study, they prepared a large
library of gene sequences expressed by Glycyrrhiza
to identify previously uncharacterized P450s2. This time around, Saito
and Muranaka performed a functional assay in which they expressed
several of these putative P450s in cultured cells so they could identify
enzymes that act on specific intermediates in glycyrrhizin manufacture.
They
identified one protein, CYP72A154, which recognized the early
glycyrrhizin intermediate 11-oxo-?-amyrin as a substrate. Remarkably,
this enzyme appears to perform multiple sequential oxidation reactions
on this compound, effectively moving the synthetic process forward three
steps. To confirm these findings, they tested the function of CYP72A154
by co-expressing it alongside other enzymes known to participate in
this biological process. “We achieved biotechnological production of
glycyrrhetinic acid, an intermediate of glycyrrhizin, by means of
synthetic biology in yeast,” says Muranaka.
This
demonstration of partial glycyrrhizin biosynthesis represents an
important step in the right direction: even though this valuable
molecule is easily purified from licorice plants, scientists may
ultimately find themselves forced to resort to laboratory production
methods. “There is a potential risk of a shortage of natural resources
in the near future,” says Saito. “Another problem is that China, the
dominant supplier of licorice, is setting restrictions on licorice
exports as a governmental policy.”
Several
pieces are still missing from the puzzle, but Saito and Muranaka are
excited to learn what remains to be found, both from a biotechnology
perspective and in terms of understanding aspects of plant evolutionary
history. “We still don’t know why and how higher plants have evolved the
production systems for such interesting compounds,” says Muranaka.
Source: RIKEN
