As essential structural molecules for plant plasma membranes, phytosterols are key intermediates for the synthesis of many downstream specialized metabolites of pharmaceutical or agricultural significance, such as brassinosteroids and withanolides. Saccharomyces cerevisiae has been widely used as an alternative producer for plant secondary metabolites. Establishment of heterologous sterol pathways in yeast, however, has been challenging due to either low efficiency or structural diversity, likely a result of crosstalk between the heterologous phytosterol and the endogenous ergosterol biosynthesis. For example, in this study, we engineered campesterol production in yeast using plant enzymes; although we were able to enhance the titer of campesterol to ∼40 mg/L by upregulating the mevalonate pathway, no conversion to downstream products was detected upon the introduction of downstream plant enzymes. Further investigations uncovered two interesting observations about sterol engineering in yeast. First, many heterologous sterols tend to be efficiently and intensively esterified in yeast, which drastically impedes the function of downstream enzymes. Second, yeast can overcome the growth deficiency caused by altered sterol metabolism through repeated culture. By employing metabolic engineering, strain evolution, fermentation engineering, and pathway reconstitution, we were able to reconstruct the multienzyme pathways for the synthesis of a set of phytosterols: campesterol (∼7 mg/L), β-sitosterol (∼2 mg/L), 22-hydroxycampesterol (∼1 mg/L), and 22-hydroxycampest-4-en-3-one (∼4 mg/L). This work identified and addressed some of the technical bottlenecks in phytosterol-derived pathway reconstitution in the baker's yeast and opens up opportunities for efficient bioproduction and metabolic pathway elucidation of this group of phytochemicals.
Keywords: brassinosteroids; metabolic engineering; phytosterols; yeast.