Carbohydrates

Definition and General Characteristics

Basic Definition

Carbohydrates: organic compounds with formula (CH2O)n. Composed of carbon, hydrogen, oxygen. Serve as energy source, structural components.

General Properties

Hydrophilic due to multiple hydroxyl groups. Soluble in water. Exhibit optical activity. Form ring and open-chain structures.

Molecular Formula and Elements

Empirical formula: Cn(H2O)n. Elements: C, H, O. Variations exist with derivatives like deoxy sugars.

Classification of Carbohydrates

Monosaccharides

Simple sugars, single polyhydroxy aldehyde or ketone unit. Examples: glucose, fructose.

Oligosaccharides

2-10 monosaccharide units linked. Common forms: disaccharides, trisaccharides.

Polysaccharides

Long chains of monosaccharides. Homopolysaccharides or heteropolysaccharides. Examples: starch, cellulose.

Based on Carbon Number

Trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7C).

Monosaccharides: Structure and Properties

Functional Groups

Contain aldehyde (aldoses) or ketone (ketoses) groups. Multiple hydroxyl groups.

Ring and Open-Chain Forms

Exist in equilibrium between linear and cyclic hemiacetal/hemi-ketal forms. Pyranose (6-membered), furanose (5-membered) rings common.

Examples and Importance

Glucose: primary energy source. Fructose: fruit sugar. Ribose: RNA backbone.

Optical Isomerism

Chiral centers lead to stereoisomers. D- and L- forms defined by Fischer projections.

Mutarotation

Interconversion between α and β anomers in solution. Alters optical rotation.

Glucose equilibrium:α-D-glucopyranose ⇌ open-chain glucose ⇌ β-D-glucopyranose

Disaccharides: Formation and Types

Glycosidic Bond Formation

Condensation reaction between hydroxyl groups of two monosaccharides. Releases water molecule.

Common Disaccharides

Sucrose: glucose + fructose (α1→β2). Lactose: glucose + galactose (β1→4). Maltose: glucose + glucose (α1→4).

Reducing vs Non-Reducing Sugars

Reducing sugars: free anomeric carbon. Non-reducing: glycosidic bond involves anomeric carbons on both monosaccharides.

Hydrolysis

Enzymatic or acid catalyzed cleavage of glycosidic bond. Yields monosaccharides.

Polysaccharides: Structure and Function

Homopolysaccharides

Single type monosaccharide repeated. Starch (α-glucose), cellulose (β-glucose).

Heteropolysaccharides

Multiple monosaccharide types. Examples: hyaluronic acid, chondroitin sulfate.

Branching Patterns

Linear or branched chains. Amylopectin branched (α1→6), glycogen highly branched.

Biological Roles

Energy storage (starch, glycogen). Structural support (cellulose, chitin).

Stereochemistry and Isomerism

Chirality in Carbohydrates

Chiral centers: usually all except carbonyl carbon. Determine D- or L- configuration based on C-atom furthest from carbonyl.

Enantiomers and Diastereomers

Enantiomers: mirror-image stereoisomers. Diastereomers: non-mirror-image stereoisomers (epimers, anomers).

Anomers

Isomers differing at the anomeric carbon in cyclic form: α and β forms.

Epimers

Differ in configuration at a single chiral center other than anomeric carbon.

Example:D-Glucose ↔ D-Mannose (C-2 epimer)D-Glucose ↔ D-Galactose (C-4 epimer)

Glycosidic Bonds and Linkages

Definition

Covalent bond formed between anomeric carbon of a sugar and hydroxyl group of another molecule.

Types of Glycosidic Linkages

α-glycosidic and β-glycosidic linkages. Influence polysaccharide properties.

Bond Formation Mechanism

Condensation reaction catalyzed by glycosyltransferases. Water released.

Importance in Polysaccharide Structure

Determines digestibility, solubility, and biological function.

Chemical Reactions of Carbohydrates

Oxidation

Aldehyde groups oxidized to carboxylic acids (aldonic acids). Tollens' and Benedict's tests detect reducing sugars.

Reduction

Conversion of aldehyde/ketone to alcohol groups forming sugar alcohols (e.g., sorbitol).

Isomerization

Conversion between aldoses and ketoses under basic conditions (Lobry de Bruyn–Alberda van Ekenstein transformation).

Esterification and Etherification

Hydroxyl groups can form esters and ethers, modifying properties.

Biological Functions of Carbohydrates

Energy Source and Storage

Glucose oxidation yields ATP. Storage as glycogen (animals) and starch (plants).

Structural Components

Cellulose in plant cell walls. Chitin in fungal and arthropod exoskeletons.

Cell Signaling and Recognition

Glycoproteins and glycolipids mediate cell-cell interactions, immune response.

Metabolic Intermediates

Precursors in biosynthesis of nucleotides, amino acids, and vitamins.

Carbohydrate Metabolism

Glycolysis

Glucose breakdown to pyruvate. ATP and NADH production. Cytosolic pathway.

Gluconeogenesis

Formation of glucose from non-carbohydrate precursors. Occurs in liver and kidney.

Glycogenesis and Glycogenolysis

Glycogen synthesis and breakdown. Regulated by insulin and glucagon.

Pentose Phosphate Pathway

Generates NADPH and ribose-5-phosphate for anabolic reactions.

Glycolysis overview:Glucose → Glucose-6-phosphate → Fructose-6-phosphate → Fructose-1,6-bisphosphate → ... → Pyruvate + ATP + NADH

Industrial and Medical Applications

Food Industry

Sweeteners (sucrose, fructose), thickeners (starch), dietary fibers.

Pharmaceuticals

Drug delivery systems, vaccine adjuvants, anticoagulants (heparin).

Biofuel Production

Fermentation of carbohydrates to ethanol and biogas.

Diagnostics

Glucose monitoring in diabetes, biochemical assays.

Analytical Methods and Techniques

Chromatography

HPLC, GC for separation and identification of carbohydrates.

Spectroscopy

NMR for structure elucidation. IR to identify functional groups.

Mass Spectrometry

Molecular weight determination, structural analysis.

Colorimetric Tests

Benedict’s, Barfoed’s, and Seliwanoff’s tests for sugar detection.

References

• Lehninger, A.L., Nelson, D.L., Cox, M.M. Principles of Biochemistry. 7th ed., W.H. Freeman, 2017, pp. 170-220.
• Voet, D., Voet, J.G. Biochemistry. 4th ed., Wiley, 2011, pp. 450-490.
• McMurry, J. Organic Chemistry. 9th ed., Cengage Learning, 2015, pp. 630-670.
• Whitfield, C. Biosynthesis and Assembly of Capsular Polysaccharides in Escherichia coli. Annual Review of Biochemistry, vol. 75, 2006, pp. 39-68.
• Esselen, M., et al. Carbohydrate Chemistry and Biochemistry: Structure and Mechanism. Royal Society of Chemistry, 2015, pp. 1-90.