Abstract:

Turanose is a reductive disaccharide which is made from glucose and fructose through the formation of α-1,3 glycosidic bond, and has a potential to replace sucrose as a new functional sweetener with a broad application in food industry. Turanose can also be produced from sucrose with high yield through the isomerization (intramolecular transglycosidation) reaction catalyzed by amylosucrases. However, by-products, maltooligosaccharide and trehalulose, are easily produced in this process. In order to solve this problem, based on previously identified sucrose hydrolase mutant S271A from Caulobacter crescentus for the isomerization reaction with high turanose yield but without the formation of the by-product maltooligosaccharide, we further developed the mutant S271A/I382Q with improved reaction specificity and increased yield of turanose through the molecular modification of the acceptor subsite. Furthermore, conditions of the enzymatic conversion were optimized. When the reaction was performed under its optimal conditions: 2 mol/L sucrose as substrate with 40 U/mL enzyme dosage under pH 5.0 and 30 ℃, the yield of turanose could reach up to 70.3% and its final concentration was 480 g/L. Most significantly, no the by-product trehalulose was detected. Molecular dynamics simulations show that the mutant S271A/I382Q could stabilize the conformation of α-1,3 glycosidic bond formed by the acceptor fructose through hydrogen bond interactions, making it more conducive to the formation of turanose. This study innovates the transformation of sucrose hydrolase into transglucosidase with high reaction specificity, and the production yield of turanose is the highest reported at present, which lays a theoretical and technical foundation for the large-scale production and application of turanose.

Key words: turanose, sucrose, trehalulose, sucrose hydrolase, mutant, enzymatic conversion

摘要：

松二糖是由葡萄糖与果糖以α-1，3糖苷键连接而成的还原性二糖，具有代替蔗糖成为新型功能性甜味剂的潜力，在食品工业中应用前景广阔。淀粉蔗糖酶能够以蔗糖为底物催化异构（分子内转苷）反应制备松二糖，产率高但易产生副产物麦芽寡糖和海藻酮糖。为解决这一问题，选用前期获得的松二糖产率高并且不产副产物麦芽寡糖的Caulobactercrescentus蔗糖水解酶突变体S271A为研究对象，进一步通过受体亚位点分子改造，获得了反应特异性和松二糖产率提升的突变体S271A/I382Q。在此基础上进行了酶转化条件优化，当以2 mol/L蔗糖溶液为底物，加酶量为40 U/mL，在pH 5.0、30 ℃的条件下，松二糖的产率达到最高为70.3%，松二糖的浓度为480 g/L，并且反应产物中不含副产物海藻酮糖。分子动力学模拟表明，突变体S271A/I382Q可通过氢键相互作用稳定受体果糖参与形成α-1，3糖苷键时的构象，从而更有利于生成松二糖。本研究创新性地将蔗糖水解酶改造为键型特异性强的转苷酶，获得的松二糖产率为目前报道的最高水平，为松二糖的规模化制备与应用奠定了理论和技术基础。

关键词: 松二糖, 蔗糖, 海藻酮糖, 蔗糖水解酶, 突变体, 酶转化