## 1 Introduction  
\(\alpha\)-Galactosidases (EC 3.2.1.22) are glycoside hydrolases (GHs) which catalyze the hydrolysis of terminal non-reducing \(\alpha\)-D-galactosyl units and are found across seven GH families with the majority of the characterized enzymes being part of families GH27 and GH36 [1]. \(\alpha\)-Galactosidases are able to act on a range of different substrates such as raffinose, stachyose and melibiose and polymeric substrates such as galactomannans (locust bean gum, LBG and guar gum) and galactoglucannannan [2] but the substrate preference varies considerably. In several cases, it was shown that GH27 \(\alpha\)-galactosidaseases display activity towards galactosyl units in galactomannan and other abundant \(\beta\)-mannans, but this is an unusual property among GH36 \(\alpha\)-galactosidaseases which often prefer to act on terminal galactoses in oligosaccharides [3, 4, 5]. \(\beta\)-Mannans from renewable sources, e.g., softwood hemicellulose, are valuable second-generation feedstocks that gain increased interest as biobased raw materials [6, 7]. GH27 and GH36 enzymes belong to clan GH-D and catalyze the hydrolysis of carbohydrates through a double-displacement mechanism [8]. In the reaction, theomeric configuration (in this case \(\alpha\)-configuration) of the substrate is retained in the product, and such enzymes are often referred to as \"retaining\". Both GH27 and GH36 utilize two aspartate residues as the catalytic residues, acting as nucleophile and acid/base catalysts, respectively [8]. A nucleophilic attack by the enzyme generates a leaving group and a covalent glycosyl-enzyme intermediate with the remaining part of the substrate (donor). The intermediate is then disrupted by a nucleophilic attack by a water molecule [8] or another molecule with an accessible hydroxyl group (acceptor) [9]. For a given reaction, the propensity for the glycosyl-enzyme intermediate to be disrupted by the acceptor (transglycosylation) or water (hydrolysis) is reflected by the ratio between the rate of synthesis and rate of hydrolysis (r\({}_{\text{S}}\)/r\({}_{\text{H}}\)), and such values can be used to judge the transglycosylation capacity of enzymes [10]. The ability to use the retaining GHs' transglycosylation capacity for the synthesis of defined glycosides and oligosaccharides has gained a lot of interest [9; 11; 12]. In contrast to chemical glycosynthesis, glycosylation with GHs can occur under mild, benign conditions, in a stereo- and regio-selective manner and in a single reaction without the use of protective groups [9; 13]. Furthermore, GHs do not require activated substrates such as nucleotide sugars used by glycosyltransferases [9] and are therefore attractive for the utilization of abundant polysaccharides [14]. A number of studies have been carried out to investigate the transglycosylation capacity of \(\alpha\)-galactosidases, exploring their ability to utilize different donor substrates, including 4-nitrophenyl-\(\alpha\)-D-galactopyranoside (pNP-Gal) [15; 16; 17; 18], di- and oligosaccharides [19; 20] and polymeric substrates such as guar gum galactomannan [21]. The acceptors which were explored include saccharides [19; 20], various alcohols [15; 17] and glycerol [21]. Recent studies showed the prebiotic potential of \(\alpha\)-galactosidasesylglycerol [22]. Furthermore, there is increased interest in enzymatically generated [23]\(\alpha\)-galactooligosaccharides for prebiotic applications [23; 24]. One of the challenges when using the transglycosylation approach is the risk of secondary hydrolysis, i.e., when the synthesized product is hydrolyzed. The propensity for this is dependant on the used enzyme and an important factor to consider when looking for promising enzymes [10]. We recently showed that a synergy approach greatly benefits transglycosylation reactions with polymeric donor substrates.