## 1 Introduction  
Leloir glycosylItransferases (GTs) that utilize nucleotide-activated monosaccharides as donor substrates are the most common enzymes to catalyze glycosylation reactions in nature [1]. GTs are, however, not well suited for in vitro glycosylation reactions, due to the requirement of expensive nucleotide-activated glycosyl donors, as well as reported recombinant expression difficulties. Instead, retaining glycoside hydrolases (GH), with high transglycosylation activity (often referred to as transglycoses) are ideal candidates for glycosylation reactions, due to natural abundance, robustness and wide acceptor specificity [2, 3]. In transglycoses, oligo- or polysaccharide acceptors react instead of water, resulting in a new glycoside bond. Therefore, transglycoses are attractive tools for developing a variety of glycoproteins [4], including oligosaccharides and glycosconjugateswith applications in food, pharma, and other industries. Some examples with main specificities for \(\alpha\)-glucosyl groups, include cyclomaltodextrin glucanotransferases (CGTases in family GH13) [5; 6]. CGTases can also be used for the synthesis of alkyl glycosides [7; 8].  
Bacterial transglycosases are involved in various functions. Periplasmic synthesis of \(\beta\)-glucans and cyclic \(\beta\)-glucans has been reported in some species [9] that may have roles in protection against antibiotics [10], plant-microbe interaction, or osmoregulation [11]. The enzymes identified to be involved in this synthesis are modular membrane-anchored proteins (NdvB), including GH17 catalytic modules. Cloning of synthetic genes, encoding the individual GH17 catalytic domains of these proteins from _Bradyrhizobium diazoefficiens_ (previously _B. japonicum_, Glt20), _Pseudomonas putida_ (Glt3) and _P. aeruginosa_ (Glt1), have shown that these enzymes are able to hydrolyze \(\beta\)(1-3)-glucosidase bonds and transfer \(\beta\)(1-3)-glucuron oligosaccharidesarides to other \(\beta\)(1-3)- or \(\beta\)(1-6)-linkages [12; 13]. Biochemical studies using oligosaccharides have shown that Glt1 and Glt3 act from the non-reducing end, while transfer by Glt20 occurs from the reducing end (after removal of a disaccharide). The main product of the respective enzyme is also different. Glt20 produces branched 1,6-linked products [13], while Glt1 and 3 mainly form 1,3-linkages [12]. To understand the molecular reasons for these differences, structural information is needed. However, no structures of these transglycosating types of bacterial enzymes are yet known, but, recently, a crystal structure of a GH17 \(\beta\)-1,3-glucuronasyltransferase from the fungus _Rhizomucor miehei_ (termed _Rm_Bgt17A) was solved [14].  
In this work, the crystal structure of the fungal enzyme was used as the main template for building hybrid models of the structures of bacterial enzymes Glt20, Glt3 and Glt1. The models were then subjected to molecular dynamics simulations, which supported the stability of the modeled structures. Subsequent docking with different ligands allowed prediction of the amino acids potentially involved in specificity. This work provides an explanation of why these highly homologous enzymes act from either the reducing or non-reducing end of oligosaccharide substrates, based on molecular dynamics simulation and ligand docking to the modeled structure.