Carbohydrates: Overview

Monosaccharides

Structural Representations

  • Open-chain Monosaccharides:

    • Fischer Projection Formula: Example of D-Glyceraldehyde.

    • Chiral Carbons: Describes carbons that have four different substituents leading to stereoisomers.

    • Enantiomers: Two compounds that are mirror images of each other, such as D-Glyceraldehyde and its enantiomer.

    • Isomers: Occur due to the arrangement of hydroxyl (−OH) groups.

    • Types of Monosaccharides:

    • Aldoses (Containing an aldehyde group)

      • Example: D-Glyceraldehyde (Aldotriose)

    • Ketoses (Containing a ketone group)

      • Example: Dihydroxyacetone (Ketotriose).

Stereochemistry

  • Chirality in Solution:

    • Light source (Polarizer) used to analyze chiral compounds.

    • Epimers: Stereoisomers differing at a single carbon atom, e.g., D-Mannose and D-Glucose differing at C-2.

    • Categories of Sugars:

    • Trioses (3 carbons)

    • Tetroses (4 carbs)

    • Pentoses (5 carbs)

    • Hexoses (6 carbs).

Ring Monosaccharides

Formation of Rings

  • Hemiacetal & Hemiketal Formation:

    • Result from nucleophilic attack of hydroxyl groups on carbonyl carbons.

    • Leads to formation of an asymmetric anomeric carbon.

    • Types of Cyclic Structures:

    • Furanose: 5-membered ring.

    • Pyranose: 6-membered ring.

Cyclisation and Mutarotation

  • Occurs in aqueous solutions leading to equilibrium between open-chain and ring forms.

  • Importance of the carbonyl oxygen forming a hydroxyl group and its position determine whether the anomer is α or β.

  • Reducing Sugars: All monosaccharides are reducing sugars due to their free anomeric carbon; some disaccharides like maltose and lactose are also reducing sugars.

  • Fehling’s Test: Assesses the concentration of sugars; involves copper sulfate mixed in alkaline solution which reduces to a carboxylic acid.

Disaccharides

Formation

  • Two monosaccharides condense to form disaccharides via a glycosidic bond.

  • Hydrolysis is the process for breaking down disaccharides, necessitating the removal of the glycosidic bond.

Nomenclature and Structure

  • Key factors for naming:

    1. Configuration of the anomeric carbon.

    2. Identify the non-reducing residue (indicate if it is furanose or pyranose).

    3. Highlight joining carbon atoms via glycosidic bond.

    4. Name second residue, considering its form (furano/pyrano).

  • Types of Glycosidic Bonds:

    • Nonreducing disaccharides are formed with glycosidic bonds between anomeric carbons resulting in two acetal groups.

    • Reducing disaccharides contain a hemiacetal group making them reducing sugars.

Polysaccharides

Composition and Types

  • Composed of several monosaccharides:

    • Homopolysaccharides: Composed of one type of monosaccharide.

    • Heteropolysaccharides: Incorporate different types of sugars (e.g., agar - agarose, peptidoglycan).

  • Notable Polysaccharides:

    • Starch (Beta - storage in plants)

    • Glycogen (Beta - storage in animals)

    • Cellulose (Linear - structural in plants)

    • Chitin (Linear - structural component in fungi and exoskeletons).

Functions and Uses

  • These polysaccharides serve vital roles in biology as:

    • Energy storage (starch and glycogen)

    • Structural components (cellulose and chitin).

  • They exhibit significant commercial and industrial value.

Chemical Reactions Involving Carbohydrates

Cyclisation and Disaccharide Formation

  • Nucleophilic substitution involves a nucleophile attacking a positively charged molecule, replaced by a functional group.

  • Condensation Reactions combine two molecules, forming a glycosidic bond while removing a small molecule (H2O).

Oligosaccharides and Polysaccharides

  • Defined based on the number of monosaccharides linked via glycosidic bonds.

    • Oligosaccharides: Typically consist of 3-15 linked sugars.

    • Polysaccharides: Composed of many linked monosaccharides.

Case Studies in Medicine

Applications of Carbohydrates

1. Rare Genetic Diseases

  • Example: Slc2a1 gene associated with Glut1 deficiency syndrome affecting glucose transport into the brain.

    • A low-carbohydrate ketogenic diet can reduce seizure occurrences.

2. Understanding Viral Infections

  • Heparan sulfate's role in COVID-19 virus binding and entry into host cells as crucial in therapeutic applications.

    • Heparan sulfate proteoglycans bind with angiotensin-converting enzyme 2 enhancing viral entry.

3. Blood Glucose Measurement in Diabetes Diagnosis

  • Various methods evolved:

    • Historical Fehling’s test for reducing sugars.

    • Modern test strips and continuous monitoring devices for glucose from interstitial fluids.

Case Studies in Commercial Value

Uses in Biofuels and Therapeutics

  • Biofuels: Derived from carbohydrates or biomass; for example, fermentation of microbial or plant materials.

  • Dextran: Commercially essential in drug delivery systems, fluorescent markers, and chromatography.

Industrial Applications

  • Carbohydrate-based therapeutics are used for:

    • Cell adhesion, signal transduction, pathogen recognition, inflammation, and stabilization of proteins.

Market Values (2022)

  • Global biofuels market: approx. 117 billion dollars.

  • Global dextran market: approx. 220 million dollars.

  • Global pharmaceutical market: approx. 148 trillion dollars.

Overview of Carbohydrates

Key Topics Covered

  1. Chemistry of sugars and their chiral centers.

  2. Open-chain and ring forms of monosaccharides.

  3. Structures and nomenclature of monosaccharides.

  4. Structures and properties of disaccharides and glycosidic bonds.

  5. Biological functions of homo- and heteropolysaccharides.

Quizzes and Answers

Key Questions

  1. Types of Carbohydrates:

    • Disaccharides are made up of two sugar units.

  2. Identifying Monosaccharides:

    • Fructose is a monosaccharide.

  3. Polymers of Sugars:

    • Starch is a polymer of glucose.

  4. Types of Polysaccharides:

    • Examples include cellulose, which cannot be digested by humans.

  5. Human Metabolism:

    • Carbohydrates are broken down during metabolism.

Additional Questions

  • Various examples related to glycosidic bonds, epimers, reduction agents, and specifications of carbohydrates.