Carbohydrates

Chapter 7: Carbohydrates

Introduction to Carbohydrates

  • Definition: Carbohydrates are referred to as "hydrates of carbon."

  • Originally used to denote glucose with the chemical formula C6H12O6.

  • Explanation: This term encompasses a large class of biomolecules sharing a similar structure, characterized by multiple –OH (hydroxyl) groups on adjacent carbon atoms, and either an aldehyde or ketone functional group.

  • Detailed Classification: Known collectively as "polyhydroxy aldehydes and ketones."

Basic Types of Carbohydrates

  • Simple Carbohydrates

    • Monosaccharides:

    • Definition: The simplest form of carbohydrates.

    • Examples: Glucose and fructose (composed of one saccharide molecule).

    • Disaccharides:

    • Definition: Formed when two monosaccharides bond together.

    • Examples: Sucrose (glucose + fructose) and lactose (galactose + glucose).

    • Linkage: The bond connecting monosaccharides in disaccharides is called a glycosidic bond.

  • Basic Formula: Cn(H2O)_n where n typically ranges from 3 to 7.

Complex Carbohydrates

  • Oligosaccharides:

    • Definition: Composed of three to ten monosaccharides linked by glycosidic bonds.

  • Polysaccharides:

    • Definition: Long chains of monosaccharides (more than ten units).

    • Examples: Starch, glycogen, and cellulose.

Naming Carbohydrates

  • Carbohydrates are named based on two criteria:

    • Functional Groups:

    • Ketone carbonyl = Ketose.

    • Aldehyde carbonyl = Aldose.

    • Number of Carbon Atoms:

    • 3 Carbons = Triose.

    • 4 Carbons = Tetrose.

    • 5 Carbons = Pentose.

    • 6 Carbons = Hexose.

  • Combining both naming criteria allows for more descriptive names of carbohydrates.

Structural Variants and Chirality of Monosaccharides

  • Examples of different monosaccharides based on carbon counts:

    • Aldotrioses (3 Carbons): D-Glyceraldehyde

    • Aldotetroses (4 Carbons): D-Erythrose, D-Threose

    • Aldopentoses (5 Carbons): D-Ribose, D-Arabinose, D-Xylose, D-Lyxose

    • Ketoses (4 & 5 Carbons): Dihydroxyacetone, D-Ribulose, D-Xylulose, D-Fructose, D-Sorbose, D-Tagatose

  • Chirality:

    • Definition: A carbon atom is chiral if it possesses four different substituents.

    • Importance of Chiral Carbons: Molecules with chiral centers can exist as pairs of enantiomers.

    • Example: Glyceraldehyde possesses a chiral carbon and thus can have two isomeric forms.

Stereoisomerism and Diastereomers

  • Enantiomers: Molecules that are mirror images of each other.

  • Diastereomers: Stereoisomers that are not mirror images, applicable when there are multiple chiral centers.

  • Example: Aldotetroses can have 4 distinct structural isomers, resulting in 2 pairs of enantiomers.

  • Relationship of Chiral Carbons to Stereoisomers:

    • For a compound with n chiral carbons, the maximum number of stereoisomers is 2^n.

    • Example: Glucose (4 chiral carbons) has 2^4 = 16 stereoisomers (8 pairs of enantiomers).

Epimers

  • Definition: Diastereomers that differ at only one chiral carbon.

  • Example: D-Mannose and D-Glucose differ at C-2, whereas D-Galactose differs from D-Glucose at C-4.

  • Visual Representation: Structural formulae demonstrating differing configurations at chiral centers.

Cyclization of Monosaccharides

  • Cyclization Mechanism: The reaction of the hydroxyl group (–OH) at the highest-numbered chiral carbon with the carbonyl group results in a hemiacetal (or hemiketal in the case of ketoses).

  • Prevalence of Forms: In solution, about 98% of sugars exist in a cyclized form while only 2% remain in the linear form.

  • Anomeric Carbon: The formation introduces a new chiral carbon at C-1 (the anomeric carbon) when cyclization occurs.

  • Types:

    • Pyranoses: Six-membered rings.

    • Furanoses: Five-membered rings.

Haworth Projections

  • Representation of Cyclic Structures: Depicted in nearly planar (flat) forms, often illustrated with Fischer, complete Haworth, and abbreviated Haworth models.

  • Conventions in Representation:

    • In Haworth projections, a downward hydroxyl group at the anomeric carbon (C1) denotes the alpha (α) configuration; upwards denotes the beta (β) configuration.

Reducing Properties of Monosaccharides

  • Monosaccharides act as reducing agents primarily in their open-chain forms.

  • Example: The reducing properties are the basis for Fehling’s reagent, where Cu$^{2+}$ is reduced to Cu$^+$.

Sugar Derivatives

  • Sugar derivatives include a variety of modifications that lead to different functional roles in biological systems.

    • Amino sugars: e.g., N-acetyl-β-D-glucosamine.

    • Deoxy sugars: sugars with one less oxygen functional group.

  • Acidic Sugars: E.g., Muramic acids or N-acetylmuramic acid.

Glycosidic Bonds and Oligosaccharides

  • Formation: Glycosidic bonds are formed when two monosaccharides undergo a condensation reaction to yield oligosaccharides, releasing a molecule of water.

  • Types and Examples:

    • O-glycosidic bonds typically link sugars; a and b configurations indicate the orientation of the linked groups.

Examples of Oligosaccharides

  1. Sucrose (glucose + fructose):

    • Common table sugar derived from sugarcane and sugar beets.

    • Hydrolyzed into glucose and fructose upon consumption.

    • Fructose is sweeter and often used as a sugar substitute in various food products.

  2. Lactose (galactose + glucose linked b 1→4):

    • Known as milk sugar; lacks sufficient lactase in some individuals, leading to lactose intolerance.

  3. Maltose (glucose + glucose linked a 1→4):

    • Produced from the breakdown of starch; important in various digestive processes.

Polysaccharides

  • Definition: Polysaccharides are polymeric chains of monosaccharides.

  • Types:

    • Homopolysaccharides: Composed of the same type of monosaccharide.

    • Heteropolysaccharides: Contain more than one type of monosaccharide.

Examples of Important Polysaccharides

  1. Cellulose:

    • Major structural component of plant cell walls.

    • Composed of linear chains of glucose units linked by b 1→4 glycosidic bonds.

    • H-bonding between cellulose chains provides structural integrity.

    • Animals lack enzymes to hydrolyze cellulose, preventing its digestion.

  2. Starch:

    • Composed of α-D-glucose units, prevalent in plants.

    • Amylose: linear chain of glucose ((1→4) linkages).

    • Amylopectin: branched form, contains α(1→6) branch points.

    • The average chain length can vary, with helices formed for amylose when complexed with iodine.

  3. Glycogen:

    • A branched polymer of α-D-glucose, serving as energy storage in animals.

    • Breakdown involvement: enzymes like glycogen phosphorylase.

  4. Chitin:

    • Homopolysaccharide consisting of N-acetylglucosamine units.

    • Important structural material in fungal cell walls and exoskeletons of arthropods.

  5. Glycoconjugates:

    • Composed of proteins and carbohydrates, important in cell signaling and structural functions.

    • Proteoglycans: proteins with glycosaminoglycan chains.

    • Glycoproteins: proteins with shorter, branched carbohydrate chains, highly diverse.

  6. Blood Typing by Glycoproteins:

    • Antigenic determinants on erythrocytes (red blood cells) determining blood type (e.g., Type A = N-acetylgalactosamine, Type B = galactose).

Conclusion

  • Carbohydrates, as essential biomolecules, play a critical role in biological functions ranging from providing energy to serving as structural components. Their diverse structures and functions underscore their importance in both biochemistry and health.