Valuable Saponins production by new Enzymatic Method

The discovery of a critical link in the complex biochemical pathway for saponin synthesis has been documented by scientists from Osaka University in collaboration with the National Agriculture and Food Research Organization (NARO), RIKEN, and Chiba University.

The researchers believe that their findings will help pave the way for improving the commercial development of these high-value, anticarcinogenic, antioxidant, antiviral and other pharmacological items.

These results were published under titled  "A cellulose synthase-derived enzyme catalyzes 3-O-glucuronosylation in saponin biosynthesis"

Triterpenoid saponins are specialised metabolites that consist of one or more sugar moieties attached to triterpenoid aglycones, widely distributed in the plant kingdom. The UGT that catalyses the transfer of the preserved glucuronic acid moiety at the C-3 position of glycyrrhizin and various soyasaponins has not been determined, despite the generally accepted view that glycosylation is catalysed by UDP-dependent glycosyltransferase (UGT), the investigators write.

Here, we report that 3-O-glucuronosylation of triterpenoid aglycones is catalysed by cellulose synthase superfamily-derived glycosyltransferase (CSyGT). Analyses of gene co-expression of three legume species (Glycyrrhiza uralensis, Glycine max, and Lotus japonicus) show the involvement of CSyGTs in saponin biosynthesis, and we use Saccharomyces cerevisiae to characterise CSyGTs in vivo.

"The CSyGT mutants in L. Japonicus does not accumulate soyasaponin, but the endoplasmic reticulum membrane-localized CSyGTs in an L. ectopic expression. Japonicus mutants successfully supplement the biosynthesis of soyasaponin. Finally, we developed glycyrrhizin de novo in yeast, paving the way for high-value saponins to be produced sustainably.

Using gene cloning and sequence comparisons, the researchers researched the co-expression gene network of saponin synthesis combined with biochemical studies in mutants and genetically modified plants of a model legume genus. In the CSyGT family, they discovered a new enzyme which is similar in structure to the cellulose-producing enzymes in plant cell walls.

They suddenly demonstrated that a crucial step in saponin synthesis, where a sugar molecule is added to the triterpenoid backbone, was the responsibility of the new member of the family. According to the researchers, this discovery challenged the widely accepted view that a different class of enzyme was likely involved in this step.

They went on to introduce into yeast cells the gene for the newly discovered CSyGT enzyme, along with genes for other steps in the biochemical pathway. Glycyrrhizin from simple sugars was successfully developed by the engineered cells, suggesting a possible route for the industrial production of saponins by growing yeast cells on a large scale.

For the first time, this multi-disciplinary team has shown that this form of enzyme is essential in saponin synthesis, said by corresponding author Toshiya Muranaka, PhD, at the Department of Biotechnology, Osaka University Graduate School of Engineering. "Our findings fill a gap in previous knowledge and also challenge the accepted perspective of how this biosynthesis pathway operates".

They have shown that when we insert the necessary plant genes, yeast cells can make glycyrrhizin, explains Soo Yeon Chung, also in the biotechnology department in Osaka. This provides the prospect of new ways of commercially processing these useful substances and of producing entirely new forms of saponin that could have more beneficial applications in the medicine or food industries.

Team members add that the production of chemicals in cell cultures will also decrease the need for natural plant resources to be exhausted and help achieve the goals of sustainable development.