ABSTRACT
The GTFR glucansucrase from S. orali. GTFR synthesizes a glucose polymer that is mainly linked by α-(1→6)-bonds and minor amounts of α-(1→3) bonds. Changes in the most conserved motif around the transition state stabilizer result in variants with different glycosidic linkages ratios. Variant R624G/V630I/D717A shows a drastic switch in regioselectivity from dextran to a mutan-type polymer with predominantly α-(1→3) glucosidic linkages. Additionally, mutations at residue S628 switch the reaction towards the synthesis of short-chain oligosaccharides. With glucose as acceptor S628D mutant dramatically increases the synthesis of the prebiotic isomaltose from 1% to 47% yield. When incubated with fructose as acceptor S628D variant produces leucrose in 64% yield (Hellmuth et al., 2008). Changes made in the catalytic domain of DSR-S vardel Δ4N
dextransucrase, specifically at the peptide sequence DYVHT, which together with S512 shapes the subsite +2 suggest that this region is involved in the promotion of α-(1→3) linkage formation (Irague et al., 2013). Gaining insights into GTFs regiospecificity has remained one longstanding goal for decades; as such knowledge would be useful in the design of tailor-made saccharides with different structural and functional properties. Variation in the degree of polymerization and type of branching of glucans also results in molecules with different characteristics. Due to their broad range of acceptors, GTF are also used to produce novel glycostructures, for example, molecules with antioxidative properties (Nakahara et al., 1995; Meulenbeld et al., 1999; Bertrand et al., 2006).
