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Steviol glycosides are the
chemical compounds A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
responsible for the sweet taste of the leaves of the
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
n plant '' Stevia rebaudiana'' (
Asteraceae Asteraceae () is a large family (biology), family of flowering plants that consists of over 32,000 known species in over 1,900 genera within the Order (biology), order Asterales. The number of species in Asteraceae is rivaled only by the Orchi ...
) and the main ingredients (or precursors) of many sweeteners marketed under the generic name
stevia Stevia () is a sweet sugar substitute that is about 50 to 300 times sweetness, sweeter than sugar. It is extracted from the leaves of ''Stevia rebaudiana'', a plant native to areas of Paraguay and Brazil. The active compounds in stevia are ...
and several
trade name A trade name, trading name, or business name is a pseudonym used by companies that do not operate under their registered company name. The term for this type of alternative name is fictitious business name. Registering the fictitious name with ...
s. They also occur in the related species '' S. phlebophylla'' (but in no other species of ''
Stevia Stevia () is a sweet sugar substitute that is about 50 to 300 times sweetness, sweeter than sugar. It is extracted from the leaves of ''Stevia rebaudiana'', a plant native to areas of Paraguay and Brazil. The active compounds in stevia are ...
'') and in the plant '' Rubus chingii'' (
Rosaceae Rosaceae (), the rose family, is a family of flowering plants that includes 4,828 known species in 91 genera. The name is derived from the type genus '' Rosa''. The family includes herbs, shrubs, and trees. Most species are deciduous, but som ...
). 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
ferment Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic compound, Organic molecules, such as glucose or other sugars, are Catabo ...
. Steviol glycosides do not induce a glycemic response when ingested, because humans cannot metabolize stevia. The
acceptable daily intake Acceptable daily intake or ADI is a measure of the amount of a specific substance (originally applied for a food additive, later also for a residue of a veterinary drug or pesticide) in food or drinking water that can be ingested (orally) daily ove ...
(ADI) for steviol glycosides, expressed as steviol equivalents, has been established to be 4 mg/kg body weight/day, and is based on no observed adverse effects of a 100 fold higher dose in a rat study.


Structure

These compounds are
glycoside In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...
s of steviol. Specifically, their molecules can be viewed as a steviol molecule, with its
carboxyl In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl group (e.g. ...
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
replaced by a
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
molecule to form an
ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
, and a
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
hydrogen with combinations of glucose and rhamnose to form an
acetal In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments n ...
. The steviol glycosides found in ''S. rebaudiana'' leaves, and their dry weight percentage, include: *
Stevioside Stevioside is a glycoside derived from the stevia plant, which can be used as a sweetener. Evidence of benefit is lacking for long-term effects on weight loss and heart disease risks. Origin Stevioside is the main sweetener (along with rebaud ...
(5–10%) * Dulcoside A (0.5–1%) * Rebaudioside A (2–4%) * Rebaudioside B * Rebaudioside C (1–2%) * Rebaudioside D * Rebaudioside E * Rebaudioside F * Rubusoside * Steviolbioside The last three are present only in minute quantities, and rebaudioside B has been claimed to be a byproduct of the isolation technique. A commercial steviol glycoside mixture extracted from the plant was found to have about 80% stevioside, 8% rebaudioside A, and 0.6% rebaudioside C. 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 Stevia () is a sweet sugar substitute that is about 50 to 300 times sweetness, sweeter than sugar. It is extracted from the leaves of ''Stevia rebaudiana'', a plant native to areas of Paraguay and Brazil. The active compounds in stevia are ...
, extracted from the leaves of the ''Stevia rebaudiana''.''COMMISSION REGULATION (EU) 2021/1156 of 13 July 2021''
/ref> 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. 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. However, a more recent evaluation found rebaudoside A to be about 240 times sweeter, and stevioside about 140 times. Rebaudioside A also had the least bitterness and aftertaste. The relative sweetness seems to vary with concentration: a mix of steviol glycosides in the natural proportions was found to be 150 times sweeter than sucrose when matching a 3% sucrose solution, but only 100 times sweeter when matching a 10% sucrose solution.


Biosynthesis

In '' Stevia rebaudiana'', the
biosynthesis Biosynthesis, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-Catalysis, catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthe ...
of the glucosides occurs only in green tissues. Steviol is first produced in the
plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...
s and in the
endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...
is glucosylated and
glycosylated Glycosylation is the reaction in which a carbohydrate (or ' glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not ...
in the
cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
, catalyzed by UDP- glucosyltransferases. Rebaudioside A, in particular, is formed from
stevioside Stevioside is a glycoside derived from the stevia plant, which can be used as a sweetener. Evidence of benefit is lacking for long-term effects on weight loss and heart disease risks. Origin Stevioside is the main sweetener (along with rebaud ...
. Though there are several
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s that fall into the category of steviol glycoside, synthesis follows a similar route. Synthesis of steviol glycoside begins with
isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. It is produced by many plants and animals (including humans) and its polymers ar ...
units created via the DXP or MEP pathway. Two molecules derived from primary metabolism,
Pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic ...
and
Glyceraldehyde 3-Phosphate Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central pathways of all organisms.Nelson, D ...
, are the initial molecules for this pathway. Upon forming IPP and DMAPP, the
diterpene Diterpenes are a class of terpenes composed of four isoprene units, often with the molecular formula C20H32. They are biosynthesized by plants, animals and fungi via the HMG-CoA reductase pathway, with geranylgeranyl pyrophosphate being a primary ...
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
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s copalyl diphosphate synthase (CDPS) and Kuarene Synthase (KS) to form -(-)Kuarene. Several
oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
steps then occur to form steviol. Steviol glycoside
biosynthesis Biosynthesis, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-Catalysis, catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthe ...
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
vacuole A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...
s.


See also

*
Glycoside In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...
*
Stevia Stevia () is a sweet sugar substitute that is about 50 to 300 times sweetness, sweeter than sugar. It is extracted from the leaves of ''Stevia rebaudiana'', a plant native to areas of Paraguay and Brazil. The active compounds in stevia are ...
*
Sugar substitute A sugar substitute or artificial sweetener, is a food additive that provides a sweetness like that of sugar while containing significantly less food energy than sugar-based sweeteners, making it a zero-calorie () or low-calorie sweetener. Arti ...


References


External links

* {{Transient receptor potential channel modulators Diterpene glycosides Sugar substitutes Carboxylic acids E-number additives