2.10.3 Glycosidic Linkage in Lactose

Formation: - Lactose is formed by the condensation of β-galactose and β-glucose.

• The -OH group at C-1 of β-galactose interacts with the H of -OH group at C-4 of β-glucose.
• A water molecule is released.
• A β-1,4-glycosidic linkage (or β-1, β-4-glycosidic linkage) is formed.

Structure:

β-Galactose + β-Glucose → Lactose + Water

• The bond is between C-1 of galactose and C-4 of glucose.
• One anomeric carbon (C-1 of galactose) is involved in the bond.
• The other anomeric carbon (C-1 of glucose) is free.
• Therefore, lactose is a reducing sugar.

Summary of Glycosidic Linkages in Disaccharides

Intermediate Level

Glycosidic Bond Formation in Detail

• Two monosaccharides react.
• A hydroxyl group (-OH) from one sugar and a hydrogen atom (-H) from the other sugar are removed.
• These combine to form water (H₂O).
• A glycosidic bond is formed between the two sugars.

Naming Convention:

Examples:

Reducing and Non-Reducing Sugars

• A sugar that has a free anomeric carbon (C-1).
• Can reduce other compounds.
• Gives a positive result with Benedict's test.
• Examples: Maltose, lactose, glucose, fructose.

• A sugar that has no free anomeric carbon.
• Both anomeric carbons are involved in the glycosidic bond.
• Does not reduce other compounds.
• Gives a negative result with Benedict's test.
• Example: Sucrose.

• In sucrose, the glycosidic bond involves C-1 of glucose and C-2 of fructose.
• Both anomeric carbons are involved in the bond.
• Therefore, there is no free anomeric carbon.
• Sucrose is a non-reducing sugar.

• In maltose, only C-1 of the first glucose is involved in the bond.
• The C-1 of the second glucose is free.
• In lactose, only C-1 of galactose is involved in the bond.
• The C-1 of glucose is free.
• Therefore, maltose and lactose are reducing sugars.

Comparison of Glycosidic Bonds

Important Points

• 1. A glycoside is basically a ring chain hexose (6C) or pentose (5C) sugar molecule that is attached usually to another sugar molecule.
• 2. Covalent chemical bonds between two glycosides that hold them together is called a glycosidic bond.
• 3. Glycosidic bond is formed due to condensation reaction between two sugar molecules mostly hexoses.
• 4. For the formation of glycosidic bond, the H-group from one ringed sugar interacts with the -OH group of another sugar and one water molecule is released.
• 5. Glycosidic linkage is present in disaccharides, oligosaccharides, and polysaccharides.
• 6. Glycosidic bonds are named on the basis of carbon number of interacting carbons of both linked sugars.
• 7. Glycosidic linkage is also named alpha or beta on the basis of alpha or beta type of first sugar involved in linkage.
• 8. Example: α-1,4 glycosidic linkage is formed when first sugar is alpha, donates -OH group from its C-1 and other sugar donates H-group from its fourth carbon.
• 9. Glycosidic linkage in sucrose:
• - Sucrose is formed by the condensation of α-glucose and β-fructose.
• - The -OH group at C-1 of α-glucose interacts with the H of -OH group at C-2 of β-fructose.
• - A water molecule is released.
• - An α-1,2-glycosidic linkage (or α-1, β-2-glycosidic linkage) is formed.
• 10. Glycosidic linkage in maltose:
• - Maltose is formed by the condensation of two molecules of α-glucose.
• - The -OH group at C-1 of first α-glucose interacts with the H of -OH group at C-4 of other α-glucose.
• - A water molecule is released.
• - An α-1,4-glycosidic linkage (or α-1, α-4-glycosidic linkage) is formed.
• 11. Glycosidic linkage in lactose:
• - Lactose is formed by the condensation of β-galactose and β-glucose.
• - The -OH group at C-1 of β-galactose interacts with the H of -OH group at C-4 of β-glucose.
• - A water molecule is released.
• - A β-1,4-glycosidic linkage (or β-1, β-4-glycosidic linkage) is formed.

Remember

• - Glycoside = sugar molecule attached to another sugar
• - Glycosidic bond = covalent bond between two sugars
• - Formation = condensation reaction; water released
• - **Naming** = carbon numbers + αβ type
• - Sucrose = α-1,2 (glucose + fructose)
• - Maltose = α-1,4 (glucose + glucose)
• - Lactose = β-1,4 (galactose + glucose)
• - Sucrose = non-reducing sugar
• - Maltose and Lactose = reducing sugars
• - Glycosidic bonds in disaccharides, oligosaccharides, polysaccharides

Common Mistakes

• 1. Confusing glycosidic bond with peptide bond – Glycosidic bond is between sugars; peptide bond is between amino acids.
• 2. Thinking all glycosidic bonds are the same – Glycosidic bonds differ in carbon numbers and αβ configuration.
• 3. Confusing the glycosidic bonds in sucrose, maltose, and lactose – Sucrose = α-1,2; Maltose = α-1,4; Lactose = β-1,4.
• 4. Thinking sucrose is a reducing sugar – Sucrose is a non-reducing sugar because both anomeric carbons are involved in the bond.
• 5. Forgetting that water is released during glycosidic bond formation – Glycosidic bond formation is a condensation reaction; water is released.
• 6. Thinking glycosidic bonds are only in disaccharides – Glycosidic bonds are also in oligosaccharides and polysaccharides.
• 7. Confusing the naming convention – The first number is the carbon of the first sugar; the second number is the carbon of the second sugar.
• 8. Thinking α and β are the same – α and β refer to the position of the -OH group on the anomeric carbon.
• 9. Forgetting that maltose and lactose are reducing sugars – They have a free anomeric carbon.
• 10. Thinking all disaccharides are non-reducing – Only sucrose is non-reducing; maltose and lactose are reducing.
• 11. Confusing the composition of disaccharides – Sucrose = glucose + fructose; Maltose = glucose + glucose; Lactose = galactose + glucose.
• 12. Forgetting the biological importance of glycosidic bonds – They are important for energy storage, structural support, cell recognition, and transport.

Quick Revision

• A glycoside is basically a ring chain hexose (6C) or pentose (5C) sugar molecule that is attached usually to another sugar molecule.
• Covalent chemical bonds between two glycosides that hold them together is called a glycosidic bond.
• Glycosidic bond is formed due to condensation reaction between two sugar molecules mostly hexoses.
• For the formation of glycosidic bond, the H-group from one ringed sugar interacts with the -OH group of another sugar and one water molecule is released.
• Glycosidic linkage is present in disaccharides, oligosaccharides, and polysaccharides.
• Glycosidic bonds are named on the basis of carbon number of interacting carbons of both linked sugars.
• Glycosidic linkage is also named alpha or beta on the basis of alpha or beta type of first sugar involved in linkage.
• e.g., α-1,4 glycosidic linkage is formed when first sugar is alpha, donates -OH group from its C-1 and other sugar donates H-group from its fourth carbon.
• Glycosidic linkage in sucrose:
• The sucrose is formed by the condensation of α-glucose and β-fructose.
• In this reaction, the -OH group at C-1 of α-glucose interacts with the H of -OH group at C-2 of β-fructose, releasing a water molecule forming α-1,2-glycosidic linkage or α-1, β-2-glycosidic linkage.
• Glycosidic linkage in maltose:
• Maltose is formed by the condensation of two molecules of α-glucose.
• In this reaction, the -OH group at C-1 of first α-glucose interacts with the H of -OH group at C-4 of other α-glucose, releasing a water molecule forming α-1,4-glycosidic linkage or α-1, α-4-glycosidic linkage.
• Glycosidic linkage in lactose:
• The lactose is formed by the condensation of β-galactose and β-glucose.
• In this reaction, the -OH group at C-1 of β-galactose interacts with the H of -OH group at C-4 of β-glucose, releasing a water molecule forming β-1,4-glycosidic linkage or β-1, β-4-glycosidic linkage.

60-Second Revision

• Glycoside = sugar molecule attached to another sugar
• Glycosidic bond = covalent bond between two sugars
• Formation = condensation reaction; water released
• **Naming** = carbon numbers + αβ type
• Sucrose = α-1,2 (glucose + fructose)
• Maltose = α-1,4 (glucose + glucose)
• Lactose = β-1,4 (galactose + glucose)
• Sucrose = non-reducing sugar
• Maltose and Lactose = reducing sugars
• Glycosidic bonds in disaccharides, oligosaccharides, polysaccharides
• Warning: Glycosidic bonds differ in carbon numbers and αβ configuration. Sucrose is non-reducing because both anomeric carbons are involved in the bond. Maltose and lactose are reducing because they have a free anomeric carbon. Do not confuse the glycosidic bonds of sucrose, maltose, and lactose. Water is released during glycosidic bond formation.