Carbohydrates Note M.N Chatterjea Monosaccharides, the simplest form of carbohydrates, play crucial roles in biological processes. They serve as building blocks for larger carbohydrate molecules and are vital for energy production, cellular signaling, and metabolic pathways.
Trioses
D-Glyceraldehyde and Dihydroxyacetone are the two main trioses. These three-carbon sugars serve as intermediates in glycolysis, a fundamental metabolic pathway that converts glucose into energy. Trioses are also precursors to glycerol, a component that gets incorporated into various lipids, thus playing a critical role in membrane structure and energy storage.
Tetroses
Erythrose-4-Phosphate is a significant tetrose that occurs as an intermediate in the hexose monophosphate (HMP) shunt, an alternative pathway for glucose oxidation. This pathway is essential for generating NADPH, which is crucial for anabolic reactions and antioxidant defense.
Pentoses
Pentoses are five-carbon sugars with several biological functions:
- D-Ribose: A vital component of RNA, ribonucleotides, and certain coenzymes like FAD, NAD, and Coenzyme A, making it critical for nucleic acid synthesis and energy metabolism.
- D-2-Deoxyribose: A key constituent of DNA, allowing for genetic information storage.
- D-Ribulose and D-Xylulose: These ketopentoses occur as intermediates in the HMP shunt, further emphasizing their metabolic importance.
- L-Xylulose: A metabolite of D-Glucuronic acid, which is excreted in the urine of individuals with pentosuria, a hereditary metabolic disorder.
- L-Fucose: A methyl pentose found in glycoproteins, contributing to cellular recognition and signaling processes.
- D-Lyxose: Found in lyxoflavin, isolated from human heart muscle, although its specific function remains unclear.
Hexoses
- D-Glucose (Dextrose, Grape Sugar):
- The primary sugar found in the bloodstream, maintaining a constant concentration of about 0.1%.
- It serves as the main energy source for all tissues, particularly crucial for erythrocytes and brain cells, which rely exclusively on glucose.
- Stored as glycogen in the liver and muscles.
- Exhibits mutarotation, altering its optical rotation when dissolved in water.
- D-Galactose:
- Rarely found free; mainly exists as a component of lactose (milk sugar) and in galactolipids and glycoproteins.
- An epimer of glucose, differing at carbon-4 in orientation.
- Less sweet and soluble than glucose, it is dextrorotatory and also shows mutarotation.
- Oxidized with hot HNO3 to yield mucic acid, a unique identifier due to its characteristic crystal shape.
- D-Fructose (Fruit Sugar):
- A ketohexose that is sweeter than sucrose and commonly found in fruits.
- Forms part of sucrose and inulin, exhibiting mutarotation.
- Seminal fluid is rich in fructose, providing energy for sperm cells.
- D-Mannose:
- Not found free in nature but exists as mannan in polysaccharides.
- It is a constituent of glycoproteins, emphasizing its importance in cell signaling and recognition.
- Sedoheptulose:
- A ketoheptose present in certain plants and involved in the HMP shunt as an intermediate, highlighting its role in photosynthesis.
Important Properties of Monosaccharides
Iodo Compounds
When an aldose is heated with concentrated HI, it loses all oxygen atoms and is transformed into an iodo compound. This reaction demonstrates that the resulting derivative has a straight-chain structure, indicating the absence of branched chains in glucose.
Acetylation or Ester Formation
Monosaccharides can react with organic acids to form sugar esters. Acetylation, for example, occurs when monosaccharides react with acetyl chloride (CH₃COCl), leading to the formation of acetylated derivatives. This property is significant as it indicates the presence of hydroxyl (-OH) groups.
Osazone Formation
Osazones are crystalline derivatives of sugars that can be used for identification purposes. The process involves reacting sugars with phenylhydrazine, resulting in the formation of distinct crystals:
- Glucosazone: Yellow needles arranged in fan-like aggregates, melting point at 204–205°C. Similar structures are formed by glucose, mannose, and fructose.
- Lactosazone: Characterized by irregular clusters of fine needles.
- Maltosazone: Star-shaped crystals resembling sunflower petals.
These distinct characteristics are valuable in identifying different sugars.
Interconversion of Sugars
Monosaccharides can undergo interconversion in weak acid or base solutions, particularly glucose, fructose, and mannose. This process contributes to the dynamic nature of sugar metabolism.
Oxidation to Produce Sugar Acids
Under oxidation conditions, aldoses can yield various types of sugar acids, including:
- Monobasic Aldonic Acids: Resulting from the oxidation of the aldehyde group.
- Dibasic Saccharic Acids: Formed through further oxidation.
- Monobasic Uronic Acids: Containing aldehyde groups, maintaining their reducing properties.
Biomedical Importance of D-Glucuronic Acid: D-Glucuronic acid is synthesized from glucose in the liver and plays a vital role in detoxification by conjugating with various substances, rendering them water-soluble for excretion.
Reduction of Sugars to Form Sugar Alcohols
Sugars can be reduced to their corresponding alcohols using reducing agents like Na-amalgam. Examples include:
- D-Glucose to D-Sorbitol
- D-Galactose to D-Dulcitol
- D-Mannose to D-Mannitol
- D-Fructose yielding both D-Mannitol and D-Sorbitol
These sugar alcohols have practical applications, including their use in microbiological identification.
Action of Acids on Carbohydrates
Polysaccharides and compound carbohydrates are hydrolyzed into monosaccharides when boiled with dilute mineral acids (0.5 to 1.0 N, such as HCl or H₂SO₄). However, concentrated acids can decompose monosaccharides.
- Pentoses: Yield cyclic aldehyde “furfural” upon hydrolysis.
Action with Alkalies
Monosaccharides can react with alkalies in the following ways:
- In dilute alkali: Monosaccharides convert to cyclic α and β forms, establishing an equilibrium between the two isomeric forms.
- In concentrated alkali: Monosaccharides caramelize, leading to the formation of decomposition products and pigments.
Reducing Action of Sugars in Alkaline Solution
Sugars with free sugar groups undergo enolization and other transformations in alkaline solutions. The resulting enediol forms are highly reactive and easily oxidized, leading to the formation of sugar acids.
Other Sugar Derivatives of Biomedical Importance
Deoxy Sugars
Deoxy sugars lack the oxygen of a hydroxyl group, resulting in a modified structure. These sugars have various physiological roles, contributing to the structure and function of nucleic acids and glycoproteins.
Amino Sugars (Hexosamines)
Amino sugars contain an amine group (-NH₂). These sugars are essential for the synthesis of glycoproteins and glycolipids, influencing cell signaling and recognition.
Amino Sugar Acids
Neuraminic Acid: This amino sugar acid serves as a key component in the structure of sialic acids, which are important for cell recognition and signaling.
Glycosides
Glycosides consist of a carbohydrate moiety linked to a non-carbohydrate residue (aglycone). They are named based on the carbohydrate component:
- Glucosides: Containing glucose
- Galactosides: Containing galactose
Glycosides play vital roles in various biological processes, including cellular signaling and plant defense mechanisms.
Conclusion
Understanding the biological importance and chemical properties of carbohydrates, particularly monosaccharides, is crucial for nurses. This knowledge aids in nutritional assessments, patient education, and the management of conditions like diabetes. As carbohydrate metabolism plays a significant role in overall health, nurses must be equipped with the information necessary to support their patients effectively.
References
- Chatterjea, M.N. (8th Edition). “The Text Book of Medical Biochemistry.”