Unit 2: Carbohydrates
Table of Contents
1. Monosaccharides: Structure, Properties, and Function
a) Structure
Monosaccharides (simple sugars) are the simplest carbohydrates. They are polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses). The general formula is Cn(H₂O)n.
- Classification by Carbon Number: Triose (3C), Tetrose (4C), Pentose (5C), Hexose (6C).
- Classification by Functional Group:
- Aldose: Contains an aldehyde group (e.g., Glucose, Galactose, Ribose).
- Ketose: Contains a ketone group (e.g., Fructose).
- Ring Structures: In solution, pentoses and hexoses form stable ring structures (Haworth projections) by reacting the carbonyl group with a hydroxyl group. This creates a new chiral center at the anomeric carbon (C1 for aldoses, C2 for ketoses), resulting in α and β anomers.
Formation of α-D-Glucose and β-D-Glucose ring structures from linear D-Glucose.
b) Properties
- Isomerism: They exhibit various forms of isomerism, including:
- Epimers: Isomers that differ in configuration at only *one* chiral center (e.g., Glucose and Galactose are C4 epimers).
- Anomers: Isomers that differ at the anomeric carbon (α vs. β).
- Reducing Sugars: Monosaccharides with a free anomeric carbon (all of them) can act as reducing agents. They can be detected by Benedict's test, which produces a brick-red precipitate.
c) Function
- Primary Energy Source: Glucose is the main fuel for cells.
- Building Blocks: Monomers for complex carbohydrates (disaccharides, polysaccharides).
- Metabolic Intermediates: e.g., in glycolysis and the pentose phosphate pathway.
2. Disaccharides: Structure, Properties, and Function
a) Structure
Disaccharides consist of two monosaccharide units joined by a glycosidic bond, which is formed via a dehydration reaction.
Common examples:
- Sucrose (Table Sugar) = Glucose + Fructose (joined by an α-1,β-2 glycosidic bond)
- Lactose (Milk Sugar) = Galactose + Glucose (joined by a β-1,4 glycosidic bond)
- Maltose (Malt Sugar) = Glucose + Glucose (joined by an α-1,4 glycosidic bond)
b) Properties
- Reducing vs. Non-reducing:
- A disaccharide is reducing if it has a free anomeric carbon (e.g., Lactose, Maltose).
- It is non-reducing if the anomeric carbons of *both* units are involved in the glycosidic bond (e.g., Sucrose).
c) Function
- Energy: Easily hydrolyzed to monosaccharides for quick energy.
- Transport: Sucrose is the main form in which sugar is transported in plants.
3. Polysaccharides: Structure, Properties, and Function
Polysaccharides are long-chain polymers of monosaccharides (or their derivatives) linked by glycosidic bonds. They can be linear or branched.
Their properties (e.g., solubility, digestibility) depend on the monomer, the type of glycosidic bond (α or β), and the degree of branching.
4. Homo- and Hetero-polysaccharides
a) Homopolysaccharides (Homoglycans)
Composed of only one type of monosaccharide unit. They are primarily used for energy storage or structural support.
| Polysaccharide | Monomer Unit | Glycosidic Bond(s) | Function |
|---|---|---|---|
| Starch | Glucose | α-1,4 (linear) and α-1,6 (branching) | Energy storage in plants. (Made of Amylose and Amylopectin) |
| Glycogen | Glucose | α-1,4 (linear) and α-1,6 (highly branched) | Energy storage in animals. (Mainly in liver and muscle) |
| Cellulose | Glucose | β-1,4 (linear) | Structural component of plant cell walls. (Humans cannot digest β-1,4 bonds). |
b) Heteropolysaccharides (Heteroglycans)
Composed of two or more different types of monosaccharide units or their derivatives (e.g., amino sugars). They often provide structural support or are involved in cell recognition.
5. Mucopolysaccharides (Glycosaminoglycans - GAGs)
Mucopolysaccharides, now more commonly called Glycosaminoglycans (GAGs), are a major class of heteropolysaccharides.
Structure and Properties
- They are long, unbranched chains made of repeating disaccharide units.
- One unit is an amino sugar (like N-acetylglucosamine) and the other is usually an uronic acid (like glucuronic acid).
- They are highly polyanionic (negatively charged) due to sulfate (—SO₃⁻) and carboxyl (—COO⁻) groups.
- This high negative charge attracts water, making them excellent lubricants and shock absorbers.
Examples and Functions
- Hyaluronic Acid: Found in synovial (joint) fluid, vitreous humor of the eye. Acts as a lubricant and shock absorber.
- Chondroitin Sulfate: The most abundant GAG. Found in cartilage, bone, and tendons, providing tensile strength.
- Heparin: A natural anticoagulant. Binds to antithrombin, preventing blood clotting.
6. Glycoproteins and their Biological Functions
a) Structure
Glycoproteins are proteins that have one or more carbohydrate chains (oligosaccharides) covalently attached to them. This process is called glycosylation.
The carbohydrate part is typically short, branched, and diverse.
- N-linked: The carbohydrate is attached to the nitrogen atom of an Asparagine (Asn) residue.
- O-linked: The carbohydrate is attached to the oxygen atom of a Serine (Ser) or Threonine (Thr) residue.
- Glycoproteins: Mostly protein, small carbohydrate part.
- Proteoglycans: A subclass of glycoproteins with GAG chains. They are mostly carbohydrate, small protein part.
b) Biological Functions
The carbohydrate portion of glycoproteins is crucial for many functions, especially in cell-to-cell communication and interactions.
- Cell-Surface Recognition: The oligosaccharides on the cell surface (the glycocalyx) act as "ID tags."
- ABO Blood Groups: The A, B, and O blood types are determined by different oligosaccharides on the surface of red blood cells.
- Cell Adhesion: Proteins called selectins (a type of glycoprotein) mediate cell-cell adhesion, for example, in the immune response.
- Protein Folding and Stability: The addition of carbohydrates can help a protein fold correctly and protect it from degradation by proteases.
- Hormones: Many hormones are glycoproteins (e.g., TSH - Thyroid-stimulating hormone, FSH - Follicle-stimulating hormone).
- Immune System: Antibodies (Immunoglobulins) are glycoproteins.