Describe the breakage of a glycosidic bond in polysaccharides and disaccharides by hydrolysis, with reference to the non-reducing sugar test

Sourav PanOctober 28, 2024

Answered

The breakage of a glycosidic bond in disaccharides and polysaccharides occurs through a process called hydrolysis. In hydrolysis, a water molecule is used to cleave the bond between monosaccharide units, reversing the condensation reaction that originally formed the glycosidic bond.

Hydrolysis of Glycosidic Bonds

1. Mechanism of Hydrolysis:
   • During hydrolysis, the addition of a water molecule provides a hydroxyl group (-OH) to one of the monosaccharides and a hydrogen atom (H) to the other, breaking the covalent bond between them.
   • This reaction is often catalyzed by specific enzymes, such as maltase (which breaks down maltose), sucrase (for sucrose), or amylase (which hydrolyzes starch and glycogen).
2. Hydrolysis in Disaccharides:
   • For example, maltose can be hydrolyzed by the enzyme maltase to produce two glucose molecules, while sucrose can be hydrolyzed by sucrase to produce one glucose and one fructose molecule.
   • In both cases, hydrolysis breaks the glycosidic bond, releasing the component monosaccharides that can be absorbed or used by the organism.
3. Hydrolysis in Polysaccharides:
   • Polysaccharides like starch and glycogen are broken down by enzymes such as amylase or glucosidase. These enzymes catalyze the hydrolysis of α(1→4) glycosidic bonds, producing shorter chains and eventually freeing glucose monomers.
   • In cellulose, the β(1→4) glycosidic bonds can be broken down by cellulase (produced by some bacteria and fungi), though most animals lack this enzyme.

Non-Reducing Sugar Test and Hydrolysis

The non-reducing sugar test is commonly used to identify the presence of non-reducing sugars, such as sucrose, which do not directly react with standard reducing sugar tests (like Benedict’s test). Here’s how it works in the context of hydrolysis:

1. Testing Procedure:
   • To test for non-reducing sugars, the sample is first treated with acid hydrolysis (typically by heating it with dilute hydrochloric acid) to break any glycosidic bonds present in non-reducing sugars like sucrose.
   • This hydrolysis step breaks sucrose into its monosaccharides, glucose and fructose, which are reducing sugars.
   • The sample is then neutralized with a base, and a reducing sugar test (like Benedict’s test) is applied.
2. Observing Results:
   • After hydrolysis, if the test now shows a positive result for reducing sugars, this indicates that the original substance contained non-reducing sugars, such as sucrose.
   • For example, if sucrose is hydrolyzed by acid, the glucose and fructose produced will yield a color change in Benedict’s test, confirming the presence of a reducing sugar after hydrolysis.