3. Metabolic and Functional Diversity of Saponins

The ‘saponin’ word is originated from Latin name ‘sāpō’ means ‘soap’, as saponins make foams when they are shaken using water. These are a varied class of surface active and nonvolatile secondary metabolites are broadly dispersed in nature, existing in diverse species of plants, including both monocot and dicot. Saponins are 30-carbon skeleton molecules derived from oxidosqualene precursor that consisted of nonpolar aglycones, to which one or more polar monosugar molecules are attached. The polar (sugar moieties) and nonpolar (aglycones) structures mixture in the saponin compounds describe their soap like behavior in water and provide the base for their biological activities. Although saponin is considered major group of plant natural products, their functions in plant biological process are not fully understood and saponins are usually recognized to have significant functions in plant defense mechanisms against pathogens, herbivores and pests. Saponin compounds have a wide array of characters, such as emulsifying and foaming, bitterness and sweetness, antimicrobial, insecticidal, as well as pharmacological and medicinal properties. Although in the early times it may be suitable to categorize saponin compounds according to their biological and/or physicochemical activities, currently with the high throughput in chemistry and mass spectrometry, the structural diversity of saponin compounds became the main classification scheme. In this chapter, we will try to describe the different types of saponin compounds and their distributions in the different plant species. The new isolated saponin compounds from different plants will also be listed as a source information for future biological studies.