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Enzymatic formation and cleavage of the bond between two sugars or between a sugar and another group can occur by hydrolysis to give the free sugar (glycosidases or glycoside hydrolases), by transglycosylation to give a new glycoside (transglycosidases), by phosphorolysis to give the sugar-1-phosphate (phosphorylases) or by elimination to give unsaturated sugar products (lyases). The principal enzymes that catalyze glycoside synthesis are nucleotide phosphosugar-dependent glycosyltransferases.
Sequence analysis methods allow the grouping of proteins into sequence-related families. Sets of sequence-related enzymes are termed families. Sequence analysis groups transglycosidases with glycoside hydrolases (e.g. Family GH13 cyclodextrin glucanotransferases and amylases). According to all available evidence transglycosidases and glycoside hydrolases use the same mechanism, except that a sugar or some other group, rather than water, acts as the nucleophile.
Phosphorylases fall into two mechanistic classes: glycoside hydrolase-like and glycosyltransferase-like, and are likewise classified into GH or GT families by sequence comparisons. Lyases fall into two mechanistic classes. The largest class is that which cleaves polymers containing uronic acids: most commonly pectins and glycosaminoglycans. These enzymes break the bond between the glycosidic oxygen and the ring carbon of the sugar in the +1 site via an elimination mechanism. This is the group separately classified in CAZY as polysaccharide lyases (PLs) A second, very small, group of alpha-glucan lyases is found within GH Family 31 and follows a cationic glycoside-hydrolase-like mechanism.
[[Category:Glycoside Hydrolase Families]]