Glycosidic bond

Overview
In chemistry, a glycosidic bond is a certain type of functional group that joins a carbohydrate (sugar) molecule to an alcohol, which may be another carbohydrate. Specifically, a glycosidic bond is formed between the hemiacetal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some alcohol. A substance containing a glycosidic bond is a glycoside.


 * [[Image:ethyl-glucoside.png|550px|Ethyl glucoside]]
 * Glucose and ethanol combine to form ethyl glucoside and water.

Chemistry
The hemiacetal group of carbohydrates (which contains the anomeric carbon) is reactive, and glycosidic bonds form readily in the presence of acid. This is a condensation reaction as one molecule of water is released. Glycosidic bonds are fairly stable; they can be broken chemically by strong aqueous acids.

A glycosidic functional group is an example of an acetal.

Saccharides in aqueous solution can exist in linear (rare) or cyclic form (more common), and these forms readily interconvert. Only the cyclic forms have an anomeric carbon and can form a glycosidic bond; once the bond has formed, the saccharide unit can no longer attain the linear form.

Polysaccharides
A glycosidic bond can join two monosaccharide molecules to form a disaccharide, as for instance in the linkage of glucose and fructose to create sucrose. More complicated polysaccharides such as starch, glycogen, cellulose or chitin consist of numerous monosaccharide units joined by glycosidic bonds.

While the cyclic structures of monosaccharide units are fairly rigid, the glycosidic bonds confer flexibility to polysaccharide molecules.

Glycosidic bonds join monosaccharides to form polysaccharides, just like peptide bonds join amino acids to form proteins.

S- and N- and O-glycosidic bonds
In analogy, one also considers S-glycosidic bonds, where the anomeric carbon of a sugar is bound to some other group via a sulfur (rather than an oxygen) atom, and N-glycosidic bonds, where the anomeric carbon is bound to some other group via a nitrogen atom. The glycosidic bonds discussed earlier are often called O-glycosidic bonds to distinguish them from S- and N-glycosidic bonds. Substances containing N-glycosidic bonds are also known as glycosylamines; the term "N-glycoside" is considered a misnomer by IUPAC and is discouraged.

α-, β-, 1,4 and 1,6 glycosidic bonds
In general, one distinguishes between α- and β-glycosidic bonds, depending on whether the substituent groups on the carbons flanking the ring oxygen are pointing in the same or opposite directions in the standard way of drawing sugars. An α-glycosidic bond for a D-sugar emanates below the plane of the sugar while the hydroxyl (or other substituent group) on the other carbon points above the plane (opposite configuration), while a β-glycosidic bond emanates above that plane (the same configuration). The alpha and beta designation is reversed for L-sugars with an opposing configuration designated beta and the same configuration designated alpha. (The figure above shows ethyl α-D-glucoside.)

In a 1,4-glycosidic bond a C1-O-C4 bond is made involving the C1 of one sugar molecule and C4 of the other; likewise a C1-O-C6 bond is called a 1,6-glycosidic bond.

Examples from biochemistry
Important examples in biochemistry include DNA (or RNA), where deoxyribose (or ribose) sugar units are joined to nucleobases via N-glycosidic bonds. The polysaccharides often used for energy storage were already mentioned above. Organisms also often form glycoproteins by attaching sugars to proteins via O- or N-glycosidic bonds in a process known as glycosylation. Animals (and pharmacists) 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.

Enzymes
Enzymes that form or break glycosidic bonds (i.e. that catalyze their hydrolysis) are called glycoside hydrolases (or glycosidases).

Glycoside hydrolases typically can act either on α- or on β-glycosidic bonds, but not on both.

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 cytosine monophosphate (CMP). Sometimes mono- or oligosaccharides are also activated by being linked to lipids through a phosphate or diphosphate group. These activated species are known as sugar donor substrates. Then enzymes known as glycosyltransferases transfer the sugar unit from the activated glycosyl donor to an accepting nucleophile (the acceptor substrate).