뒤로Disaccharides: Structure, Linkages, and Properties
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Disaccharides
Definition and General Structure
Disaccharides are carbohydrates composed of two monosaccharide units covalently linked by an O-glycosidic bond. They are a common class of oligosaccharides and play important roles in energy storage and metabolism.
Monosaccharide: The simplest form of carbohydrate (e.g., glucose, fructose, galactose).
O-glycosidic bond: A covalent bond formed between the anomeric carbon of one monosaccharide and a hydroxyl group of another.
The most abundant disaccharides include maltose, cellobiose, lactose, and sucrose.
Common Disaccharides and Their Properties
Disaccharide | Sugar 1 | Sugar 2 | Linkage | Degradable? |
|---|---|---|---|---|
Maltose | Glucose | Glucose | α-1,4 linkage | Yes |
Cellobiose | Glucose | Glucose | β-1,4 linkage | No (in humans) |
Lactose | Galactose | Glucose | β-1,4 linkage | Yes |
Sucrose | Glucose | Fructose | α-1,2 linkage | Yes |
Additional info: Cellobiose is not digestible by humans due to the lack of the enzyme β-glucosidase.
Key Glycosidic Linkages
α-1,4 linkage: Found in maltose; connects the anomeric carbon (C1) of one glucose to the C4 of another glucose in the alpha configuration.
β-1,4 linkage: Found in lactose and cellobiose; connects the anomeric carbon (C1) of one sugar to the C4 of another in the beta configuration.
α-1,2 linkage: Found in sucrose; connects the anomeric carbon (C1) of glucose to the C2 of fructose in the alpha configuration.
Examples and Applications
Lactose: Found in milk; composed of galactose and glucose with a β-1,4 linkage.
Sucrose: Table sugar; composed of glucose and fructose with an α-1,2 linkage.
Maltose: Product of starch digestion; composed of two glucose units with an α-1,4 linkage.
Practice Questions and Explanations
Which disaccharide contains galactose as one of the sugar subunits? Answer: Lactose
Which disaccharide forms a 1,1-glycosidic linkage? Answer: Trehalose (not shown in the main table, but trehalose is a glucose-glucose disaccharide with an α,α-1,1 linkage).
What is the identity of the disaccharide below? Given the structure, the answer is Lactose (galactose + glucose, β-1,4 linkage).
Ring-Chain Equilibrium
Both sugar rings in a disaccharide can be in equilibrium with their linear chain forms, especially when the anomeric carbon is free (not involved in glycosidic bond).
Identifying Monosaccharide Units and Linkages
To name the monosaccharide units and glycosidic linkage in a disaccharide, identify the ring structure and the carbons involved in the bond.
Example: In lactose, the linkage is β-1,4 between galactose (C1) and glucose (C4).
Haworth Projections
Haworth projections are used to represent the cyclic structure of sugars.
To draw a disaccharide, connect the appropriate carbons of each monosaccharide according to the linkage type.
Summary Table: Common Disaccharides
Name | Monosaccharide 1 | Monosaccharide 2 | Linkage | Source |
|---|---|---|---|---|
Maltose | Glucose | Glucose | α-1,4 | Starch hydrolysis |
Lactose | Galactose | Glucose | β-1,4 | Milk |
Sucrose | Glucose | Fructose | α-1,2 | Plants (table sugar) |
Cellobiose | Glucose | Glucose | β-1,4 | Cellulose hydrolysis |
Key Equations
General formula for a disaccharide (after condensation):
