## 3 Steviol Glycos Chemical Diversity and Biosynthesis
GGPP is converted to ent-kaurene by cyclization steps catalyzed by copalyl diphosphate synthase (CPPS) and kaurene synthase (KS). Ent-kaurene is then transported to the endoplasmic reticulum and oxidized by KO (P450 monooxygenase) enzyme to generate ent-kaurene acid (ent-KA). The ent-KA is the last common substrate for the SvGls and GA synthesis pathway. The hydroxylation of ent-KA at the C7 position favors the generation of GA, whereas the hydroxylation reaction at the C13 position leads to the formation of steviol (ent-13-hydroxy kaureonic acid), which is catalyzed by kaureonic acid 13-hydroxy (KAH). This is one of the crucial steps involved in the SvGl (Figure 1). The final stage is the series of glycosylation, which takes place in the cytosol catalyzed by the family of UDP-dependent glycosyltransferases (UGTs). The glycosylation starts at the C13 position, and then at the C19 position, UGTs transfer the sugar moiety from the activated donor to the acceptor molecule. Different forms and different numbers of sugar molecules produce diverse SGs with varying properties. Four main UGTs which are involved in the formation of SvGls are UGT85C2, UGT74G1, UGT76G1, and UGT91D2, which are then transferred to the vacuoles [21]. It was suggested that UGT85C2 is the key  
Figure 1: Schematic depiction of the biosynthesis of steviol glycosides by MEP pathway outlining the important enzymes involved in the pathway. G3P = Glyceraldehyde 3-phosphate; DXS = 1-deoxy-D-Xylulose 5-Phosphate; DXR = 1-deoxy-D-xylulose 5-phosphate reductoisomerase; MEP = 2-C-Methyl-D-erythriot 4-phosphate; CMS = CDP-ME synthase; CDP-ME = methylerythritol 4-phosphate; CMK = CDP-ME kinase; CDP-MEP = 4-diphosphocytidyl-2-C-methyl-D-erythriot-2-phosphate; MCS = 2-C-methyl-D-erythriot 2,4-cyclodiphosphate synthase; MEcPP = 2-C-methyl-D-erythriot-2,4-cyclodiphosphate; HDS = 4-hydroxy-3-methylbut-2-enyl diphosphate synthase; HMPP = 4-hydroxy-3-methyl-butenyl 1-diphosphate; HDR = 4-hydroxy-3-methylbut-2-enyl diphosphate reductase; IPP = Isopentyl diphosphate; DMAPP = dimethylallyl diphosphate; GGPP = geranyl-geranyl diphosphate; GGPPS = GGPP synthase; CPPS = copalyl diphosphate synthase; CPP = ent-copalyl diphosphate; KS = ent-kaurene synthase; KO = kaurene 19-oxidase; ent-KO = ent kaurene 19-oxidase. regulator for the synthesis of steviolmonoside, which is then glycosylated to stevioside. UGT91D2 is deciphered to play a major role in the synthesis of different rebaudiosides; in particular, it catalyzes the synthesis of Reb D from Reb A. Reb M, the newly popular sweetener, is also actively generated by three different crossways, with Reb A being the main backbone for its biosynthesis. Recently, Reb T, Reb U, and Reb Q were extracted from the leaves of stevia, although the sweetness concentration and their characterization are not well understood [22]. The detailed formation of different rebaudiosides and steviosides by different UGTs are represented in Figure 2.  
Most of the enzymes involved in the SvGIs biosynthesis have been discovered and identified, and each enzyme may be the limiting factor of the biosynthesis pathway. However, it is suggested to explore novel genes that may be involved in biosynthesis of known (Reb D, Reb M, Reb A) and other unidentified sweeter steviosides. Furthermore, scientists may utilize metabolic engineering techniques to characterize and alter the gene expression of these enzymes to increase the production of specific SvGIs.