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A glycosidic bond is formed between the hemiacetal or hemiketal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some compound such as an alcohol. A substance containing a glycosidic bond is a glycoside.
The term 'glycoside' is now extended to also cover compounds with bonds formed between hemiacetal (or hemiketal) groups of sugars and several chemical groups other than hydroxyls, such as -SR (thioglycosides), -SeR (selenoglycosides), -NR1R2 (N-glycosides), or even -CR1R2R3 (C-glycosides).
Particularly in naturally occurring glycosides, the compound ROH from which the carbohydrate residue has been removed is often termed the aglycone, and the carbohydrate residue itself is sometimes referred to as the 'glycone'.
S-, N-, C-, and O-glycosidic bonds
Glycosidic bonds of the form discussed above are known as O-glycosidic bonds, in reference to the glycosidic oxygen that links the glycoside to the aglycone or reducing end sugar. In analogy, one also considers S-glycosidic bonds (which form thioglycosides), where the oxygen of the glycosidic bond is replaced with a sulfur atom. In the same way, N-glycosidic bonds, have the glycosidic bond oxygen replaced with nitrogen. Substances containing N-glycosidic bonds are also known as glycosylamines; the term "N-glycoside" is considered a misnomer by IUPAC and is discouraged. C-glycosyl bonds have the glycosidic oxygen replaced by a carbon. All of these modified glycosidic bonds have different susceptibility to hydrolysis, and in the case of C-glycosyl structures, they are typically more resistant to hydrolysis.
Numbering, and α/β distinction of glycosidic bonds
One distinguishes between α- and β-glycosidic bonds based on the relative stereochemistry (R or S) of the anomeric position and the stereocentre furthest from C1 in the saccharide. An α-glycosidic bond is formed when both carbons have the same stereochemistry, whereas a β-glycosidic bond occurs when the two carbons have different stereochemistry. One complicating issue is that the alpha and beta conformations were originally defined based on the relative orientation of the major constituents in a Haworth projection. In this case, for D-sugars, a beta conformation would see the major constituent at each carbon drawn above the plane of the ring (nominally the same conformation), while alpha would see the anomeric constituent below the ring (nominally opposite conformations). For L-sugars, the definitions would then, necessarily, reverse. This is worth noting as these older definitions still permeate the literature and can lead to confusion.
Pharmacologists often join substances to glucuronic acid via glycosidic bonds in order to increase their water solubility; this is known as glucuronidation. Many other glycosides have important physiological functions.
Before monosaccharide units are incorporated into glycoproteins, polysaccharides, or lipids in living organisms, they are typically first "activated" by being joined via a glycosidic bond to the phosphate group of a nucleotide such as uridine diphosphate (UDP), guanosine diphosphate (GDP), thymidine diphosphate (TDP), or cytidine monophosphate (CMP). These activated biochemical intermediates are known as sugar nucleotides or sugar donors. Many biosynthetic pathways use mono- or oligosaccharides activated by a diphosphate linkage to lipids, such as dolichol. These activated donors are then substrates for enzymes known as glycosyltransferases, which transfer the sugar unit from the activated donor to an accepting nucleophile (the acceptor substrate).
- Definition of glycosides, from the IUPAC Compendium of Chemical Terminology, the "Gold Book"
- Varki A et al. Essentials of Glycobiology. Cold Spring Harbor Laboratory Press; 1999. Searchable online