Production of the saponin-based adjuvant QS-21 in engineered yeast

• Applicant: Dr. Lukas Ernst
• Subject Area: Biochemistry ; Analytical Chemistry ; Pharmacy
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 565645208

Adjuvants are crucial components of modern vaccines with the ability to enhance the immunogenicity of co-administered antigens by provoking a strong immune response. Due to its efficacy and safety profile, the triterpene saponin fraction QS-21 from the Chilean soapbark tree (Quillaja saponaria) has emerged as a coveted immunostimulant used in vaccines against shingles, malaria, and COVID-19. Despite the low overall yield and a tedious extraction process, the supply of QS-21 is currently derived from bark extracts of Q. saponaria, which has raised concerns about the sustainability of the natural plant source. Biotechnology offers a powerful way to produce such complex natural products with multiple stereocenters, which are difficult to synthesize chemically. In a recent study, the host laboratory of the proposed project succeeded in reconstructing the complex biosynthetic pathway of QS-21 in engineered yeast, but the final product yield ultimately did not exceed the titer of the traditional bark extract. The aim of this project is to improve the efficiency of this microbial production platform by resolving major bottlenecks in the biosynthetic process. The first objective will be to improve the biosynthesis of the fully oxidized triterpene intermediate, quillaic acid, by screening cytochrome P450 mono-oxigenases and alcohol/aldehyde dehydrogenases as alternative catalysts. To further enhance P450 activity, various aspects of their catalytic microenvironment, such as redox cofactor supply, reactive oxygen species accumulation, and membrane lipid composition, will be targeted by engineering approaches. As the second objective, unresolved problems with the soluble expression of enzymes in the subsequent glycosylation cascade will be addressed by extensive protein engineering, either through methods of rational protein design or directed evolution. As the final objective, efforts will be made to reconstruct the biosynthesis of several natural and unnatural QS-21 variants that have been described to possess promising adjuvant activities. Due to the simplified structure of these analogs, it is likely that they can be produced in larger quantities than QS-21.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Dr. Jay D. Keasling