Isomerases perform biotransformations without cofactors but often cause an undesirable mixture of substrate and product due to unfavorable thermodynamic equilibria. We demonstrate the feasibility of using an engineered yeast strain harboring oxidoreductase reactions to overcome the thermodynamic limit of an isomerization reaction. Specifically, a yeast strain capable of consuming lactose intracellularly is engineered to produce tagatose from lactose through three layers of manipulations. First, GAL1 coding for galactose kinase is deleted to eliminate galactose utilization. Second, heterologous xylose reductase (XR) and galactitol dehydrogenase (GDH) are introduced into the ∆gal1 strain. Third, the expression levels of XR and GDH are adjusted to maximize tagatose production. The resulting engineered yeast produces 37.69 g/L of tagatose from lactose with a tagatose and galactose ratio of 9:1 in the reaction broth. These results suggest that in vivo oxidoreaductase reactions can be employed to replace isomerases in vitro for biotransformation.

异构酶在没有辅因子的情况下进行生物转化，但是由于不利的热力学平衡，常常导致底物和产物的不合需要的混合物。我们证明了使用经过工程改造的酵母菌株携带氧化还原酶反应来克服异构化反应的热力学极限的可行性。具体地，将能够在细胞内消耗乳糖的酵母菌株改造成通过三层操作从乳糖产生塔格糖。首先，删除编码半乳糖激酶的GAL1，以消除半乳糖的利用。其次，将异源木糖还原酶（XR）和半乳糖醇脱氢酶（GDH）引入∆gal1拉紧。第三，调节XR和GDH的表达水平以最大化塔格糖的产生。所得的工程酵母从乳糖中产生37.69 g / L的塔格糖，反应液中塔格糖和半乳糖的比例为9：1。这些结果表明，体内氧化还原酶反应可用于代替异构酶在体外进行生物转化。