explain how glycosidic bonds are formed
Answers
Glycosidic bonds are the covalent chemical bonds that link ring-shaped sugar molecules to other molecules. They form by a condensation reaction between an alcohol or amine of one molecule and the anomeric carbon of the sugar and, therefore, may be O-linked or N-linked.
In chemistry, a glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
They are in the sugar that you eat, the trunks of trees, the hard exoskeleton of lobsters, and even in your DNA. Glycosidic bonds are important for the structure of all of these substances and many others. Indeed, life would not exist without glycosidic bonds. So, what are they?
Glycosidic bonds are covalent chemical bonds that hold together a glycoside. A glycoside is simply a ring-shaped sugar molecule that is attached to another molecule. The sugar ring may be either a 5-membered ring or a 6-membered ring, and the other molecule can be - and often is - another sugar. Look at this figure showing a sucrose molecule.
Sucrose is the sugar that you put in your tea and use to make cookies. It is composed of two sugar units, a glucose (left) and a fructose (right), linked by a glycosidic bond. The glycosidic bond is shown in blue.
How Does a Glycosidic Bond Form?
A glycosidic bond forms by a condensation reaction, which means that one water molecule is produced during formation of a glycoside. The reverse reaction, the breakage of a glycosidic bond, is a hydrolysis reaction. One water molecule is used up in the reverse reaction.
The condensation reaction occurs when an alcohol group, or OH, from a molecule attacks the anomeric carbon of a sugar. The anomeric carbon is the central carbon of a hemiacetal. That is, the carbon has single bonds to two oxygen atoms.
One of the oxygen atoms is part of the sugar ring, and the other is an OH group. When the alcohol attacks the anomeric carbon, the OH group bonded to that carbon is replaced by the O of the alcohol, and the H of the alcohol is removed. As you can see, both an H and an OH (shown in red) are removed from the original molecules during the reaction. Together they make H2O, or water.
The result of a glycosidic bond is a sugar molecule linked to another molecule via an ether group. An ether is an oxygen atom bonded to two carbon atoms, which is relatively unreactive compared to other chemical groups, such as alcohols. Therefore, glycosides tend to be more stable than free sugars.
Are All Glycosidic Bonds the Same?
All glycosidic bonds are not the same, however. Glycosidic bonds can either be O-linked or N-linked. In the sucrose example we just saw, an alcohol group attacked the anomeric carbon to form an ether. However, a glycosidic bond can also occur if the nitrogen atom of an amine group attacks the anomeric carbon instead. One example of an N-linked glycosidic bond is in the molecule deoxyadenosine shown here.