Biosynthesis of Structurally Diverse Triterpenes in Plants: the Role of Oxidosqualene Cyclases

Triterpenes, thirty-carbon compounds derived from five-carbon isoprene units, are among the largest and diverse classes of natural products. To date, over 20,000 distinct triterpene structures having more than 100 skeletal variations are indentified from natural sources. The majority of this triterpene diversity is found within the plant kingdom. Plants often accumulate triterpenes in their glycosylated forms that are known as saponins. Plants usually produce triterpenes in tissue-specific, organ-specific and developmentally specific manners, and in response to environmental perturbation, and pest and pathogen attacks. Although, the biological function of majority of the triterpenes is yet to be determined, some triterpenes are suggested to play specialized/secondary function as defence compounds. On the contrary, sterols and steroid hormones that share common biosynthetic precursor (i.e. 2,3-oxidosqualene) with triterpenes play primary function during plant growth and development. The first diversifying step in triterpene/sterol biosynthesis is catalyzed by a family of enzymes known as oxidosqualene cyclases (OSCs) that convert 2,3-oxidosqualene into various triterpene/sterol scaffolds. In recent years, remarkable progress has been made in enzymatic characterization of OSCs from diverse plant species, besides isolation, structural elucidation and studying pharmacological activities of triterpenes. These efforts led to the identification of OSCs with diverse product specificities and, also helped us understand the evolution of OSC family in plants. In this review article, besides presenting an overview of triterpenes with respect to their structural diversity, biological functions and biosynthetic pathways, the role of OSCs in generating triterpene skeletal diversity in plants is highlighted.