Glucansucrase Gtf180-[DELTA]N of Lactobacillus reuteri 180: enzyme and reaction engineering for improved glycosylation of non-carbohydrate molecules

Glucansucrases have a broad acceptor substrate specificity and receive increased attention as biocatalysts for the glycosylation of small non-carbohydrate molecules using sucrose as donor substrate. However, the main glucansucrase-catalyzed reaction results in synthesis of [alpha]-glucan polysaccharides from sucrose, and this strongly impedes the efficient glycosylation of non-carbohydrate molecules and complicates downstream processing of glucosylated products. This paper reports that suppressing [alpha]-glucan synthesis by mutational engineering of the Gtf180-[DELTA]N enzyme of Lactobacillus reuteri 180 results in the construction of more efficient glycosylation biocatalysts. Gtf180-[DELTA]N mutants (L938F, L981A, and N1029M) with an impaired [alpha]-glucan synthesis displayed a substantial increase in monoglycosylation yields for several phenolic and alcoholic compounds. Kinetic analysis revealed that these mutants possess a higher affinity for the model acceptor substrate catechol but a lower affinity for its mono-[alpha]-d-glucoside product, explaining the improved monoglycosylation yields. Analysis of the available high resolution 3D crystal structure of the Gtf180-[DELTA]N protein provided a clear understanding of how mutagenesis of residues L938, L981, and N1029 impaired [alpha]-glucan synthesis, thus yielding mutants with an improved glycosylation potential.