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Glycosidic Linkage: Definition & Overview

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Instructor
Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Expert Contributor
Ronald Okoth

Ronald received his PhD. from Brown University in Providence RI. Ronald has taught college level chemistry.

Glycosidic linkage, or bonds, are the formation between sugar, carbohydrates, and -OR group, in four possible forms: Carbon, oxygen, nitrogen, or sulfur. See an overview of the chemistry involved, and its application in biology. Updated: 10/14/2021

What Are Glycosidic Bonds?

This is not your ordinary bond. Oh no, this bond is one of a kind. A bond that can link carbohydrates to any molecule and dare to be different by taking on different forms is a special type of bond. What type of bonds are these? These are glycosidic bonds.

Glycosidic bonds are formed between a sugar molecule, or carbohydrate, and -OR group. There are many forms of glycosidic bonds such as C-, O-, N-, and S-. These forms are differentiated by the atom (carbon, oxygen, nitrogen, or sulfur) bonding the sugar and -OR group together.

We'll focus on O-glycosidic bonds. The '-OR' refers to when an oxygen atom is attached to any R group, which is any molecule containing a carbon and hydrogen atom. Examples of -OR groups include ketone, carboxylic acid, and ester. See if you can identify the -OR groups in these functional groups by looking at Diagram 1.

Diagram 1. Compounds Containing -OR Groups
or groups

The term glycosidic linkage is the same as glycosidic bond. In biology, glycosidic bonds are commonly seen in carbohydrate molecules, such as simple sugars and complex starches. But before we dive into the biological importance of these bonds, let's focus on its chemistry.

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  • 0:01 What Are Glycosidic Bonds?
  • 1:23 Chemistry of a Glycosidic Bond
  • 3:43 Glycosidic Bonding In Biology
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The Chemistry of a Glycosidic Bond

Now on to the fun part, the chemistry of a glycosidic bond. This bond is considered to be a covalent bond. Covalent bonds are chemical bonds formed between atoms through the sharing of electrons. These type of bonds are formed because atoms willingly exchange their electrons with each other to create this link. Remember, we discussed that a sugar molecule is required when forming a glycosidic bond. Chemically, this sugar molecule is referred to as a hemiacetal or hemiketal group.

Hemiacetal is created when an aldehyde and alcohol molecule combine. Hemiketal is created when a ketone and alcohol molecule combine. Think of hemiacetals and hemiketals as being formed when you chemically synthesize two smaller molecules (acetaldehyde/ketone and alcohol) into a larger molecule. Diagram 2 shows a simplified formation of both groups.

Diagram 2: Chemical Synthesis of an (a) Hemiacetal and (b) Hemiketal
hemiexample1

Going back to sugar molecules, these contain hemiacetals or hemiketals. The difference is that these chemical groups are cyclic which means that they look like a ring structure. The sugars glucopyranose and fructopyranose, shown in Diagram 3, are great examples of hemiacetals and hemiketals.

Diagram 3: Example Of (a) Hemiacetal Glucopyranose and (b) Hemiketal Fructopyranose
hemiexample2

Do they look familiar? They should, as these are cyclic structures of the sugars glucose and fructose, both containing a hemiacetal and hemiketal. The biology folks attach the word 'pyranose' to the ending of a carbohydrate name. Pyranose are carbohydrates with structures that are cyclic, either 5 or 6 membered rings. Can you guess what type of ring is glucopyranose and fructopyranose? They are 6 member rings as 6 carbon atoms are used to form the cyclic structure.

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Additional Activities

Hemiacetals and Hemiketals

A hemiacetal is a structure in which a carbon atom is bonded to (1) a carbon atom, (2) a hydrogen atom, (3) a hydroxyl group and (4) an ether group. Structure A below is an example of a hemiacetal. In a hemiketal, a carbon atom is bonded to (1) a carbon atom, (2) another carbon atom, (3) a hydroxyl group and (4) an ether group. Structure B below is an example of a hemiketal.

1. Identify each of the compounds below as either hemiacetals or hemiketals.


Anomeric Carbon

Carbohydrates also referred to as sugars can exist in cyclic form. During cyclization, carbohydrates form a new chiral carbon. This new chiral carbon is known as the anomeric carbon. The structure shown below is the cyclic form of mannose. The anomeric carbon of mannose is indicated in the structure.

2. Identify the anomeric carbon in each of the carbohydrates given below.


Glycosidic Bond

Glyosidic bond can be identified as O-gycosidic, N-glycosidic, S-glycosidic or C-glycosidic. The structure below is an example of a N-glycosidic bond.

3. Identify the glycosidic bond in each of the carbohydrates below and label it as either O-glycosidic, N-glycosidic, S-glycosidic or C-glycosidic.


Disaccharides

A disaccharide is formed when two monosaccharides combine. For example, as illustrated below, two glucose molecules (monosaccharides) combine to form maltose. The bond between the anomeric carbon of one monosaccharide and the oxygen atom of the other monosaccharide is the glycosidic bond.

4. Lactose is a disaccharide formed when two monosaccharides, galactose and glucose, combine. Use the structure of lactose given below to deduce the structures of the two monosaccharides.


Answer Key:

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