Productive Amyrin Synthases for Efficient α‑Amyrin Synthesis in Engineered <i>Saccharomyces cerevisiae</i>

α-Amyrin is a plant-derived pentacyclic triterpenoid, with a lot of important physiological and pharmacological activities. The formation of α-amyrin from (3S)-2,3-oxidosqualene is catalyzed by α-amyrin synthase (α-AS), a member of the oxidosqualene cyclase (OSC) protein family. However, α-amyrin is not yet commercially developed due to its extremely low productivity in plants. The engineered Saccharomyces cerevisiae with efficient α-amyrin production pathway could be used as an alternative and sustainable solution to produce α-amyrin from renewable raw materials. To efficiently improve α-amyrin production in S. cerevisiae, we identified two α-ASs, EjAS and MdOSC1 from Eriobotrya japonica and Malus × domestica, respectively, through strict bioinformatics screening criteria and phylogenetic analysis. The specific activities of purified EjAS and MdOSC1 were 0.0032 and 0.0293 μmol/min/mg, respectively. EjAS produced α-amyrin and β-amyrin at a ratio of 17:3, MdOSC1 produced α-amyrin, β-amyrin and lupeol at a ratio of 86:13:1, indicating MdOSC1 had significantly higher specific activity and higher ratio of α-amyrin than EjAS. Furthermore, MdOSC1 was introduced into S. cerevisiae combining with the increased supply of (3S)-2,3-oxidosqualene to achieve the encouraging α-amyrin production, and the titer of α-amyrin achieved 11.97 ± 0.61 mg/L, 5.8 folds of the maximum production reported