Glycoside phosphorylases: structure, catalytic properties and biotechnological potential.

Glycoside phosphorylases (GPs) are the enzymes that reversibly phosphorolytically process glycosidic bond in sucrose (6'-phosphate), α-1,4-glucan and maltodextrins, α-glucobioses, α-1,3-oligoglucan, β-glucobioses and β-glucodextrins, chitobiose, β-galactosides and β-mannosides, and transfer non-reducing end terminal glycosyl residue to inorganic phosphate. They are modular enzymes that form biologically active homooligomers. From a mechanistic as well as structural point of view, they are similar to glycoside hydrolases or glycosyltransferases. Regardless the stereochemical outcome of the phosphorylase-catalyzed reaction (inversion or retention) the phosphorolytic cleavage of glycosidic bond is reversible, therefore glycosyl phosphates may efficiently be used for oligosaccharide synthesis. Although majority of GPs show very high substrate and positional selectivity, they might be employed for a green, inexpensive and often one-pot conversion of one sugar (cheap) to another one (expensive). This fascinating capability is due to the fact that pathways of several GPs share the same glycosyl phosphate, i.e. a product of one phosphorylase is simultaneously consumed as a substrate by another one, or even the same enzyme in a second step if the phosphorylase possesses a relaxed acceptor specificity. In some cases glycosyl phosphates may be interconverted using other auxiliary carbohydrate-active enzymes, achieving for example galactoside synthesis from gluco-configured sugar donors, thus widening synthetic potential of these biocatalysts. In comparison with common hydrolysis, the energy of glycosidic bond is not annihilated during phosphorolysis. This energetic aspect of the reactions catalyzed by GPs and their physiological role is discussed in relation to often concurrently occurring glycoside hydrolases.