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 α‐glucans with both α‐1,6‐ and α‐1,4‐glycosidic linkages (reuteran) from sucrose. The crystal structure of GTFA‐ΔN, a 118 kDa fragment of GTFA comprising residues 745–1763 and including the catalytic domain, was determined at 3.6 Å resolution by molecular replacement. The crystals have large solvent channels and an unusually high solvent content of 85%. GTFA‐ΔN has the same domain arrangement and domain topologies as observed in previously determined GH70 glucansucrase structures. The architecture of the GTFA‐ΔN active site and binding pocket confirms that glucansucrases have a conserved substrate specificity for sucrose. However, this first crystal structure of an α‐1,6/α‐1,4‐specific glucansucrase shows that residues from conserved sequence motif IV (1128–1136 in GTFA‐ΔN) 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.