Amylase 2018; 2: 17–29 Review Article Mary Casa-Villegas, Julia Marín-Navarro, Julio Polaina* Amylases and related glycoside hydrolases with transglycosylation activity used for the production of isomaltooligosaccharides https://doi.org/10.1515/amylase-2018-0003 Abbreviations: AIG, α-glucosidase from Acremonium Received March 26, 2018; accepted May 20, 2018 implicatum; ANG, α-glucosidase from Aspergillus niger; Abstract: Isomaltooligosaccharides (IMOS) are sugars AOG, α-glucosidase from Aspergillus oryzae; BcCITase, with health promoting properties that make them relevant CITase from Bacillus circulans T-3040; CBM, carbohydrate- for the pharmaceutical and food industries. IMOS have binding module; CI, cyclic isomaltooligosaccharide; ample chemical diversity achieved by different α-glucosidic CITase, cycloisomaltooligosaccharide glucanotransferase; linkages and polymerization degrees, forming linear, CMM, cyclic maltosylmaltose; CTS, cyclic tetrasaccharide; branched and cyclic structures. Enzymatic synthesis of DP, degree of polymerization; FOS, fructooligosaccharides; these compounds can be carried out by glycoside hydrolases GH, glycoside hydrolases; GOS, galactooligosaccharides; (GHs) with transglycosylating activity. Different substrates IMOS, isomaltooligosaccharides; OPMA-N, maltogenic are used for the synthesis: combinations of disaccharides amylase from Bacillus sp.; PsCITase, CITase from and monosaccharides, or polymeric carbohydrates such Paenibacillus sp. 598K; SI, sucrose isomerase; SmuA, as starch or dextran, which are converted to IMOS by a sucrose isomerase from Protaminobacter rubrum; SOG, combination of hydrolysis and transglucosylation. In this α-glucosidase from Schwanniomyces occidentalis; ThMA, review, the structural features of different enzyme families maltogenic amylase from Thermus sp.; XDG, α-glucosidase (GH31, GH13, GH70, GH57 and GH66) involved in IMOS from Xanthophyllomyces dendrorhous. synthesis are analysed. Focus is placed on structural traits that affect substrate and product specificity, and on the relative efficiency of transglucosylation and hydrolysis. 1 Introduction Information resulting from site-directed mutagenesis and Isomaltooligosaccharides (IMOS) are important sequence alignments complements structural data to compounds due to their health-associated properties understand the role of specific residues in the performance and potential industrial applications. IMOS could be of the enzymes. Altogether, these studies provide a frame of used as prebiotics, low calorie sweeteners or cariostatic knowledge which may be used to design new enzymes with compounds [1-4]. From a strict chemical point of view, improved properties. IMOS are linear short molecules of d-glucose units linked by α-1,6-linkages. However, in a wider definition Keywords: Glycoside transferases; maltose; prebiotics; they are also accepted as IMOS branched and cyclic starch. glucooligosaccharides with glucose units bound by α-1,6-, α-1,3- or α-1,2-linkages even in combination with α-1,4- linkages. IMOS structure and properties depend on their *Corresponding author: Julio Polaina, Instituto de Agroquímica degree of polymerization (DP) (usually between 2 and y Tecnología de los Alimentos (IATA-CSIC), Catedrático Agustín 10 glucose units), linkage types (α-1,2, 3, 4 or 6) and the Escardino 7, Paterna (Valencia) E-46980, Spain; E-mail: jpolaina@ proportion and position of each type of linkage [5-7]. IMOS iata.csic.es prebiotic properties are due to the fact that glucosidic Mary Casa-Villegas, Julia Marín-Navarro, Instituto de Agroquímica y Tecnología de los Alimentos (IATA-CSIC), Catedrático Agustín Escar- linkages, other than α-1,4, not easily hydrolysed by dino 7, Paterna (Valencia) E-46980, Spain intestinal enzymes, are suitable substrates for enzymes Julia Marín-Navarro, Department of Biochemistry and Molecular of beneficial species of the intestinal microbiota, such as Biology, Faculty of Biology, University of Valencia, Dr. Moliner 50, lactobacilli and bifidobacteria. Polymerization degree and Burjassot (Valencia) E-46100, Spain the ratio between α-1,4 and other linkages are important Open Access. © 2018 Mary Casa-Villegas et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution- NonCommercial-NoDerivs 4.0 License. 18 M. Casa-Villegas, et al. aspects in IMOS digestibility and metabolism by intestinal 2 Glycoside hydrolases used for microbiota [2,3,8]. The list of linear and branched IMOS described IMOS production in the literature includes isomaltose, isomaltotriose, Industrial production of IMOS frequently uses starch isomaltotetraose, panose, isopanose, linear panose series, as the substrate and the combined action of different glucose-maltotriose, nigerose, nigerotriose, kojibose, glycoside hydrolases (GH) enzymes with amylolytic, centose, and isomalto/maltooligosaccharides [6-22]. branching or transferase activity, yielding a complex An important group of cyclic isomaltooligosaccharides mixture of α-glucosides [32-34]. An alternative process (CIs) are those formed by glucose units bonded only to obtaining linear or branched IMOS with defined by α-1,6-linkages, also called cyclodextrans. The DP of composition is enzymatic synthesis, using maltose or cyclodextrans ranges from 7 to 12 [4,23-27]. In addition sucrose as substrates and the catalytic action of GH to cyclodextrans, there are CIs with both α-1,6 and α-1,4- enzymes with transferase activity [6,8,9,11,12,14,16,20]. In linkages, like the cyclic maltosylmaltose (CMM) [28,29], the case of CIs, most of them are produced from dextran or with both α-1,6 and α-1,3-linkages, such as the cyclic with the concourse of cycloisomaltooligosaccharide tetrasaccharide (CTS) cyclo{-6)-α-d-Glcp-1,3-α-d-Glcp- glucanotransferases (CITases) [23,24]. 1,6-α-d-Glcp-1,3-α-d-Glcp-(1-} and its branched variants It should be pointed out that the transferases used for 4-O-α-d-glucopyranosyl-CTS and 3-O-α-isomaltosyl-CTS IMOS synthesis are not “glycosyl transferases” labelled [30,31]. A schematic representation of the different types as “GT” enzymes in the CAZy classification [35], but GH of IMOS is shown in Figure 1. Figure 1. Chemical diversity of IMOS and related oligosaccharides. Enzymatic synthesis of isomaltooligosaccharides 19 enzymes, in which the transferase activity is high or even (family GH31), mutant sucrose isomerase (subfamily predominant. Whereas glycosyl transferases are enzymes GH13_31) and dextransucrases (GH70). Alternatively, that require the use of nucleotide or phosphate-activated maltogenic amylase (GH13_20) and branching enzymes sugars for their function, IMOS synthesis is carried out by (GH57) have been used to produce linear IMOS from non-Leloir transferases that belong phylogenetically to polysaccharides, such as starch by the combined action GH enzyme families. These enzymes are characterized by of their hydrolytic and transferase activities. On the other a retaining reaction mechanism. From a structural point hand, CIs are produced by CITases (GH66) using dextran of view, in this kind of enzymes the distance between the as the substrate. In this work, structural and functional two carboxylic catalytic residues (either aspartic acid or aspects of relevant enzymes from these GH families, able glutamic acid) is around 5 Å [36]. Hydrolysis occurs in to synthesize IMOS as a result of a dual hydrolytic and two steps. In the first one (glycosylation), the acid/base transglycosylating activity, are described. residue (acting as acid) protonates the oxygen of the glycosidic linkage, while the nucleophile residue breaks the linkage by acting on the anomeric carbon and forms 3 Family GH31 a covalent enzyme-glycoside intermediate. In the second Some GH31 α-glucosidases (EC 3.2.1.20) with step (deglycosylation), a water molecule, deprotonated transglycosylation activity can be used for IMOS synthesis. by the acid/base residue (acting as base), hydrolyses From a structural point of view, a representative member the covalent enzyme-glycoside intermediate and the of this family is sugar beet α-glucosidase [44]. This glycosyl moiety is released keeping the configuration of enzyme (Fig. 2A) is composed by four protein domains: the original substrate [37,38]. Transglycosylation, as an an N-terminal β-sandwich, a (β/α) -barrel (i.e. TIM-barrel) alternative to hydrolysis, occurs when instead of water, 8 catalytic domain, a proximal C-terminal domain and a other molecule with an OH functional group, such as a distal C-terminal domain, both with a β-fold. The pocket- sugar, acts as a nucleophile on the covalent intermediate. shaped active site is formed mainly by loops linking the In this case, depending on the acceptor, new sugar β-strands to the α-helices of the catalytic domain and also molecules, with variable polymerization degree and/or by a long loop that bulges from the N-terminal domain new glycosidic linkages, can be synthesized. Hydrolysis called N-loop [20]. and transglycosylation reactions are performed by GH31 enzymes use maltose as the substrate to the same enzyme. The competition between the two produce IMOS with different chemical constitutions. mechanisms basically depends on the enzyme’s nature The α-glucosidases from Aspergillus niger (ANG), and the concentration of potential substrates in the Aspergillus oryzae (AOG) and Schwanniomyces reaction environment. The catalytic pocket of the enzyme occidentalis (SOG) synthesize isomaltose and panose harbours different binding subsites for each of the glycosyl as the main transglycosylation