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CARBOHYDRATES

LEARNING OBJECTIVES

The learning objectives of this section include understanding the following concepts:

  • General structures and nomenclature of carbohydrates
  • Biological functions and types of carbohydrates
  • The role of carbohydrates in bacterial cell walls

WHAT ARE CARBOHYDRATES?

Carbohydrates are defined as carbon-based molecules that are rich in hydroxyl (−OH) groups. They serve as fuel molecules and are essential components of living systems. The empirical formula for many carbohydrates can be represented as (CH₂O)ₙ where n is a whole number greater than or equal to 3.

  • Monosaccharides are simple carbohydrates consisting of single units.
  • Polysaccharides are polymers formed from monosaccharides.
  • There are also disaccharides (two monosaccharide units) and oligosaccharides (a few monosaccharide units).

MONOSACCHARIDES

Monosaccharides are characterized as carbon chains that typically have three to seven carbon atoms and contain two or more hydroxyl groups. The classification of monosaccharides based on carbon atom number includes:

  • Trioses: Monosaccharides containing three carbon atoms
  • Tetroses: Monosaccharides containing four carbon atoms
  • Pentoses: Monosaccharides containing five carbon atoms
  • Hexoses: Monosaccharides containing six carbon atoms
  • Heptoses: Monosaccharides containing seven carbon atoms
    The most common hexoses include glucose and fructose:
  • Glucose is an essential energy source for virtually all forms of life.
  • Fructose is found abundantly in honey, corn syrup, and fruits; it is considered the sweetest of all sugars.
Monosaccharides Continued

Monosaccharides may be classified as aldehydes or ketones based on their functional group presence. The smallest monosaccharides, which contain three carbon atoms, are glyceraldehyde (an aldose) and dihydroxyacetone (a ketose).

  • Glyceraldehyde: An aldose with an aldehyde functional group.
  • Dihydroxyacetone: A ketose containing a keto functional group.
  • The suffix “-ose” designates a sugar while prefixes specify the structure (e.g., gluco-, fruco-).

ISOMERIC FORMS OF MOLECULES

Isomers are defined as molecules that share the same molecular formula but differ in structure, thus constituting different compounds.

Types of Isomers

  1. Constitutional Isomers: Molecules with the same molecular formula but different connectivity (the arrangement of bonded atoms), leading to distinct structures.
       - Example: Butane (C₄H₁₀) and Isobutane (C₄H₁₀).

  2. Stereoisomers: Molecules with the same molecular formula and connectivity but different spatial orientations of groups. They can further be categorized into:
       - Enantiomers: Chiral molecules that are mirror images of each other and are not superimposable.
       - Diastereomers: Stereoisomers that are not mirror images and are also not superimposable.

ISOMERIC FORMS OF CARBOHYDRATES

  • Glyceraldehyde and Dihydroxyacetone are constitutional isomers owing to their identical molecular formulas (C₃H₆O₃) but differing in connectivity (atomic arrangement). Dihydroxyacetone is unique as it is the only monosaccharide without at least one asymmetric (chiral) carbon atom.
  • The number of possible stereoisomers for a compound can be calculated using the formula 2n2^n, where n is the number of asymmetric (chiral) carbon atoms. Glyceraldehyde contains one chiral carbon, thus having two stereoisomers: D-glyceraldehyde and L-glyceraldehyde.
Cyclic Structures of Monosaccharides

Monosaccharides primarily exist in cyclic forms rather than their open-chain equivalents in biological systems. This cyclization occurs through a spontaneous intramolecular reaction where a hydroxyl group attacks the carbonyl carbon, forming stable cyclic structures (hemiacetals from aldoses and hemiketals from ketoses).

  • For glucose, the open-chain form cyclizes when the oxygen from the C-5 hydroxyl group attacks the carbonyl carbon at C-1, producing two anomeric forms, designated α and β.
  • For fructose, the cyclization occurs when the hydroxyl oxygen at C-5 attacks the carbonyl carbon (C-2), also resulting in two anomeric forms (α and β).

DISACCHARIDES

Disaccharides are formed from the linkage of two monosaccharide units via a glycosidic bond. Common disaccharides include:

  • Sucrose
  • Maltose
  • Lactose
  • Isomaltose
  • Cellobiose
Formation of Maltose

Maltose is formed through an α-(1,4)-glycosidic linkage, which is present in fermenting grains, and is hydrolyzed by maltase in the human intestine.

Formation of Lactose

Commonly referred to as milk sugar, lactose accounts for about 7% of human milk, and is hydrolyzed by lactase in the human intestine. Lactose intolerance arises from a deficiency of lactase, leading to gastrointestinal discomfort upon milk consumption. Lactose is formed through a β-(1,4)-glycosidic linkage.

Formation of Sucrose

Known as table sugar, sucrose is hydrolyzed by sucrase in the human intestine. The glycosidic linkage for sucrose is designated as α,β-(1,2).

POLYSACCHARIDES

Polysaccharides, or glycans, are polymers made from large numbers of monosaccharides linked by glycosidic linkages. These molecules can also serve as covalently linked structures with amino acids, peptides, proteins, lipids, etc.

  • They are categorized into two classes:
       - Homopolysaccharides (Homoglycans): Composed of one kind of monosaccharide.
       - Heteropolysaccharides (Heteroglycans): Composed of two or more kinds of monosaccharides.
Homopolysaccharides - Starch

Starch comprises two forms: amylose and amylopectin, which function as energy storage in plants.

  • Amylose: A linear homopolymer of 50 to 5000 D-glucose residues linked by α-(1,4)-glycosidic bonds. It accounts for 10%-30% of starch content, and gives a black-blue color when iodine is applied.
  • Amylopectin: A highly branched polymer of D-glucose linked by α-(1,4)-glycosidic bonds with side branches formed by α-(1,6)-glycosidic linkages. It contributes 70%-90% of starch content.
Homopolysaccharides - Glycogen

Glycogen is the primary polysaccharide stored in animals, found in liver (10% of mass) and skeletal muscle (2-3% of mass). Its structure is similar to amylopectin but more highly branched (every 8-12 residues). Liver glycogen helps maintain blood sugar levels while muscle glycogen serves as fuel for muscle activity.

Homopolysaccharides - Cellulose

Cellulose is known as the most abundant natural polymer and provides structural support in plant cells.

  • It consists of β-(1,4)-glycosidic linkages, is insoluble in water, and is found in the cell walls of nearly all plants (e.g., cotton is 98% pure cellulose).
  • Cellulose's structural differences from starch significantly alter its properties. The enzyme cellulase, secreted by certain bacteria, can hydrolyze cellulose, and some animals, such as cows and termites, can digest cellulose due to the presence of cellulase-producing microorganisms in their digestive systems.
Homopolysaccharides - Chitin

Chitin is the second most abundant biopolymer globally, found in the exoskeletons of crustaceans, insects, spiders, and cell walls of fungi. It is similar to cellulose in structure and forms water-insoluble sheets.

Summary of Homopolysaccharides

A comparative summary of polysaccharides including:

  • Starch: Contains both amylose (unbranched α-1,4) and amylopectin (branched α-1,4 and α-1,6).
  • Glycogen: Similar to amylopectin but more branched.
  • Cellulose: Composed of β-1,4 linkages and provides structural support.
  • Chitin: Similar to cellulose but composed of N-acetylglucosamine.

HETEROPOLYSACCHARIDES

Glycosaminoglycans (GAGs)

Glycosaminoglycans are long, unbranched polysaccharide chains consisting of repeating disaccharide units. These disaccharides typically feature an amino sugar alternating with an acidic sugar, forming highly polyanionic structures.

Peptidoglycan

Peptidoglycan is a major structural component of bacterial cell walls, characterized by heteropolysaccharide chains cross-linked by short peptide bridges, forming a robust three-dimensional network that contributes to mechanical strength.

  • Backbone Components: These include N-acetyl-D-glucosamine (NAG) and N-acetylmuramic acid (NAM) which is joined to a tetrapeptide chain (Ala-Glu-Lys-Ala).
Cell Wall Composition in Bacteria
  • Gram-positive bacterial cell walls feature a thick layer of peptidoglycan with inter-peptide cross-linking structures.
  • Gram-negative bacterial cell walls have a thinner layer with direct cross-linking.

SUMMARY

  • Monosaccharides and many disaccharides are generally water-soluble due to the numerous hydroxyl groups that can bind with water.
  • Polysaccharides tend to be insoluble in water because their polymer linkages tie up reactive groups, limiting their interaction with water.

Prepared by Siu Wai (Phyllis) Tsang, PhD, Tung Wah College