Glycoside Hydrolases
Glycosidases (O-glycoside hydrolases; EC 3.2.1.-) are in
vivo purely hydrolytic enzymes. Their subclass in the
IUBMB system (International Union of Biochemistry and
Molecular Biology) comprises over 150 entries. In the
CAZy database (Carbohydrate Active Enzymes, http://
www.cazy.org/) glycosidases are structurally divided into
over 130 families.
molecule may be virtually any structure possessing a hydroxyl
group, allowing the formation of a new glycosidic bond,
instead of the naturally occurring hydrolysis reaction. Two
strategies for such synthetic processes may be applied. First,
two reducing sugars react in a thermodynamically controlled
condensation process, usually called “reverse hydrolysis”.
This approach has been preferentially used for the glycosylation
of alcohols.[43] Besides primary and secondary alcohols,
successful glycosylations of sterically hindered tertiary
alcohols were accomplished, such as of 2-methylbutan-2-ol,
2-methylpentan-2-ol or tert-butyl alcohol.[44, 45] More complex
structures are efficiently glycosylated under kinetic control
in so-called transglycosylation reactions. In this case, glycoside
donors require activation by a good leaving group; this
Scheme 2. Glycosylation reactions catalyzed by the various classes of carbohydrate-
active enzymes (Copyright Wiley-VCH Verlag GmbH & Co.
KGaA. Adapted and reproduced with permission from reference [41]).
Scheme 1. Prominent glycosyl donors used in chemical synthesis (Ac=
Acetyl, NBS=N-bromosuccinimide).[30]
Scheme 3. Synthetic and hydrolytic reactions catalyzed by glycosidases.
Chem. Eur.
Glycosidases split saccharidic chains by transferring the
cleaved glycosyl moiety to water as an acceptor substrate
(Scheme 3). In laboratory conditions, however, the acceptor