Improving squalene production by enhancing the NADPH/NADP(+) ratio, modifying the isoprenoid-feeding module and blocking the menaquinone pathway in Escherichia coli

BACKGROUND: Squalene is currently used widely in the food, cosmetics, and medicine industries. It could also replace petroleum as a raw material for fuels. Microbial fermentation processes for squalene production have been emerging over recent years. In this study, to study the squalene-producing potential of Escherichia coli (E. coli), we employed several increasing strategies for systematic metabolic engineering. These include the expression of human truncated squalene synthase, the overexpression of rate-limiting enzymes in isoprenoid pathway, the modification of isoprenoid-feeding module and the blocking of menaquinone pathway. RESULTS: Herein, human truncated squalene synthase was engineered in Escherichia coli to create a squalene-producing bacterial strain. To increase squalene yield, we employed several metabolic engineering strategies. A fivefold squalene titer increase was achieved by expressing rate-limiting enzymes (IDI, DXS, and FPS) involved in the isoprenoid pathway. Pyridine nucleotide transhydrogenase (UdhA) was then expressed to improve the cellular NADPH/NADP(+) ratio, resulting in a 59% increase in squalene titer. The Embden–Meyerhof pathway (EMP) was replaced with the Entner–Doudoroff pathway (EDP) and pentose phosphate pathway (PPP) to feed the isoprenoid pathway, along with the overexpression of zwf and pgl genes which encode rate-limiting enzymes in the EDP and PPP, leading to a 104% squalene content increase. Based on the blocking of menaquinone pathway, a further 17.7% increase in squalene content was achieved. Squalene content reached a final 28.5 mg/g DCW and 52.1 mg/L. CONCLUSIONS: This study provided novel strategies for improving squalene yield and demonstrated the potential of producing squalene by E. coli. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s13068-019-1415-x) contains supplementary material, which is available to authorized users.