Carbohydrates Note M.N Chatterjea For Nurses Part V

M.N Chatterjea For Nurses Part V: Polysaccharides

Polysaccharides are complex carbohydrates composed of long chains of monosaccharide units. They are classified into two main types: homopolysaccharides (homoglycans), which are made up of a single type of monosaccharide, and heteropolysaccharides (heteroglycans), which contain different monosaccharides or other functional groups.

Homopolysaccharides (Homoglycans)

1. Starch
   • Structure: Starch is a polymer of glucose and is primarily found in plants as a storage form of energy. It consists of two types of molecules: amylose and amylopectin.
     • Amylose: Linear chains of glucose units connected by α(1→4) glycosidic bonds.
     • Amylopectin: Branched structure with both α(1→4) and α(1→6) linkages.
   • Properties:
     • Solubility: Insoluble in cold water but forms a gel or paste upon heating.
     • Iodine Reaction: Starch reacts with iodine to produce a blue color due to amylose, while amylopectin yields a blue-violet color.
   • Biomedical Importance:
     • Starch is a major dietary carbohydrate, providing a significant source of energy.
     • It is utilized in various food products and is easily digestible due to its hydrolysis in the human gastrointestinal tract.
2. Glycogen
   • Structure: Glycogen is the primary storage form of glucose in animals. It is highly branched, resembling amylopectin but with more frequent branching.
   • Properties:
     • Composed of glucose units linked by α(1→4) bonds with α(1→6) branches occurring every 8-12 glucose units.
   • Biomedical Importance:
     • Stored primarily in the liver and muscles, glycogen serves as an immediate energy source during physical activity.
     • It is synthesized from glucose through a process called glycogenesis and broken down into glucose via glycogenolysis.
3. Inulin
   • Structure: Inulin is a polymer of D-fructose and serves as a storage carbohydrate in certain plants, particularly tubers.
   • Properties:
     • Low molecular weight, tasteless, and does not give color with iodine.
     • Hydrolyzed by inulinase to produce D-fructose.
   • Biomedical Importance:
     • Inulin is used in medical settings to assess renal function, specifically the glomerular filtration rate (GFR).
     • It can also help determine extracellular fluid volume.
4. Cellulose
   • Structure: Cellulose is a polymer of glucose units linked by β(1→4) glycosidic bonds. It forms the structural component of plant cell walls.
   • Properties:
     • Not easily hydrolyzed by dilute acids but can be broken down by stronger acids to yield cellobiose and glucose.
   • Biomedical Importance:
     • While humans cannot digest cellulose due to the lack of cellulase, it plays a crucial role as dietary fiber, aiding in digestion by adding bulk to stool and promoting peristalsis.
5. Dextrins
   • Structure: Dextrins are products of starch hydrolysis, consisting of shorter chains of glucose units.
   • Properties:
     • They resemble starch and can be precipitated by alcohol.
   • Biomedical Importance:
     • Dextrin solutions are often used as mucilages and in infant foods due to their digestibility.
6. Dextrans
   • Structure: Dextrans are polysaccharides synthesized by certain bacteria and consist of glucose units linked by α(1→6), α(1→4), or α(1→3) bonds.
   • Properties:
     • They are used clinically as plasma expanders due to their ability to increase blood volume when administered intravenously.
   • Biomedical Importance:
     • Dextrans can be involved in various medical applications, but care must be taken as they can interfere with blood typing.
7. Agar
   • Structure: Agar is derived from red algae and is composed of polysaccharides, primarily agarose and agaropectin.
   • Properties:
     • It dissolves in hot water and forms a gel upon cooling.
   • Biomedical Importance:
     • Agar serves as a solid medium for culturing bacteria in microbiology and is also used as a laxative due to its fiber content.

Heteropolysaccharides (Heteroglycans)

Heteropolysaccharides are composed of different monosaccharide units and often include additional functional groups.

Mucopolysaccharides (Glycosaminoglycans)

Mucopolysaccharides, now often referred to as glycosaminoglycans (GAGs), are composed of repeating disaccharide units, typically containing an amino sugar and an uronic acid. They play significant roles in various biological functions.

1. Hyaluronic Acid
   • Structure: A non-sulfated GAG found in various connective tissues.
   • Properties:
     • Acts as a lubricant and shock absorber in joints and maintains tissue hydration.
   • Biomedical Importance:
     • Hyaluronic acid is involved in tissue repair and inflammation processes. It can enhance the absorption of injected substances in clinical settings.
2. Keratan Sulfate
   • Structure: A sulfated GAG found in cartilage, cornea, and other connective tissues.
   • Biomedical Importance:
     • Plays a crucial role in maintaining tissue structure and function, particularly in the cornea.
3. Chondroitin Sulfates
   • Structure: Principal GAGs in cartilage that consist of alternating units of glucuronic acid and N-acetylgalactosamine.
   • Biomedical Importance:
     • Chondroitin sulfate supports cartilage structure and is often used in supplements for joint health.
4. Heparin
   • Structure: A highly sulfated GAG that acts as an anticoagulant.
   • Biomedical Importance:
     • Heparin is crucial in preventing blood clot formation and is commonly used in medical settings to manage various conditions.

Proteoglycans: Chemistry and Functions

Chemistry

Proteoglycans are complex molecules composed of a core protein linked to glycosaminoglycans (GAGs). They can vary significantly in composition, with some containing a high percentage of carbohydrate content, sometimes up to 95%.

• Glycosaminoglycan Types: The GAGs involved can include hyaluronic acid, keratan sulfate, chondroitin sulfate, and heparin, among others.
• Structure: The core protein serves as a scaffold for the attachment of GAG chains, forming a versatile structure that can interact with various biological molecules.

Functions of Proteoglycans

1. Extracellular Matrix Component: Proteoglycans are essential components of the extracellular matrix (ECM), where they interact with collagen and elastin to provide structural support and stability to tissues.
2. Osmotic Regulation: Due to their polyanionic nature, GAGs within proteoglycans can attract cations (such as Na⁺ and K⁺) and water, contributing to tissue hydration and turgor.
3. Barrier Function: In tissues, proteoglycans like hyaluronic acid provide a barrier that allows nutrients to pass while preventing the entry of pathogens and other harmful agents.
4. Lubrication: Hyaluronic acid is particularly important in synovial fluid, acting as a lubricant to reduce friction in joint movement.
5. Hormone Release: Proteoglycans participate in the release of hormones from secretory granules, influencing various physiological processes.
6. Cell Migration and Morphogenesis: In embryonic tissues, proteoglycans facilitate cell migration during development and wound healing.
7. Glomerular Filtration: Proteoglycans in the kidney’s glomerular basement membrane are crucial for selective filtration of blood, playing a role in the kidney’s function.
8. Anticoagulant Properties: Heparin is utilized in clinical settings as an anticoagulant, enhancing the activity of antithrombin III and preventing thrombus formation.
9. Receptor Activity: Proteoglycans on cell membranes can act as receptors for various ligands, contributing to cell signaling and communication.
10. Tissue Compressibility: In cartilage, proteoglycans such as chondroitin sulfate and hyaluronic acid contribute to the compressibility and elasticity of the tissue, allowing it to withstand mechanical stress.

Clinical Implications

Understanding the biochemical roles of carbohydrates, particularly polysaccharides and proteoglycans, is crucial for nurses and healthcare professionals. Disorders related to carbohydrate metabolism, such as mucopolysaccharidoses, underscore the importance of recognizing these compounds’ physiological and pathological roles.

• Mucopolysaccharidoses: This group of inherited disorders arises from enzyme deficiencies that lead to the accumulation of glycosaminoglycans in tissues, resulting in various clinical manifestations, including skeletal abnormalities, mental retardation, and organ dysfunction.

Conclusion

The study of carbohydrates, particularly polysaccharides and their derivatives, is essential in nursing practice. Knowledge of their structure, function, and biomedical importance allows healthcare providers to better understand patient nutrition, metabolism, and the implications of various diseases. This comprehensive understanding can enhance patient care and foster effective communication within healthcare teams.

References

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