Carbohydrates: Structure, Stereochemistry, Cyclization, and Biological Roles

Carbohydrates: Structure and Classification

Definition and General Properties

Carbohydrates are polyhydroxy aldehydes or ketones, with the general formula Cn(H2O)n. They are produced from CO2 and H2O via photosynthesis in plants and serve as energy sources, structural components, and informational molecules in cell signaling. Carbohydrates can be covalently linked to proteins and lipids.

• Classification: Based on the number of carbon atoms (triose, tetrose, pentose, hexose, etc.) and the type of carbonyl group (aldose or ketose).

• Functional Groups: All carbohydrates initially contain a carbonyl group, either as an aldehyde (aldose) or a ketone (ketose).

Examples of Simple Carbohydrates

• Glyceraldehyde: The simplest aldose (triose).

• Dihydroxyacetone: The simplest ketose (triose).

Stereochemistry of Carbohydrates

Chirality and Isomerism

Carbohydrates often contain chiral centers, leading to multiple stereoisomers. The D- and L- forms are distinguished by their ability to rotate polarized light: D (dextrorotatory) rotates light clockwise, L (levorotatory) rotates light counterclockwise. Most natural sugars are D-isomers.

• Enantiomers: Mirror-image isomers, opposite at all chiral centers.

• Diastereomers: Stereoisomers that are not mirror images, differ at some but not all chiral centers.

• Epimers: Diastereomers that differ at only one chiral center.

Epimers of Glucose

D-mannose and D-galactose are epimers of D-glucose, differing at C-2 and C-4 respectively. Epimers are not mirror images and have distinct physical properties.

Physical Properties and Reactivity of Carbohydrates

Reducing Properties

Aldehydes and ketones are strong reductants, and thus aldoses and ketoses are generally reducing sugars. However, their reducing ability is less than expected due to their structure. Carbonyl groups absorb UV and IR light, but carbohydrate solutions lack strong absorbance bands in these regions.

Cyclization and Glycosidic Bond Formation

Hemiacetals and Hemiketals

Carbohydrates contain both carbonyl and alcohol groups, allowing intramolecular reactions to form hemiacetals (from aldoses) and hemiketals (from ketoses). This reactivity forms the basis for sugar cyclization.

Cyclization of Monosaccharides

Pentoses and hexoses readily cyclize, creating a new chiral center called the anomeric carbon. The position of the hydroxyl group on the anomeric carbon determines the α or β anomer.

• α-anomer: Hydroxyl group opposite to the CH2OH moiety.

• β-anomer: Hydroxyl group on the same side as the CH2OH moiety.

Mutarotation

When glucose is dissolved in water, the optical rotation changes as equilibrium is reached between α, β, and linear forms. This phenomenon is called mutarotation.

Polymerization and Glycosidic Bonds

Formation of Glycosidic Bonds

Monosaccharides can react via their hemiacetal/hemiketal and hydroxyl groups to form glycosidic bonds, resulting in disaccharides and polysaccharides. The glycosidic bond is more stable and less reactive than the original hemiacetal/hemiketal.

• Reducing disaccharides: One free hemiacetal/hemiketal group remains, retaining some reducing ability.

• Non-reducing disaccharides: Both anomeric carbons are involved in the bond, no reducing ends.

Polysaccharides: Structure and Function

Types of Polysaccharides

Polysaccharides are natural carbohydrate polymers, classified as homopolysaccharides (one repeating monomer) or heteropolysaccharides (multiple monomers). They can be linear or branched, and their molecular weight is not defined due to dynamic polymerization.

Starch and Glycogen

• Starch: Main storage polysaccharide in plants, composed of amylose (unbranched, α1→4) and amylopectin (branched, α1→6 every 24–30 residues).

• Glycogen: Main storage polysaccharide in animals, branched (α1→6 every 8–12 residues).

Cellulose

Cellulose is a linear homopolysaccharide of glucose with β1→4 linkages. Hydrogen bonds between monomers and chains provide structural strength and water insolubility, making cellulose the most abundant polysaccharide in nature.

Chitin

Chitin is a linear homopolysaccharide of N-acetylglucosamine, with β1→4 linkages. It forms tough, flexible, water-insoluble fibers found in fungal cell walls and arthropod exoskeletons.

Agar and Agarose

Agar is a branched heteropolysaccharide from seaweeds, used in bacterial culture media. Agarose, a component of agar, forms gels for DNA electrophoresis. Agarose consists of D-galactose and 3,6-anhydro-L-galactose units joined by alternating β1→4 and α1→3 bonds.

Table: Structures and Roles of Some Polysaccharides

Polysaccharide	Type	Repeating Unit	Size	Role/Significance
Starch (Amylose/Amylopectin)	Homo	α1→4 Glc, α1→6 branches	50–5,000 / up to 106	Energy storage in plants
Glycogen	Homo	α1→4 Glc, α1→6 branches	Up to 50,000	Energy storage in animals
Cellulose	Homo	β1→4 Glc	Up to 15,000	Structural in plants
Chitin	Homo	β1→4 GlcNAc	Very large	Structural in exoskeletons
Dextran	Homo	α1→6 Glc, α1→3 branches	Wide range	Structural in bacteria
Peptidoglycan	Hetero	Mur2Ac(β1→4)GlcNAc	Very large	Structural in bacteria
Agarose	Hetero	D-Gal(β1→4)3,6-anhydro-L-Gal(α1)	1,000	Structural in algae
Hyaluronan	Hetero	GlcA(β1→3)GlcNAc(β1)	Up to 100,000	Structural in vertebrates

Monosaccharide Derivatives and Glycoconjugates

Monosaccharide Derivatives

Carbohydrates are highly functionalized due to their hydroxyl groups. Important derivatives include deoxyribose (DNA), glucosamine (dietary supplement), and various hexose derivatives involved in biological processes.

Glucosamine

Glucosamine is a functionalized form of glucose, with an amine substituting a hydroxyl group. It is commonly used as a dietary supplement and forms linkers between carbohydrates and proteins/lipids.

Glycoconjugates: Glycolipids and Glycoproteins

Glycolipids are lipids with covalently bound oligosaccharides, important in cell signaling and recognition. Glycoproteins are proteins with attached oligosaccharides, playing roles in protein-protein recognition and immune evasion by viruses.

Extracellular Matrix (ECM)

Structure and Function

The ECM is a structural material outside cells, providing strength, elasticity, and a physical barrier in tissues. Its main components are proteoglycan aggregates, collagen fibers, and elastin. ECM acts as a barrier to tumor cell invasion.

Summary

• Structures and properties of important monosaccharides

• Monosaccharide cyclization and mutarotation

• Glycosidic bond formation and types of disaccharides

• Structures and biological roles of key polysaccharides and sugar derivatives