Description
In nature, 2-PE is mainly extracted from the essential oil of flowers and plant tissues, such as rose, jasmine, tomato, and buckwheat (Wang et al., 2019). However, the extraction process is very complicated and costly, because the 2-PE concentration in these plants is very low (Wang et al., 2018a). Besides, the harvest of flowers is greatly influenced by the weather, plant diseases, and trade restrictions. In 2008, the price of Bulgarian rose oil has climbed to 4600 Euro per kilogram. All these lead to the short supply and exorbitant cost of natural 2-PE (Qian et al., 2018).
Currently, the global market output of 2-PE is in excess of 10,000 tons annually, most of which is chemically synthesized through ethylene oxidation of benzene or reduction of styrene oxide (Wang et al., 2018b). Chemical synthesis processes are generally operated under harsh conditions, such as high temperature, high pressure, and strong acid or alkali environments, causing many undesirable by-products, such as ethylbenzene and styrene, which not only increase the downstream costs, but also seriously debase the grade of 2-PE.
Fig. 1 Grow roses for extracting 2-PE
Many wild-type strains have been isolated and characterized with a capacity for 2-PE production, most of which are eukaryotes, including Saccharomyces sp., Kluyveromyces marxianus, Yarrowia lipolytica, Aspergillus oryzae, A. niger, Pichia sp., Zygosaccharomyces rouxii, etc (Wang et al., 2019). Moreover, biologically produced 2-PE is recognized as “natural” according to the US Food and Drug Administration and similar European legislations, if the substrate used for the production process is natural.
Fig. 2 Industrial biomanufacturing for fuel and chemical production (Clomburg et al., 2017)
Y. lipolytica is emerging as the model non-conventional oleaginous yeast. As an organism with “generally regarded as safe” (GRAS) status, it has been widely recognized as a potential industrial workhorse for the production of lipid-based biofuels and oleochemicals (Abdel-Mawgoud et al., 2018; Markham & Alper, 2018). In particular, Y. lipolytica is well suited for industrial production of oleochemicals as wild type strains can accumulate lipids up to 70% of dry biomass. Key metabolic traits that contribute to this oleaginous phenotype include high acetyl-CoA flux, high tricarboxylic acid (TCA) cycle flux, and lack of fermentative capacity (Lazar et al., 2018). Moreover, Y. lipolytica has the ability to utilize diverse protein and hydrophobic substrates, which may be provided as cheap renewable carbon sources, and it grows at a wide range of pH and salinity conditions. With these unique metabolic traits and recently developed metabolic engineering tools, this industrial host shows great promise for economic and renewable production of a plethora of new products in the future.
Fig. 3 Macroscopic appearance of Y. lipolytica colonies (Abdel-Mawgoud et al., 2018) Colonies of Y. lipolytica Po1f strain grown on (A) yeast-peptone-glucose (YPD) solid medium at 30 °C for 4 days, showing the convoluted white surface, (B) YPD at 30 °C for 2 days, showing the pale matte surface (C) YPD at 30 °C for 2 days then kept at 4 °C for 1 week, showing the smooth surface.
In Y. lipolytica, production of 2-PE is mainly obtained by two routes, namely, the de novo pathway from glucose and bioconversion from L-phenylalanine by the Ehrlich pathway(Qian et al., 2018). Owing to the hard-wired, tightly complex feedback regulation33 and lengthy reaction steps (>20 steps) of the de novo pathway, bioconversion by the Ehrlich pathway is considered as the preferred biological route to synthesize 2-PE. .
Fig. 4 The synthesis pathway of 2-PE in Y. lipolytica (Wang et al., 2019) L-phe, L-phenylalanine; PPY, phenylpyruvate; PAH, phenylacetaldehyde; 2-PE, 2-phenylethanol; ylARO8, the transaminase; ylARO9, the transaminase; ylARO10, the phenylpyruvate decarboxylase.
In the Ehrlich pathway (Fig. 2), L-phenylalanine is converted to 2-PE through three enzymatic dependent processes: (i) L-phenylalanine is transaminated to phenylpyruvate by amino transferase with 2-oxoglutarate (aKG) as the amine-receptor; (ii) phenylpyruvate is further decarboxylated to phenylacetaldehyde by phenylpyruvate decarboxylases; (iii) and finally, phenylacetaldehyde is reduced to 2-PE by phenylacetaldehyde alcohol dehydrogenases with NADH as cofactor. However, in Y. lipolytica, transaminases and the phenylpyruvate decarboxylase are ylARO8 (encoded by gene YALI0E20977g) or ylARO9 (encoded by gene YALI0C05258g) and ylARO10 (encoded by gene YALI0D06930g), respectively. However, the specific phenylacetaldehyde alcohol dehydrogenase has not been characterized. Therefore, we plan to identify the specific phenylacetaldehyde alcohol dehydrogenases in Y. lipolytica, and further improve the performances of 2-PE production produced by Y. lipolytica.
Abdel-Mawgoud, A.M., Markham, K.A., Palmer, C.M., Liu, N., Stephanopoulos, G., Alper, H.S. 2018. Metabolic engineering in the host Yarrowia lipolytica. Metab Eng, 50, 192-208.
Clomburg, J.M., Crumbley, A.M., Gonzalez, R. 2017. Industrial biomanufacturing: The future of chemical production. Science, 355(6320), 11.
Lazar, Z., Liu, N., Stephanopoulos, G. 2018. Holistic Approaches in Lipid Production by Yarrowia lipolytica. Trends Biotechnol, 36(11), 1157-1170.
Markham, K.A., Alper, H.S. 2018. Synthetic Biology Expands the Industrial Potential of Yarrowia lipolytica. Trends Biotechnol, 36(10), 1085-1095.
Qian, X., Yan, W., Zhang, W., Dong, W., Ma, J., Ochsenreither, K., Jiang, M., Xin, F. 2018. Current status and perspectives of 2-phenylethanol production through biological processes. Crit Rev Biotechnol, 1-14.
Wang, J., Shen, X., Rey, J., Yuan, Q., Yan, Y. 2018a. Recent advances in microbial production of aromatic natural products and their derivatives. Appl Microbiol Biotechnol, 102(1), 47-61.
Wang, Y., Zhang, H., Lu, X., Zong, H., Zhuge, B. 2019. Advances in 2-phenylethanol production from engineered microorganisms. Biotechnol Adv, 37(3), 403-409.
Wang, Z., Jiang, M., Guo, X., Liu, Z., He, X. 2018b. Reconstruction of metabolic module with improved promoter strength increases the productivity of 2-phenylethanol in Saccharomyces cerevisiae. Microb Cell Fact, 17(1), 60.