2-PhenylethanolAmong the various attractant compounds available, there is one compound called 2-Phenylethanol (PEA). 2-Phenylethanol (PEA) is an aromatic alcohol with a rose-like odor and occurs in many essential oils and fermented foods. An annual production of 7000 tons is generated by chemical processes, mainly by the Friedel–Craft reaction of ethylene oxide with benzene, or by hydrogenation of styrene oxide with Raney nickel as a catalyst. It can be also biologically produced through the Ehrlich pathway, which originally exists in Saccharomyces cerevisiae. Through this pathway, ‘natural’ PEA can be obtained at high purity by an environmentally friendly process. 2-Phenylethanol can work very well as an attractant towards fruit flies, and it demonstrates higher attraction when mixed with some other compounds (Figure 1).
Figure 1. Mean catches of Drosophila melanogaster in a trap assay. This shows different chemicals have attraction towards insects. Adopted from J. Zhu, et al, 2003.
Therefore, based on the above information, we choose 2-phenylethanol as our target attractant molecule, which may also be potentially applied to attract other pests in a biological pesticide system.
Yeast Ehrlich pathwaySaccharomyces cerevisiae has been used for at least 8 millennia in the production of alcoholic beverages. Along with ethanol and carbon dioxide, fermenting cultures of this yeast produces many low-molecular-weight flavor compounds, including fusel alcohols. Fusel alcohols are derived from amino acid catabolism via an yeast endogenous pathway that was first proposed a century ago by Ehrlich.
The Ehrlich pathway contains three steps, transamination, decarbonxylation and reduction. The first step, transamination, is mainly catalyzed by Aro8p and Aro9p, which are initially characterized as the aromatic amino acid aminotransferases I and II. 2-Phenylethanol production is mainly catalyzed by ARO9p. The second step, decarbonxylation, can be catalyzed by no fewer than 5 genes that show sequence similarity with thiamine diphosphate dependent decarboxylase genes. Three of these (PDC1, PDC5 and PDC6) encode pyruvate decarboxylases, while ARO10 and THI3 represent alternative candidates for Ehrlich-pathway decarboxylases. The third step, reduction, is catalyzed by dehydrogenase to yield the final product, 2-phenylethanol.
In our project, we plan to adopt this yeast endogenous pathway to produce 2-phenylethanol from 2-phenylalanine, as well as control this pathway for our future application.
Figure 2. 2-Phenylethanol biosynthetic pathway in Saccharomyces cerevisiae