Isoprenoid Structure Classification and Biological Importance

Isoprenoid or terpenoid refers to a class of organic compounds that are made up of isoprene. These hydrocarbons can have two or more repeating isoprene units arranged in a specific pattern. Along with the carbon backbone, one or more functional groups can also be attached to form the isoprenoid structure. This makes isoprenoids the most structurally diverse class of natural resins.

These naturally occurring organic chemicals function as secondary metabolites in biological systems. Their structural diversity allows them to be applicable in a variety of biological and commercial contexts. There are more than 30,000 compounds known to be derived from terpenoids. 

Isoprene

The hydrocarbon that makes up isoprenoid is termed as isoprene (2-methyl-1,3-butadiene). This is a five-carbon compound that has a branched-chain structure. Isoprene is unsaturated, which means that the isoprene structure has double bonds between the carbon atoms. One isoprene unit has two double bonds between carbon atoms. Isoprenoids can be made up of any number of isoprene units, ranging from two to thousands.

 Structure of Isoprene

[Image will be Uploaded Soon]

Classes of Isoprenoids

The basic building block of isoprenoid is isoprene which is a five-carbon compound. Isoprenoids can be classified based on the number of isoprene (C₅H₈) units that they are made up of. Based on this, the following classes of isoprenoids are observed:

1. Monoterpenes: C₁₀H₁₆ 

2. Sesquiterpenes: C₁₅H₂₄

3. Diterpenes: C₂₀H₃₂;

4. Triterpenes: C₃₀H₄₈

5. Tetraterpenes: C₄₀H₆₄ 

6. Polyterpenes: (C₅H₈)_n

Another basis for the classification of isoprenoids is the number and type of cyclic units that they have. Based on this, terpenoids can be classified into the following categories:

• Linear

• Acyclic

• Monocyclic

• Bicyclic

• Tricyclic

• Tetracyclic

• Pentacyclic

• Macrocyclic

[Image will be Uploaded Soon]

Structure of Isoprenoid

1. Head to Tail: Several isoprenoids have a head to tail linking of the carbon skeleton of isoprene units. This involves bonding between the carbon atom 1 of one unit and carbon atom 4 of the next. 

2. Tail to Tail: This type of bond is seen in several triterpenes and tetraterpenes, which have a bond between the carbon atom 4 of both the isoprene units.

Formation of Isoprenoids

Several terpenoids were originally found in plants and extracted from them. 

Two biosynthetic pathways that lead to the formation of isoprenoids are:

1. Mevalonate pathway

2. Deoxyxylulose Phosphate Pathway

In nature, isoprenoids are derived from Mevalonic acid in a series of enzyme-controlled reactions. Mevalonate is a precursor to isoprenoid units formed from acetyl coenzyme A (acetyl-CoA or activated acetate).

acetyl‐CoA → mevalonate

This reaction occurs in the presence of the HMG-CoA reductase enzyme. 

Other intermediaries in the process include dimethylallyl pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP). These are isomers that are used in the production of isoprenoids, including carotenoids, steroids, quinones, chlorophyll.

Terpenoids that have one or more alcohol functional groups are formed by the hydrolysis of intermediaries produced from geranyl pyrophosphate. 

Uses of Isoprenoids

Isoprenoids perform a variety of functions in biological systems. The isoprene uses range from physiological processes of plants and animals to commercial usage. In living systems, isoprenoids are required for the survival of cells by performing essential functions such as electron transport. Some of the uses of isoprenoids are:

• Pigments in plants and animals

• Fragrances or perfumes

• Flavouring, e.g. spices

• Varnishes

• Pharmacological properties help in preparing medicines

• Amber

• Vitamins

• Precursors of sex hormones

• Solvents

• Preparation of chemicals

Terpenoids in Plants 

In plants, terpenoids found in essential oils, growth affecting hormones and pigments. Terpenoids help plants send signals required for attracting pollinators, seed dispersal etc. They also help in the direct and indirect defence of plants. For e.g. Peppermint trichomes have monoterpenes and sesquiterpenes.

The flavour of cinnamon, ginger, the scent of eucalyptus and the colour of tomatoes and sunflower are also caused by isoprenoids. Plant terpenoids are usually used due to their aromatic and flavouring qualities. They also find a lot of usage in traditional herbal medicines.

Other examples of isoprenoids found in plants include menthol, salvinorin A, citral, cannabinoids, ginkgolide, curcuminoids, carotenoids, hermiterpenes in leaves etc.

Terpenoids in Animals

In animals, terpenoids occur as oily substances. These include waxy material, fish liver oil, egg yolk, butterfat, wool wax etc. The steroids in animals are also derived from terpenoids. 

Isoprenoids are also found in feathers of animals and scales of fishes. 

Properties of Terpenoids

The variety of isoprenoid compounds that occur naturally or are produced synthetically show varying physical and chemical properties. Some of the general properties of isoprenoids are provided below:

Physical Properties:

• Monoterpenes are highly volatile and are fragrant. Other terpenoids of higher molecular weight do not show such volatile properties.

• They are colourless and present in a liquid state.

• Terpenoids can be dissolved in organic solvents but not in water. 

• Terpenoids show optical activity. 

• The boiling point of terpenoids ranges from 150- 1800 ºC.

Chemical Properties:

• Terpenoids are unsaturated compounds. 

• They undergo addition, polymerization and dehydrogenation reactions.

• Isoprene is produced when terpenoids undergo thermal decomposition.

Examples of Isoprenoids

Some of the most common isoprenoids examples that have physiological uses in plants and animals, as well as commercial uses, are:

• Camphor

• Rubber

• Cholesterol

• Menthol

• Citral 

• Limonene

• Carotene

• Phytol

• Retinol (Vitamin A)

• Tocopherol (Vitamin E)

• Dolichols

• Squalene