亚位点+1处突变提高软化类芽胞杆菌环糊精糖基转移酶底物麦芽糊精特异性

Improving maltodextrin specificity by site-saturation engineering of subsite +1 in cyclodextrin glycosyltransferase from Paenibacillus macerans

摘要:

通过改造来源于软化类芽胞杆菌Paenibacillus macerans的环糊精糖基转移酶 (Cyclodextrin glycosyltransferase, CGT酶) 的+1亚位点提高其对麦芽糊精的底物特异性，并进一步提高以麦芽糊精为糖基供体催化合成2-O-D-吡喃葡糖基-L-抗坏血酸 (AA-2G) 的效率。首先对+1亚位点附近的3个氨基酸残基Leu194、Ala230和His233分别进行定点饱和突变，得到3个优势突变体L194N (亮氨酸→天冬酰胺)，A230D (丙氨酸→天冬氨酸)，H233E (组氨酸→谷氨酸)，然后以这3个优势突变体为模板进一步进行两点和三点复合突变，获得7个复合突变体。研究结果表明，突变体L194N/A230D/H233E以麦芽糊精为底物合成AA-2G的产量最高，达到1.95 g/L，比野生型CGT酶提高了62.5%。对获得的突变体进行动力学分析，发现高浓度的底物L-AA对突变型CGT酶催化的酶促反应具有抑制作用。确定了突变体酶促反应的最适温度、pH和反应时间。模拟突变体的三维结构并进行分析，突变体底物特异性的改善可能与CGT酶第194位、230位和233位的氨基酸残基的亲水性及与底物分子间的作用力的改变有关。

Abstract:

By engineering the subsite +1 of cyclodextrin glycosyltransferase (CGTase) from Paenibacillus macerans, we improved its maltodextrin specificity for 2-O-D-glucopyranosyl-L-ascorbic acid (AA-2G) synthesis. Specifically, we conducted site-saturation mutagenesis on Leu194, Ala230, and His233 in subsite +1 separately and gained 3 mutants L194N (leucine →asparagine), A230D (alanine →aspartic acid), and H233E (histidine → glutamic acid) produced higher AA-2G yield than the wild-type and the other mutant CGTases. Therefore, the 3 mutants L194N, A230D, and H233E were further used to construct the double and triple mutations. Among the 7 obtained combinational mutants, the triple mutant L194N/A230D/H233E produced the highest AA-2G titer of 1.95 g/L, which was increased by 62.5% compared with that produced by the wild-type CGTase. Then, we modeled the reaction kinetics of all the mutants and found a substrate inhibition by high titer of L-AA for the mutants. The optimal temperature, pH, and reaction time of all the mutants were also determined. The structure modeling indicated that the enhanced maltodextrin specificity may be related with the changes of hydrogen bonding interactions between the side chain of residue at the three positions (194, 230 and 233) and the substrate sugars.