Structure of the α-1,6/α-1,4-specific glucansucrase GTFA from Lactobacillus reuteri 121

The reuteransucrase GTFA from Lactobacillus reuteri 121, which belongs to glycosyl hydrolase family GH70, synthesizes branched alpha-glucans with both alpha-1,6-and alpha-1,4-glycosidic linkages (reuteran) from sucrose. The crystal structure of GTFA-Delta N, a 118 kDa fragment of GTFA comprising residues 745-1763 and including the catalytic domain, was determined at 3.6 angstrom resolution by molecular replacement. The crystals have large solvent channels and an unusually high solvent content of 85%. GTFA-Delta N has the same domain arrangement and domain topologies as observed in previously determined GH70 glucansucrase structures. The architecture of the GTFA-Delta N active site and binding pocket confirms that glucansucrases have a conserved substrate specificity for sucrose. However, this first crystal structure of an alpha-1,6/alpha-1,4-specific glucansucrase shows that residues from conserved sequence motif IV (1128-1136 in GTFA-Delta N) contribute to the acceptor-binding subsites and that they display differences compared with other structurally characterized glucansucrases. In particular, the structure clarifies the importance of residues following the transition-state stabilizer for product specificity, and especially residue Asn1134, which is in a position to interact with sugar units in acceptor subsite +2.