Biosynthesis of saponins in the genus Medicago

Saponins from Medicago species are glycosidic compounds with an aglycone moiety formed through the enzymatic cyclization of 2,3-oxidosqualene by the β-amyrin cyclase. All the saponins from Medicago genus possess the triterpenic pentacyclic nucleus belonging to the class of β-amyrin. The so formed β-amyrin skeleton can be further modified by oxidative reactions, mediated by cytochromes belonging to the class of cytochrome P450, to give different saponin compounds, characterized by the presence of hydroxyl or carboxyl groups located in specific positions of the triterpenic skeleton. Based on the position and the oxidation degree of the substituents, it is possible to distinguish two groups of saponins (sapogenins) in Medicago spp: (1) sapogenins possessing an OH group on C-24 (soyasapogenols A, B and E) without any substituent at the C-28 atom, and (2) sapogenins possessing the COOH group at C-28 that are associated with different oxidation degrees (zero, OH, CHO, COOH) at C-23. These results seem to indicate that the oxidation at C-24 and the presence of the COOH group at C-28 are mutually exclusive. The subdivision in the aglycone moiety is reflected also in the sugar moiety, operated by glycosyltranferases, as the saponins of the two groups differ for the position and the nature of the sugar chains. Based on these findings, new considerations on the biosynthesis of saponins in the genus Medicago can be drawn and a biosynthetic scheme is proposed.