ABSTRACT
Terpenoids are a large group of oily compounds that are composed of isoprene units. The biological functions of terpenoids range from light harvesting in photosynthesis, electron transport, growth regulation, and as volatile attractants and toxic deterrents in animal—plant interactions. Terpenoids are classified according to the number of five-carbon units from which they are formed. Although terpenoids are often referred to as secondary metabolites many of them are essential components of primary metabolic pathways. Many of the terpenoids are important in human health, either as essential components of our diet, or as medicines. Plant sterols (phytosterols) are triterpenoids that serve the same function in plants as the animal steroid, cholesterol. The formation of the 30-carbon triterpenoids begins with the head—head condensation of two molecules of arnesyl diphosphate to form squalene. The reaction is catalyzed by squalene synthase, which is classed as both a prenyltransferase and a terpene synthase as it carries out both reactions.
