Transglycosylases

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• Author: Spencer Williams
• Responsible Curator: Spencer Williams

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Overview

Transglycosylases (also transglycosidases) are a class of GH enzymes that can catalyze the transformation of one glycoside to another. That is, these enzymes catalyze the intra- or intermolecular substitution of the anomeric position of a glycoside. Mechanistically, transglycosylases utilize the same mechanism as various retaining glycoside hydrolases [1, 2]. Thus, reaction of the nucleophile of a retaining glycoside hydrolase with a substrate gives a glycosyl-enzyme intermediate that can be intercepted either by water to give the hydrolysis product, or by another acceptor (often another carbohydrate alcohol), to give a new glycoside or oligosaccharide [3]. Alternatively, transglycosylation can occur by neighboring group participation, wherein a neighboring 2-acetamido group participates in the reaction to generate an oxazolinium ion intermediate, which again can react with another acceptor other than water. Some transglycosidases possess substantial glycoside hydrolase activity, and some glycoside hydrolases possess transglycosylase activity [4]. Indeed, in many cases it is unclear what the major role of an enzyme that possesses both activities may be. Transglycosylases are classified as glycoside hydrolases into various GH families on the basis of sequence similarity.

Figure. Generalized mechanism of a transglycosylase. Enzymatic cleavage of a substrate through a classical Koshland retaining mechanism results in formation of a glycosyl enzyme intermediate. This can partition to react with either water to cause hydrolysis (glycoside hydrolase activity) or to an alternative acceptor, often a sugar, to cause transglycosylation (transglycosylase activity).

Families

GH families with notable transglycosylase activity include:

• GH2, for example LacZ β-galactosidase converts lactose to allolactose [5].
  
• GH13, for example cyclodextran glucanotransferases that convert linear amylose to cyclodextrins [6]; glycogen debranching enzyme, which transfers three glucose residues from the four-residue glycogen branch to a nearby branch [7]; and trehalose synthase, which catalyzes the interconversion of maltose and trehalose [8].
  
• GH16, for example xyloglucan endotransglycosylases, which cuts and rejoins xyloglucan chains in the plant cell wall [9].
• GH31, for example α-transglucosidases, which catalyze the transfer of individual glucosyl residues between α-(1→4)-glucans [10].
• GH70, for example glucansucrases, which catalyse the synthesis of high molecular weight glucans, from sucrose [11].
• GH77, for examples amylomaltase, which catalyzes the synthesis of maltodextrins from maltose [12].
• GH23, GH102, GH103, and GH104 lytic transglycosylases, which convert peptidoglycan to 1,6-anhydrosugars [13].

Inverting transglycosylases

An anomer-inverting transglycosylase has been reported that is a member of Family GH186, namely the cyclic glucohexadecaose-producing enzyme from Xanthomonas sp. [14]. This enzyme converts linear β-1,2-glucan to cyclic-(β-1,2-Glc)₁₅-1,6α. The reaction involves transglycosylation of a β-1,2-Glc bond to generate an α-1,6-Glc bond. Enzymes of this family are inverting glycosidases, and presumably, this product will be a substrate for the enzyme, possibly reversing the reaction or catalyzing hydrolysis, although this was not studied.

References

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1. Sinnott, M.L. (1990) Catalytic mechanisms of enzymic glycosyl transfer. Chem. Rev. 90, 1171-1202. DOI: 10.1021/cr00105a006

4. Vocadlo, D. J. and Withers, S. G. (2008) Glycosidase-Catalysed Oligosaccharide Synthesis, Chapter 29 in Carbohydrates in Chemistry and Biology, Ernst, B., Hart, G. W. and Sinaý, P., eds., Wiley-VCH Verlag GmbH, Weinheim, Germany. DOI:10.1002/9783527618255.ch29