Carbohydrates: Structure, Isomerism, and Biochemical Significance

Biochemical Substances

Classification of Biochemical Substances

Biochemical substances are the chemical compounds that constitute living organisms. They are broadly classified into two categories: bioinorganic and bioorganic substances. This classification is based on the presence or absence of carbon atoms in their molecular structure.

• Bioinorganic Substances: Do not contain carbon. Examples include water (about 70% of body mass) and inorganic salts (about 5%).

• Bioorganic Substances: Contain carbon. Major classes include proteins (about 15%), lipids (about 8%), carbohydrates (about 2%), and nucleic acids (about 2%).

Carbohydrates: Structure and Classification

Monosaccharides: Aldoses and Ketoses

Monosaccharides are the simplest carbohydrates and are classified based on the functional group present:

• Aldoses: Contain an aldehyde group (–CHO) at one end of the molecule. Example: Glucose.

• Ketoses: Contain a ketone group (C=O) typically at the second carbon atom. Example: Fructose.

Ring Structures of Monosaccharides

Monosaccharides with five or more carbon atoms commonly exist in cyclic (ring) forms in aqueous solutions. The ring formation involves a reaction between the carbonyl group and a hydroxyl group within the same molecule.

• Pyranose: Six-membered ring structure (e.g., β-D-glucose).

• Furanose: Five-membered ring structure (e.g., α-D-fructose).

Stereochemistry of Carbohydrates

Chirality and Stereoisomerism

Many carbohydrates contain chiral centers, leading to the existence of stereoisomers. A chiral center is a carbon atom bonded to four different groups. Stereoisomers are molecules with the same molecular formula and sequence of bonded atoms, but different three-dimensional orientations.

• Enantiomers: Stereoisomers that are nonsuperimposable mirror images of each other.

• Diastereomers: Stereoisomers that are not mirror images.

Chiral Carbons in Carbohydrates

Each chiral carbon in a carbohydrate can give rise to different stereoisomers. For example, glyceraldehyde is the simplest carbohydrate with one chiral center.

Examples of Chiral Centers

• 2-Butanol: Has one chiral center (marked with an asterisk).

• 1-Chloro-1-iodoethane: Another example of a molecule with a chiral center.

Isomerism in Carbohydrates

Carbohydrates can exist as different types of isomers:

• Constitutional Isomers: Differ in the order of attachment of atoms.

• Stereoisomers: Atoms are bonded in the same order but differ in spatial arrangement.

• Enantiomers: Nonsuperimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

Fischer Projections and D/L Notation

Fischer Projections

Fischer projections are a two-dimensional representation of three-dimensional organic molecules. They are commonly used to depict carbohydrates and amino acids.

• Horizontal lines represent bonds projecting out of the plane (toward the viewer).

• Vertical lines represent bonds projecting behind the plane (away from the viewer).

D- and L- Isomers

The D- and L- notation is based on the configuration of the chiral carbon farthest from the carbonyl group. In D-isomers, the hydroxyl group on this carbon is on the right; in L-isomers, it is on the left.

Classification of Monosaccharides by Carbon Number

Trioses, Tetroses, Pentoses, and Hexoses

Monosaccharides are classified by the number of carbon atoms:

• Trioses: 3 carbons (e.g., glyceraldehyde, dihydroxyacetone)

• Tetroses: 4 carbons

• Pentoses: 5 carbons (e.g., ribose, deoxyribose)

• Hexoses: 6 carbons (e.g., glucose, galactose, fructose)

Common Monosaccharides and Their Structures

Glucose, Galactose, Fructose, Ribose, and Deoxyribose

These monosaccharides are essential for biological processes:

• D-Glucose: The most common monosaccharide, also known as blood sugar.

• D-Galactose: An epimer of glucose, differing at carbon 4.

• D-Fructose: A ketohexose, found in fruit and honey.

• D-Ribose: A component of RNA.

• 2-Deoxy-D-ribose: A component of DNA.

Cyclic Forms and Anomerism

Formation of Cyclic Structures

Monosaccharides with five or more carbons form cyclic hemiacetals or hemiketals. The carbonyl group reacts with a hydroxyl group to form a ring, creating a new chiral center called the anomeric carbon.

• α-Anomer: The OH group on the anomeric carbon is trans to the CH2OH group.

• β-Anomer: The OH group on the anomeric carbon is cis to the CH2OH group.

Pyranose and Furanose Rings

Pyranose rings are six-membered, while furanose rings are five-membered. These names are derived from the parent heterocyclic compounds pyran and furan.

Oxidation and Reduction of Monosaccharides

Redox Reactions of Carbohydrates

Monosaccharides can undergo oxidation and reduction reactions. Aldoses can be oxidized to form carboxylic acids, while reduction can convert carbonyl groups to alcohols.

• Oxidation: Aldehyde group (–CHO) is oxidized to a carboxylic acid (–COOH).

• Reduction: Carbonyl group is reduced to an alcohol.

Summary Table: Major Classes of Biochemical Substances

Class	Example(s)	Approximate % in Body
Water	H2O	70%
Inorganic Salts	NaCl, KCl, etc.	5%
Proteins	Enzymes, structural proteins	15%
Lipids	Fats, oils	8%
Carbohydrates	Glucose, starch	2%
Nucleic Acids	DNA, RNA	2%