Have and hemiketals. Diagram 3: Example Of

Have you ever wondered what type of bond is used to link molecules, such as sugar, together? Explore this lesson to understand what a glycosidic bond is, its many forms, and importance in everyday living.

What Are Glycosidic Bonds?

This is not your ordinary bond.

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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.

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

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.When a sugar molecule, such as glucopyranose, combines with an -OR group (let’s use alcohol in this example) a glycosidic bond can form. The alcohol group (seen in Diagram 4a) will attack the anomeric carbon on glucopyranose (seen in Diagram 4b) to form a glycosidic bond (as seen in Diagram 4c).

Diagram 4: the formation of a glycosidic bond
glycosidic bond

An anomeric carbon is a carbon atom in a cyclic ring surrounded by two atoms containing an oxygen. It is readily willing to share its electrons to form a glycosidic bond with an -OR group. Now that we understand the basic chemistry of glycosidic bond formation, we can apply this to a few biological examples.

Glycosidic Bonding In Biology

In biology, carbohydrates love to use glycosidic bonds when linking monomers together.

Here are two examples:Sugars: This is a no brainer as the definition of a glycosidic bond requires the use of a sugar molecule. But did you know that sugar monomers can be linked together, through glycosidic bonds, to form larger sugar molecules called disaccharides? Diagram 5 illustrates the chemical structure of the disaccharide maltose. Notice that the chemical process to form a glycosidic bond, in maltose, involves the same steps we discussed earlier. However, rather than a simple -OR group, a glucose monomer binds with a sugar molecule.

Diagram 5: Glycosidic Linkage In Maltose
Diagram 6: Glycosidic Linkage In Amylose

Lesson Summary

Let’s review. Glycosidic bonds are covalent bonds used to link a sugar molecule to an -OR group.

The -OR group can be a sugar molecule or non-sugar molecule. There are different forms of the glycosidic bond: C-, N-, S-, and O-. These atoms (C-, N-, S-, and O-) are determined by their location within a glycosidic bond. Chemically, glycosidic bonds require a hemiacetal or hemiketal bonded to an -OR group.

Glycosidic linkages are commonly found in carbohydrates, such as sugars and starches.


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