Engineering, Synthesis

High-Yielding Semi-Synthesis of an Artemisinin Precursor

“Production of amorphadiene in yeast, and its conversion to dihydroartemisinic acid, precursor to the antimalarial agent artemisinin” Westfall, P.J.; Pitera, D.J.; Lenihan, J.R.; Eng, D.; Woolard, F.X.; Regentin, R.; Horning, T.; Tsuruta, H.; Melis, D.J.; Owens, A.; Fickes, S.; Diola, D.; Benjamin, K.R.; Keasling, J.D.; Leavell, M.D.; McPhee, D.J.; Renninger, N.S.; Newman, J.D.; Paddon, C.J. Proc. Natl. Acad. Sci. U.S.A. 2012109, E111-E118. DOI: 10.1073/pnas.1110740109.

Malaria, caused mainly by the parasite Plasmodium falciparum, leads to nearly a million deaths and 250 million new infections each year. The sesquiterpene lactone endoperoxide artemisinin, derived from Artemisia annua, is very effective as an antimalarial drug, and widespread resistance hasn’t yet developed. Artemisinin is the only high-volume drug that is still isolated by extraction from its native plant producer in a low-yielding (around 10 μg per g plant material), resource-intensive process that uses volatile solvents (most commonly hexane).

Artemisia annua. Photo credit: Jorge Ferreira via Wikimedia Commons.

As a result, supplies of the drug are short, and those who need it often can’t afford it. The development of new processes for artemisinin production would therefore advance both public health and green chemistry interests. Total synthesis of the drug hasn’t been considered as a viable alternative because of low yields, but a lot of effort has been directed toward developing semi-synthetic sources of artemisinin using a combination of microbial fermentation and chemical synthesis. Toward this end, the Keasling lab reported a few years ago that they had constructed a biosynthetic pathway for the artemisinin precursor amorpha-4,11-diene in yeast with yields of ~200 mg/L—already impressive given the complexity of the molecule. Amorphadiene synthase (ADS) comes from Artemisia annua; the rest of the genes are from yeast. Here is the existing pathway:

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