Carbohydrates Note M.N Chatterjea For Nurses Part IV

Carbohydrates Note M.N Chatterjea: Disaccharides

Disaccharides are carbohydrates formed by the union of two monosaccharides through a glycosidic bond, with the elimination of one molecule of water. The general molecular formula for disaccharides is C₁₂H₂₂O₁₁. The three most common disaccharides of biological importance are maltose, lactose, and sucrose.

1. Maltose

Maltose is produced from the hydrolysis of starch and consists of two D-glucose molecules linked by an α(1→4) glycosidic bond. The hydrolysis reaction can be summarized as follows:

$\text{Maltose}\to \text{D-Glucose}+\text{D-Glucose}$

Maltose is a reducing sugar, which means it can participate in oxidation-reduction reactions. It is commonly found in malted foods and beverages and plays a significant role in the fermentation process.

2. Lactose

Lactose, commonly known as milk sugar, consists of one D-glucose molecule and one D-galactose molecule linked by a β(1→4) glycosidic bond. The hydrolysis reaction is:

$\text{Lactose}\to \text{D-Glucose}+\text{D-Galactose}$

Lactose is a crucial energy source for infants and is fermented by specific bacteria in the gut. Lactose intolerance, a common condition, arises from the deficiency of lactase, the enzyme responsible for breaking down lactose.

3. Sucrose

Sucrose is table sugar and consists of one D-glucose molecule and one D-fructose molecule linked by an α(1→2) glycosidic bond. The hydrolysis reaction is:

$\text{Sucrose}\to \text{D-Glucose}+\text{D-Fructose}$

Unlike maltose and lactose, sucrose does not have a free aldehyde or ketone group and therefore does not exhibit reducing properties. It is commonly found in many plants, especially sugar cane and sugar beets.

Invert Sugars and Inversion

When sucrose is hydrolyzed, it produces glucose and fructose, resulting in a mixture that is levorotatory due to the greater specific rotation of fructose compared to glucose. This process is known as inversion, which alters the optical activity of the solution. Sucrose has a dextrorotatory specific rotation of +62.5°, while the hydrolytic products exhibit a negative rotation due to the dominance of fructose.

Biomedical Importance of Disaccharides

Disaccharides play essential roles in nutrition and metabolism:

• Maltose: Found in baby foods and nutritional supplements, maltose is easily digestible and provides a quick source of energy.
• Lactose: Produced in the lactating mammary gland, lactose is a vital source of energy for newborns. Its fermentation by non-pathogenic coliform bacteria, such as E. coli, aids in maintaining gut health. Additionally, lactose contributes to the souring of milk through lactic acid production by various organisms.
• Sucrose: While not utilized when introduced parenterally, sucrose can alter osmotic conditions in the blood. It is often used in clinical settings to manage conditions like cerebral edema, drawing water from tissues into the bloodstream.

Comparison of Lactose and Sucrose

Characteristic	Lactose	Sucrose
Known as	Milk sugar	Common table sugar (cane sugar)
Structure	D-Glucose + D-Galactose (β 1→4 linkage)	D-Glucose + D-Fructose (α 1→2 linkage)
Hydrolysis Products	Glucose + Galactose	Glucose + Fructose
Hydrolyzing Enzyme	Lactase	Sucrase (Invertase)
Optical Rotation	Dextrorotatory	Dextrorotatory (+66.5°), products are levorotatory
Mutarotation	Exhibits mutarotation	Does not exhibit mutarotation
Reducing Sugar	Can reduce alkaline copper sulfate solution	Cannot reduce alkaline copper sulfate
Osazone Formation	Forms osazone crystals	Cannot form osazones
Mucic Acid Formation	Forms mucic acid upon oxidation	Cannot form mucic acid
Clinical Significance	Can appear in urine (lactosuria)	Not applicable

Oligosaccharides

Oligosaccharides are carbohydrates that consist of 3 to 10 monosaccharide units linked by glycosidic bonds. They play significant roles in biological systems, particularly in cell signaling and recognition.

Biomedical Importance of Oligosaccharides

Oligosaccharides are crucial in various biological functions:

1. Cell Membrane Structure: Integral membrane proteins often have covalently attached oligosaccharides on their extracellular faces. These carbohydrates contribute to cell-cell recognition and signaling processes.
2. Glycoproteins: Many secreted proteins, including antibodies and coagulation factors, contain oligosaccharide units. These carbohydrates are typically linked to amino acid residues through O-glycosidic linkages (to serine or threonine) or N-glycosidic linkages (to asparagine).
3. Diversity and Complexity: The unique arrangements of oligosaccharides on glycoproteins suggest that they carry rich information, which is functionally important for molecular targeting and recognition.
4. Removal of Glycoproteins: Surface protein receptors on liver cells, such as the asialoglycoprotein receptor, recognize and internalize glycoproteins after they have been trimmed of terminal sialic acid residues. This mechanism regulates the lifespan of glycoproteins in circulation.
5. Physiological Regulation: The rate of removal of sialic acid from glycoproteins can be influenced by the structure of the protein, allowing for precise control over protein longevity based on physiological and biological needs.

Conclusion

The study of carbohydrates, particularly disaccharides and oligosaccharides, is crucial for nursing practice. Understanding their structure, function, and biomedical importance enables nurses to make informed decisions regarding patient nutrition and health care management. Knowledge of carbohydrates’ roles in metabolism, energy provision, and cellular communication is essential for optimizing patient outcomes and fostering effective interdisciplinary collaboration.

References

• Chatterjea, M.N. (8th Edition). “The Text Book of Medical Biochemistry.”