High Titer of (S)-Equol Synthesis from Daidzein in Escherichia coli

(S)-Equol is the terminal metabolite of daidzein and plays important roles in human health. However, due to anaerobic inefficiency, limited productivity in (S)-equol-producing strains often hinders (S)-equol mass production. Here, a multi-enzyme cascade system was designed to generate a higher (S)-equol titer. First, full reversibility of the (S)-equol synthesis pathway was found and a blocking reverse conversion strategy was established. As biosynthetic genes are present in the microbial genome, an effective daidzein reductase was chosen using evolutionary principles. And our analyses showed that NADPH was crucial for the pathway. In response to this, a novel NADPH pool was redesigned after analyzing a cofactor metabolism model. By adjusting synthesis pathway genes at the right expression level, the entire synthesis pathway can take place smoothly. Thus, the cascade system was optimized by regulating the gene expression intensity. Finally, after optimizing fermentation conditions, a 5 L bioreactor was used to generate a high (S)-equol production titer (3418.5 mg/L), with a conversion rate of approximately 85.9%. This study shows a feasible green process route for the production of (S)-equol.