Everything about Glycosidic Bond totally explained
In
chemistry, a
glycosidic bond is a certain type of
functional group that joins a
carbohydrate (sugar) molecule to another, which may be another carbohydrate.In specific terms, a glycosidic bond is formed between the
hemiacetal group of a
saccharide (or a molecule derived from a saccharide) and the
hydroxyl group group of some organic compound such as an
alcohol. In the literature, the bond between an
amino group or other nitrogen-containing group and the sugar is often referred to as a glycosidic bond (although IUPAC seems to suggest that the term is a misnomer). For example, the sugar-base bond in a
nucleoside may be referred to as a glycosidic bond (for example, Andre and Guschlbauer 1974). A substance containing a glycosidic bond is a
glycoside.
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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-, N-, C-, 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.
There is also a C-glycosidic bond which is simply a covalent bond between a sugar and an aglycon. This type is very resistant to hydrolysis.
α-, β-, 1,4 and 1,6 glycosidic bonds
| |
L |
D |
| α |
same |
opposite |
| β |
opposite |
same |
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, whereas 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 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 break glycosidic bonds (for example, 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're 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).
Further Information
Get more info on 'Glycosidic Bond'.
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