Biochemistry - Lesson 7: Carbohydrate Structure and Function
Carbohydrates are a diverse group of biomolecules essential for energy storage, structural integrity, and cellular recognition. They range from simple monosaccharides to complex polysaccharides with distinct chemical and physical properties. This lesson explores monosaccharide stereochemistry, disaccharide formation, and the structural and functional roles of polysaccharides, highlighting how variations in glycosidic linkages impact functionality.
Monosaccharides and Stereochemistry
Monosaccharides (simple sugars) typically follow the empirical formula (CH2O)n. They can be classified by:
- Number of carbons (triose, tetrose, pentose, hexose)
- Aldose vs. ketose (presence of an aldehyde or ketone group)
- Stereochemistry (D or L designation, based on the chiral carbon farthest from the carbonyl)
Fischer projections depict monosaccharides in linear form, while ring (Haworth) projections reveal their common cyclic structures in solution. During cyclization, the carbonyl group reacts with a hydroxyl group, forming either a pyranose (6-membered ring) or a furanose (5-membered ring). The newly generated chiral center at the carbonyl carbon creates α or β anomers.
Cyclic Forms and Anomers
In ring form, the α-anomer places the anomeric OH opposite the ring substituent (e.g., OH down in a Haworth projection if the reference substituent is up), while the β-anomer places it on the same side. These anomers interconvert in solution (mutarotation), influencing sugar reactivity and crystallization behaviors.
Disaccharides
Disaccharides arise from a condensation reaction between the anomeric carbon of one monosaccharide and a hydroxyl group on another, forming a glycosidic bond. Three common examples:
| Name | Monosaccharide Units | Glycosidic Linkage | Key Characteristics |
|---|---|---|---|
| Sucrose | Glucose + Fructose | α(1→2)β | Common table sugar, non-reducing |
| Lactose | Galactose + Glucose | β(1→4) | Milk sugar, can be hydrolyzed by lactase |
| Maltose | Glucose + Glucose | α(1→4) | Product of starch breakdown, reducing sugar |
Whether a disaccharide is reducing or non-reducing depends on whether the anomeric carbon is free (capable of opening to the linear form) or involved in the glycosidic bond. Non-reducing sugars (e.g., sucrose) lack a free anomeric hydroxyl.
Polysaccharides
Polysaccharides can be homopolymers (repeating one type of monosaccharide) or heteropolymers (multiple monosaccharides). Their biological roles include:
- Energy Storage: Starch in plants (amylose + amylopectin) and glycogen in animals. Both generally use α(1→4) main-chain linkages, with α(1→6) branches in amylopectin and glycogen.
- Structural Support: Cellulose in plant cell walls (β(1→4) glucose) forms tough fibers. Chitin in arthropod exoskeletons incorporates N-acetylglucosamine residues.
- Modified Roles: Peptidoglycan in bacterial walls (alternating N-acetylglucosamine and N-acetylmuramic acid) ensures rigidity. Glycosaminoglycans in extracellular matrices provide hydration and resilience.
The difference between α and β linkages drastically alters physical properties—cellulose’s β(1→4) linkages promote a rigid, extended conformation, while starch’s α linkages favor helices suitable for compact energy storage.
Polysaccharide Summary
| Name | Monomer Unit | Main Linkage | Function | Occurrence |
|---|---|---|---|---|
| Starch | Glucose | α(1→4), α(1→6) branches | Energy storage | Plants (seeds, tubers) |
| Glycogen | Glucose | α(1→4), frequent α(1→6) branches | Energy storage | Animals (liver, muscle) |
| Cellulose | Glucose | β(1→4) | Structural support | Plant cell walls |
| Chitin | N-acetylglucosamine | β(1→4) | Structural support | Arthropod exoskeleton, fungal cell walls |
Carbohydrates in Nucleotides and Cell Recognition
Ribose and deoxyribose (5-carbon sugars) form the backbone of RNA and DNA nucleotides, respectively. Their ring forms (furanoses) attach to phosphate groups and nitrogenous bases, shaping the genetic material’s structure and function.
Complex carbohydrates extend beyond just polymers of glucose. Glycoproteins and glycolipids contain covalently bound oligosaccharides, contributing to cell surface architecture. These carbohydrate moieties enable:
- Cell Recognition and Signaling: ABO blood group antigens differ by terminal sugar residues on red blood cells.
- Immune Responses: Pathogen recognition by lectins, modulated by sugar epitopes.
- Protein Targeting and Clearance: Specific sugar residues on glycoproteins direct them to lysosomes or mark them for degradation.
Summary
Carbohydrates range from single-ring monosaccharides to complex branched polysaccharides, each fulfilling distinct roles—from quick fuel to robust scaffolding and molecular recognition tags. Their structural variety arises from stereochemical configurations, anomeric forms, and diverse glycosidic linkages. In addition to being vital energy stores (starch, glycogen), carbohydrates underpin plant cell wall strength (cellulose) and insect exoskeletons (chitin), as well as guiding cell interactions via glycoproteins and glycolipids. Understanding carbohydrate chemistry is fundamental for grasping metabolism, physiology, and cell communication.
Suggested Reading:
Lehninger Principles of Biochemistry (chapters covering carbohydrates and polysaccharides)
Biochemistry by Berg, Tymoczko, and Stryer (sections on sugar structure, glycosidic bonds, and cell-surface carbohydrates)
