Steviol glycosides are the chemical compounds responsible for the sweet taste of the leaves of the plant Stevia rebaudiana (Asteraceae) and the main ingredients (or precursors) of many sugar substitute marketed under the generic name stevia and several . They also occur in the related species S. phlebophylla (but in no other species of Stevia) and in the plant Rubus chingii (Rosaceae).[
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Steviol glycosides from Stevia rebaudiana have been reported to be between 30 and 320 times sweeter than sucrose,[ although there is some disagreement in the technical literature about these numbers.][ They are heat-stable, pH-stable, and do not fermentation.][
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Steviol glycosides do not induce a blood sugar when ingested, because humans cannot metabolize stevia.[
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Structure
These compounds are of steviol. Specifically, their molecules can be viewed as a steviol molecule, with its carboxyl hydrogen atom replaced by a glucose molecule to form an ester, and a hydroxyl hydrogen with combinations of glucose and rhamnose to form an acetal.
The steviol glycosides found in S. rebaudiana leaves, and their Dry matter percentage, include:
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Stevioside (5–10%)
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Dulcoside A (0.5–1%)
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Rebaudioside A (2–4%)
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Rebaudioside B
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Rebaudioside C (1–2%)
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Rebaudioside D
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Rebaudioside E
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Rebaudioside F
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Rubusoside
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Steviolbioside
The last three are present only in minute quantities, and rebaudioside B has been claimed to be a byproduct of the isolation technique.[
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The Chinese plant Rubus chingii produces rubusoside, a steviol glycoside not found in Stevia.[ According to the EU Stevia Regulation of 13 July 2021, however, rubusoside is one of the eleven major glycoside components of Stevia,
extracted from the leaves of the Stevia rebaudiana.][ COMMISSION REGULATION (EU) 2021/1156 of 13 July 2021]
Stevioside and rebaudioside A were first isolated in 1931 by French chemists, Bridel and Lavielle.[ Both compounds have only glucose subgroups: stevioside has two linked glucose molecules at the hydroxyl site, whereas rebaudioside A has three, with the middle glucose of the triplet connected to the central steviol structure.
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Early sensory tests led to claims that rebaudioside A was 150 to 320 times sweeter than sucrose, stevioside was 110 to 270 times sweeter, rebaudioside C 40 to 60 times sweeter, and dulcoside A 30 times sweeter.[ Rebaudioside A also had the least bitterness and aftertaste.][
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Biosynthesis
In Stevia rebaudiana, the biosynthesis of the glucosides occurs only in green tissues. Steviol is first produced in the and in the endoplasmic reticulum is glucosylated and glycosylation in the cytoplasm, catalyzed by UDP-glucosyltransferases. Rebaudioside A, in particular, is formed from stevioside.
Though there are several that fall into the category of steviol glycoside, synthesis follows a similar route.[Huxtable, R.J., 2002. Pharmacology and toxicology of stevioside, rebaudioside A, and steviol. In: Kinghorn, A.D. (Ed.), Stevia: The Genus Stevia. Taylor and Francis, London and New York, pp.160–177.] Synthesis of steviol glycoside begins with isoprene units created via the DXP or MEP pathway.[Lichtenhalter, H.K., 1999. The 1-deoxy-D-xylulose-5-phosphate pathway of isoprenoid biosynthesis in plants. Annu. Rev. Plant Physiol. PlantMol. Biol. 50, 47–65.][Totté, N., Charon, L., Rohmer, M., Compernolle, F., Baboeuf, I., Geuns, J.M.C., 2000. Biosynthesis of the diterpenoid steviol, an ent-kaurene derivative from Stevia rebaudiana Bertoni, via the methylerythritol phosphate pathway Tetrahedron Letters 41, 6407–6410] Two molecules derived from primary metabolism, Pyruvic acid and Glyceraldehyde 3-Phosphate, are the initial molecules for this pathway.
Upon forming IPP and DMAPP, the diterpene GGPP is formed by via head-to-tail addition by an Sn1 mechanism. Elongation begins when IPP and DMAPP form Geranyl Pyrophosphate (GPP). GPP elongates through the same Sn1 mechanism to create Farnesyl Pyrophosphate (FPP), and FPP elongates to form GGPP.
With the formation of GGPP cyclization occurs by copalyl diphosphate synthase (CDPS) and Kuarene Synthase (KS) to form -(-)Kuarene.[Richman, A.S., Gijzen, M., Starratt, A.N., Yang, Z., Brandle, J.E., 1999. Diterpene synthesis in Stevia rebaudiana: recruitment and up-regulationof key enzymes from the gibberellin biosynthetic pathway The Plant Journal 19, 411–421.] Several oxidation steps then occur to form steviol.
Steviol glycoside biosynthesis then follows several modifications from steviol that regioselectively select for sugar molecules to be placed.[Richman, A., Swanson, A., Humphrey, T., Chapman, R., McGarvey, B., Pocs, R., Brandle, J., 2005. Functional genomics uncovers threeglucosyltransferases involved in the synthesis of the major sweetglucosides of Stevia rebaudiana Plant J. 41, 56–67] Once these molecules are fully glycosylated, the glycosides are then stored in .
See also
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External links
