Difference Between Glucose Galactose and Mannose

The key difference between glucose galactose and mannose is that glucose is a six-carbon structure and galactose is the C4 epimer of glucose, whereas mannose is the C2 epimer of glucose.

An epimer is an organic compound that is described along with the isomerism of organic compounds. We can define an epimer as an isomer of a particular compound which has an asymmetric carbon atom. For example, galactose and mannose are epimers of glucose.

CONTENTS

1. Overview and Key Difference
2. What is Glucose 
3. What is Galactose
4. What is Mannose
5. Side by Side Comparison – Glucose Galactose vs Mannose in Tabular Form
6. Summary

What is Glucose?

Glucose is an organic compound having the chemical formula C6H12O6. It is a simple sugar molecule. We can find glucose as the most abundant monosaccharide molecule among carbohydrate compounds. Its sources are plants (where glucose is produced) and algae, in which photosynthesis can take place to produce glucose from carbon dioxide and water, using the energy from sunlight. Moreover, in the energy metabolism of animals, glucose is the most important source of energy. This energetic form of glucose is called starch and amylopectin when stored in the plant, and it is known as glycogen in animals. We can extract glucose from its sources, and it is also available as a commercial product in the market. This substance appears as a white powder and is very sweet tasting.

Glucose molecule is called a hexose because it contains six carbon atoms per molecule. Hexose is a subcategory of the group of monosaccharide. There are two isomeric forms of glucose as D-glucose and L-glucose and D-isomer is the most stable and abundant isomeric form. It occurs widely in nature. We can obtain glucose through the hydrolysis of carbohydrates such as milk sugar or lactose, from sucrose that occurs in sugar cane, from maltose, from cellulose from plants, etc.

When considering the chemical structure of the glucose molecule, it contains five hydroxyl groups (-OH). These hydroxyl groups are arranged in a specific pattern along its six-carbon backbone. Apart from having two isomeric forms, there are two different arrangement of hydroxyl groups in the carbon ring structure of the glucose; these are named as alpha structure and beta structure.

What is Galactose?

Galactose is the C4 epimer of the glucose molecule. Therefore, galactose also has the same chemical formula as glucose (C6H12O6). Sometimes this compound name is abbreviated as Gal. It is a monosaccharide sugar molecule having a sweet taste similar to glucose. We can name this compound as an aldohexose because it is a hexose sugar having an aldehyde functional group. When a galactose molecule is combined with a glucose molecule, it forms a lactose molecule. Moreover, the polymeric form of galactose is galactan, and we can find in hemicellulose.

When considering the chemical structure of galactose, it can occur in both open-chain and cyclic form. There is a carbonyl group at the end of the chain. There are four isomers of galactose molecule that exist in the cyclic form. These cyclic forms have two anomeric forms as alpha and beta forms.

We can find the sources of galactose mainly among dairy products such as fresh milk, yoghurt, in avocados, sugar beets, etc. Moreover, galactose molecules can be synthesized in our body.

What is Mannose?

Mannose is the C2 epimer of the glucose molecule. It is a sugar molecule in the aldohexose series of carbohydrates. This compound is very important in human metabolism, such as in glycosylation of certain protein molecules.

We can find mannose molecule occurring in two forms as the pyranose ring structure and the furanose ring structure. The pyranose ring contains six carbon atoms while the furanose ring structure has five carbon atoms in the ring. However, each ring closure tends to have either an alpha or beta configuration at the anomeric carbon centre.

We can produce mannose through the oxidation of mannitol or from glucose through the Lory-de Bruyn-van Ekenstein transformation route. These two methods are very important because we need to produce mannose to be used as a nutritional supplement that is important in preventing urinary tract infections.

When considering the chemical structure of mannose molecule, it differs from the structure of glucose at the second carbon chiral centre (C2 position). The mannose molecule shows a 4C1 pucker in the solution ring form. This difference between glucose and mannose leads to very different biochemical features of these two hexoses.

What is the Difference Between Glucose Galactose and Mannose?

Galactose and mannose are epimers of the glucose molecule. The key difference between glucose galactose and mannose is that glucose is a six-carbon structure and galactose is the C4 epimer of glucose whereas mannose is the C2 epimer of glucose. Moreover, glucose is naturally produced through photosynthesis in plants. Galactose is produced through the hydrolysis of lactose that is catalyzed by the lactase enzyme while mannose is produced through the oxidation of mannitol or from glucose through the Lory-de Bruyn-van Ekenstein transformation route.

Below infographic tabulates the differences between glucose galactose and mannose in detail.

Summary – Glucose Galactose vs Mannose

An epimer is an isomer of a particular compound which has an asymmetric carbon atom. Galactose and mannose are epimers of glucose. The key difference between glucose galactose and mannose is that glucose is a six-carbon structure and galactose is the C4 epimer of glucose whereas mannose is the C2 epimer of glucose.

Reference:

1. “Galactose.” ScienceDirect, Available here.
2. “Glucose.” ScienceDirect Topics, Available here.

Image Courtesy:

1. “Alpha-D-Glucopyranose” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia
2. “D-Galactose Haworth” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia
3. “Mannose structure” By Ed (Edgar181) – Own work (Public Domain) via Commons Wikimedia