Carbohydrates Study Guide

Carbohydrates

Unit 1: Carbohydrates Overview

• Definition and classification of carbohydrates
• Types of carbohydrates: monosaccharides, disaccharides, oligosaccharides, and polysaccharides
• Features of cyclic forms, including anomeric carbon, Fischer projections, and Haworth projections.
• Structural and functional significance of monosaccharides and disaccharides:
    - Sucrose
    - Lactose
    - Maltose
    - Cellobiose
• Types and functions of polysaccharides:
    - Starch
    - Glycogen
    - Cellulose
• Overview of artificial sweeteners:
    - Brief chemistry of saccharin
    - Brief chemistry of aspartame

Introduction to Carbohydrates

• Carbohydrates are the most prevalent biomolecules on Earth.
• Photosynthesis converts over 100 billion metric tons of CO2 and H2O into carbohydrates annually.
• Carbohydrates, particularly sugars and starches, are staple parts of diets worldwide.
• Oxidation of carbohydrates is the primary energy-generating pathway in non-photosynthetic cells.
• Insoluble carbohydrate polymers provide structural and protective roles in organisms:
    - Cell walls in bacteria and plants
    - Connective tissues in animals.
• Certain carbohydrate polymers help lubricate joints and mediate cell recognition and adhesion.
• More complex carbohydrate polymers (glycoconjugates) are linked to proteins or lipids, acting as signals for intracellular processes.

Definition of Carbohydrates

• Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds upon hydrolysis.
• Many carbohydrates conform to the empirical formula (CH2O)n, with some also containing nitrogen, phosphorus, or sulfur.

Classification of Carbohydrates

• Monosaccharides: Simple sugars that cannot be hydrolyzed further (e.g., glucose, fructose).
• Disaccharides: Formed from two monosaccharides (e.g., sucrose, maltose).
• Oligosaccharides: Composed of 3 to approximately 10 monosaccharides (oligosaccharides include trisaccharides, tetrasaccharides).
• Polysaccharides: Composed of more than 10 monosaccharide units (e.g., cellulose, starch).
    - Homopolysaccharides: Only one type of monosaccharide unit.
    - Heteropolysaccharides: Different monosaccharide units.

Monosaccharides

• Characteristics:
    - Simplest carbohydrates, colorless crystalline solids, soluble in water but not in nonpolar solvents.
    - Typically sweet in taste.
    - Types based on carbon content:
      - Triose (C3): Glyceraldehyde
      - Tetrose (C4): Erythrose
      - Pentose (C5): Ribose, Xylose
      - Hexose (C6): Glucose, Galactose, Fructose
      - Heptose (C7): Sedoheptulose

Common Monosaccharides

• No. of Carbon Atoms & Names:
    - 3 Carbon (Triose):
      - Aldoses: Glyceraldehyde
      - Ketoses: Dihydroxyacetone
    - 4 Carbon (Tetrose):
      - Aldoses: Erythrose
    - 5 Carbon (Pentose):
      - Aldoses: Ribose, Arabinose, Xylose
      - Ketoses: Ribulose, Xylulose
    - 6 Carbon (Hexose):
      - Aldoses: Glucose, Galactose, Mannose
      - Ketoses: Fructose
    - 7 Carbon (Heptose):
      - Aldoses: Sedoheptulose

Biological Importance of Monosaccharides

• Glyceraldehyde: Commonly found as phosphate; involved in glycolysis.
• Dihydroxyacetone: Also an intermediate in glycolysis.
• Ribose: Found in RNA and nucleotides, important for structure of RNA.
• Deoxyribose: Constituent of DNA.
• Xylose: Component of glycoproteins and gums.
• Glucose: Essential for energy as it forms part of multiple polysaccharides.
• Galactose: Component of lactose; converted to glucose.
• Mannose: Found in polysaccharides and glycoproteins.
• Fructose: Common in fruits and honey; participates in glycolysis.

Structural Aspects of Monosaccharides

• Stereoisomerism: Important for monosaccharides; dependent on the presence of asymmetric carbon atoms.
• Asymmetric carbon determines potential isomers (2^n, where n is the number of asymmetric carbons).
• Glucose: 4 asymmetric carbons yields 16 isomers.

Glyceraldehyde - The Reference Carbohydrate

• Glyceraldehyde (triose) is the simplest monosaccharide, with one asymmetric carbon atom, existing as two stereoisomers.

D- and L-Isomers

• D and L isomers are mirror images of each other.
• The configuration is based on the position of the OH group relative to the terminal CH2OH group:
    - Right = D
    - Left = L
• D-series monosaccharides are primarily metabolized in mammalian tissues.

Optical Activity of Sugars

• Optical activity arises from asymmetric carbon atoms.
• Compounds can be dextrorotatory (d+) or levorotatory (l-), affecting light rotation.
• Designations of optical isomers are based on relation to glyceraldehyde.

Racemic Mixture

• Mixture of equal parts d- and l-isomers; exhibits no optical activity as the rotations cancel.
• Dextrose is often used for glucose in solutions due to its dextrorotatory nature.

Configuration of D-Aldoses

• Starting from D-glyceraldehyde, the carbon chain can be extended in successive units to yield various aldohexoses, including glucose, mannose, and galactose.

Epimers

• Epimers are monosaccharides differing at one specific carbon (C-2 or C-4).
• Examples include:
    - Glucose and galactose (C4-epimers)
    - Glucose and mannose (C2-epimers).
• Epimerization: The process through which epimers interconvert, catalyzed by specific enzymes (epimerases).

Enantiomers

• Special case of stereoisomers that are non-superimposable mirror images.
• D- and L-sugars are examples.
• Diastereomers: Non-mirror image stereoisomers.

Structure of Glucose

• Reaction between aldehydes/ketones and alcohol forms hemiacetals/hemiketals.
• Cyclic Forms of Glucose: The aldehyde group can react with the hydroxyl group to create cyclic structures:
    - α- and β-D-glucose based on the orientation of the OH group at anomeric carbon (C1).

Pyranose and Furanose Structures

• Haworth projection formulae depict cyclic forms:
    - Pyranose: 6-membered ring
    - Furanose: 5-membered ring
• Cyclic Forms of Glucose: Identified as: α-D-glucopyranose and α-D-glucofuranose.

Anomers and Mutarotation

• Anomers: α- and β-cyclic forms of D-glucose differing around C1 (anomeric carbon).
• Mutarotation: The change in optical rotation, representing interconversion between α and β forms.
• The equilibrium mixture composition: 63% β-anomer, 36% α-anomer, and 1% open-chain form.

Disaccharides Overview

• Disaccharides consist of two monosaccharide units linked by glycosidic bonds (crystalline, water-soluble, sweet).

Types of Disaccharides

• Reducing Disaccharides: Have a free aldehyde or keto group (e.g., maltose, lactose).
• Non-Reducing Disaccharides: Lack free aldehyde or keto group (e.g., sucrose).

Glycosidic Bonds

• Formed from the reaction of an anomeric carbon with OH group of another monosaccharide.
• These linkages can be categorized based on the orientation of the anomeric carbon:
    - α-glycosidic bonds: Hydroxyl group in alpha position.
    - β-glycosidic bonds: Hydroxyl group in beta position.

Maltose

• Composed of two glucose residues with an alpha-1,4 linkage.
• This reducing disaccharide is involved in starch and glycogen hydrolysis.
• Maltose provides limited sweetness (30% of sucrose).

Lactose

• Found in milk; it is a reducing disaccharide that yields glucose and galactose upon hydrolysis.
• Linked by a beta-glycosidic bond; present only in mammalian milk.
• Human milk approximately contains 7.5% lactose, while cow's milk has about 4.5%.

Sucrose

• Known as normal table sugar; commonly derived from sugarcane and sugar beets.
• Contains an α-1,β-2-glycosidic linkage.
• Hydrolysis into glucose and fructose results in inversion, changing optical rotation from dextrorotatory to levorotatory, producing invert sugar.
• Has significant roles in plants for energy transport and acts as a signaling molecule.

Biological Role of Sucrose

• Integral to plant metabolism:**
    - Energy source for non-photosynthetic parts (roots, fruits).
    - Signaling molecule regulating metabolic processes and gene expression.
• In animals, sucrose provides rapid energy after digestion into glucose and fructose, important for quick energy but not essential for nutrition.