Carbohydrates: Structure, Classification, and Biological Roles

Carbohydrates

Carbohydrates are one of the four major classes of biomolecules and play essential roles in energy storage, structural integrity, and cellular recognition. They are the most abundant organic molecules in nature and are vital for life processes.

General Features of Carbohydrates

• Photosynthesis converts more than 250 billion kg of CO2 into carbohydrates daily.

• Carbohydrates are characterized by:

  • At least one, and often two or more, asymmetric (chiral) centers.

  • Existence in linear or ring structures.

  • Ability to form polymeric structures via glycosidic bonds.

  • Capacity to form multiple hydrogen bonds with water or other molecules, making them highly soluble and interactive.

Classification of Carbohydrates

• Carbohydrates are hydrates of carbon with the general formula $(CH_2O)_n$.

• They are classified into three main groups:

  • Monosaccharides (simple sugars): Cannot be broken down into simpler sugars by hydrolysis.

  • Oligosaccharides: Composed of 2 to 10 monosaccharide units linked by glycosidic bonds.

  • Polysaccharides: Polymers containing more than 10 monosaccharide units; can be hundreds or thousands of units long.

Carbohydrate Nomenclature and Structure

Naming and Classification

• Carbohydrates are named based on:

  • The number of carbon atoms: triose (3), tetrose (4), pentose (5), hexose (6), etc.

  • The type of carbonyl group:

    • Aldose: Contains an aldehyde group (e.g., glucose).

    • Ketose: Contains a ketone group (e.g., fructose).

• Examples:

  • D-Ribose: A pentose aldose.

  • D-Glucose: A hexose aldose.

  • D-Fructose: A hexose ketose.

Chirality in Carbohydrates

• Monosaccharides with at least three carbons (aldoses) or four carbons (ketoses) have one or more chiral centers.

• Chiral centers give rise to stereoisomers (molecules with the same formula but different spatial arrangements).

• Example: D-Glucose has four chiral centers; D-Fructose has three.

Stereoisomerism in Carbohydrates

• Enantiomers: Non-superimposable mirror images (D- and L- forms).

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

• Anomers: Isomers that differ at the new chiral center formed on ring closure (the anomeric carbon).

Type	Definition	Example
Enantiomers	Non-superimposable mirror images	D-Glucose and L-Glucose
Epimers	Isomers differing at one chiral center	D-Glucose and D-Mannose (C-2 epimers)
Anomers	Isomers differing at the anomeric carbon (C-1 in aldoses)	α-D-Glucose and β-D-Glucose

Fischer and Haworth Projections

• Fischer projection: A two-dimensional representation of three-dimensional organic molecules, used to depict the configuration of carbohydrates.

• Haworth projection: A common way to represent the cyclic structure of monosaccharides.

• In Fischer projections:

  • If the hydroxyl group on the chiral carbon farthest from the carbonyl is on the right, it is a D-sugar; if on the left, it is an L-sugar.

Cyclic Forms and Anomeric Carbons

Formation of Cyclic Structures

• Monosaccharides with five or more carbons commonly form cyclic (ring) structures in aqueous solution.

• Aldoses form hemiacetals and ketoses form hemiketals by reaction of the carbonyl group with a hydroxyl group on the same molecule.

• The carbon that was the carbonyl carbon becomes the anomeric carbon in the ring.

Anomers and Mutarotation

• Two possible configurations at the anomeric carbon: α (alpha) and β (beta).

• α-anomer: The OH group on the anomeric carbon is trans (opposite side) to the CH2OH group.

• β-anomer: The OH group is cis (same side) to the CH2OH group.

• Mutarotation: The interconversion between α and β anomers in solution.

Derivatives and Functions of Monosaccharides

Reducing and Non-Reducing Sugars

• Reducing sugars have a free anomeric carbon and can reduce mild oxidizing agents (e.g., Cu2+, Ag+).

• Non-reducing sugars have their anomeric carbons involved in glycosidic bonds and cannot act as reducing agents.

Common Monosaccharide Derivatives

• Sugar acids: Formed by oxidation of the terminal aldehyde or primary alcohol group.

• Sugar alcohols: Formed by reduction of the carbonyl group (e.g., sorbitol, xylitol).

• Deoxy sugars: One or more hydroxyl groups replaced by hydrogen (e.g., 2-deoxyribose in DNA).

• Amino sugars: One or more hydroxyl groups replaced by an amino group (e.g., glucosamine).

• Phosphate esters: Monosaccharides with phosphate groups, important in metabolism (e.g., glucose-6-phosphate).

Oligosaccharides and Polysaccharides

Oligosaccharides

• Short chains of monosaccharide units (2–10) linked by glycosidic bonds.

• Common examples: Maltose (glucose + glucose), Lactose (glucose + galactose), Sucrose (glucose + fructose).

• Oligosaccharides are important in cell recognition and signaling.

Polysaccharides

• Long chains of monosaccharide units; can be linear or branched.

• Classified as:

  • Homopolysaccharides: Composed of only one type of monosaccharide (e.g., starch, glycogen, cellulose).

  • Heteropolysaccharides: Composed of two or more different monosaccharides (e.g., hyaluronan, chondroitin sulfate).

Storage Polysaccharides

• Starch (plants): Mixture of amylose (linear, α(1→4) linkages) and amylopectin (branched, α(1→6) linkages every 12–30 residues).

• Glycogen (animals): Similar to amylopectin but more highly branched (α(1→6) branches every 8–12 residues); main storage form in animals.

• Both starch and glycogen are energy reserves, rapidly mobilized by enzymatic cleavage.

Structural Polysaccharides

• Cellulose: Linear polymer of β-D-glucose with β(1→4) linkages; forms rigid, insoluble fibers in plant cell walls.

• Chitin: Similar to cellulose but with N-acetylglucosamine units; found in exoskeletons of arthropods and fungal cell walls.

• Agarose: Galactose polymer from red algae, used in laboratory gel electrophoresis.

Glycosaminoglycans (GAGs)

• Linear polymers of repeating disaccharide units, often containing amino sugars and uronic acids.

• Highly negatively charged; function as structural components in connective tissue, synovial fluid, and the extracellular matrix.

• Examples: Heparin (anticoagulant), Hyaluronic acid (joint lubricant), Chondroitin sulfate (cartilage), Keratan sulfate (cornea, cartilage).

Cell Surface Carbohydrates and Glycoconjugates

Peptidoglycan in Bacterial Cell Walls

• Composed of alternating N-acetylglucosamine and N-acetylmuramic acid residues cross-linked by short peptides.

• Provides structural strength to bacterial cell walls.

• Gram-positive bacteria: Thick peptidoglycan layer; Gram-negative bacteria: Thin peptidoglycan layer plus outer membrane.

Glycoproteins and Proteoglycans

• Glycoproteins: Proteins with covalently attached oligosaccharide chains; involved in cell recognition, signaling, and immune response.

• Proteoglycans: Proteins with one or more glycosaminoglycan chains; major components of the extracellular matrix.

• Glycoproteins can be N-linked (attached to asparagine) or O-linked (attached to serine/threonine).

• Functions include altering protein solubility, stability, and directing protein trafficking.

Type	Carbohydrate Content	Main Function
Glycoprotein	Short, branched oligosaccharides	Cell recognition, signaling, immunity
Proteoglycan	Long, unbranched glycosaminoglycans	Structural support, hydration, cell adhesion

Biological Roles of Carbohydrates

• Energy storage (e.g., starch, glycogen).

• Structural support (e.g., cellulose, chitin, peptidoglycan).

• Cell-cell recognition and signaling (e.g., glycoproteins, glycolipids).

• Protection and lubrication (e.g., glycosaminoglycans in synovial fluid).

Additional info: The notes above synthesize and expand upon the provided slides and text, filling in standard biochemistry context for clarity and completeness.