L-Tagatose, a promising building block in the production of many value-added chemicals, is generally produced by chemical routes with a low yield, which may not meet the increasing demands. Synthesis of l -tagatose by enzymatic oxidation of d -galactitol has not been applied on an industrial scale because of the high cofactor costs and the lack of efficient cofactor regeneration methods. In this work, an efficient and environmentally friendly enzymatic method containing a galactitol dehydrogenase for d -galactitol oxidation and a water-forming NADH oxidase for regeneration of NAD + was first designed and used for l -tagatose production. Supplied with only 3 mM NAD + , subsequent reaction optimization facilitated the efficient transformation of 100 mM of d -galactitol into l -tagatose with a yield of 90.2 % after 12 h (obtained productivity: 7.61 mM.h −1 ). Compared with the current chemical and biocatalytic methods, the strategy developed avoids by-product formation and achieves the highest yield of l -tagatose with low costs. It is expected to become a cleaner and more promising route for industrial biosynthesis of l -tagatose.
Li Xue-Yong,Xu Meng-Qiu,Liu Hui,Zhou Qiang,Gao Jian,Zhang Ye-Wang
The combined cross-linked enzyme aggregates (combi-CLEAs) containing galactitol dehydrogenase (Gdh) and NADH oxidase (Nox) were prepared for l -tagatose synthesis. To prevent the excess consumption of cofactor, Nox in the combi-CLEAs was used to in situ regenerate NAD + . In the immobilization process, ammonia sulfate and glutaraldehyde were used as the precipitant and cross-linking reagent, respectively. The preparation conditions were optimized as follows: 60% ammonium sulfate, 1:1 (molar ratio) of Gdh to Nox, 20:1 (molar ratio) of protein to glutaraldehyde, and 6 h of cross-linking time at 35 °C. Under these conditions, the activity of the combi-CLEAs was 210 U g −1 . The combi-CLEAs exhibited higher thermostability and preserved 51.5% of the original activity after eight cycles of reuses at 45 °C. The combi-CLEAs were utilized for the preparation of l -tagatose without by-products. Therefore, the combi-CLEAs have the industrial potential for the bioconversion of galactitol to l -tagatose. Graphical abstract
危险品化学品经营许可证（带存储）