Sucrose is broken down into glucose and fructose prior to glucan and fructan formation

False. The important point to understand is that extracellular polysachharides (EPS) are not made inside the cell and then exported to the outside. EPS are, in fact, synthesised outside of the bacterial cell by two families of enzymes called Glucosyltransferases (GTF) and Fructosyltransferases (FTF) both of which use sucrose as the substrate.

Synthetic (anabolic) reactions characteristically use phosphorylated precursors which contain within their structure sufficient energy for the synthesis to proceed. However, it would be foolhardy for a cell to export these high energy precursors out of the cell in order to synthesise EPS because the cell runs a serious risk of losing control of the valuable energy they contain.

Where then does the energy come from which is needed to build EPS?
The answer lies in the structure of sucrose. Sucrose differs from many other disaccharides in the type of glycosidic bond which links the two hexoses. In disaccharides such as maltose and lactose the glycosidic bonds link carbons C1 and C4. In all aldose sugars, C1 is the carbon with the aldehyde group attached and is known as the anomeric carbon atom because this structure allows for two different configurations known as anomers . A similar situation is found in ketose sugars except the anomeric carbon atom is C2.

The free energy associated with the glycosidic bond in maltose is 4 kcal/mol whereas the free energy associated with the glycosidic bond of sucrose is 7 kcal/mol. The reason for this is that the glycosidic bond of sucrose links both anomeric carbon atoms whereas the bond in maltose links the anomeric carbon of one glucose with C4 of another. In lactose the glycosidic bond links the anomeric carbon, C1, of galactose with C4 of glucose.

The free energy associated with the glycosidic bond of sucrose is, therefore, equivalent to that of phosphorylated precursors used in intracellular synthetic pathways and is sufficient to provide 3.2 kcal/mol required to sequentially add glucose or fructose units to the growing EPS chain. Insufficient energy is released by the hydrolysis of, say, maltose because significant energy is lost during the process.

GTFs and FTFs
GTFs and FTFs work by sequentially adding either glucose or fructose units to a growing chain of glucan or fructan respectively. The energy required to form the new glycosidic bond on the growing EPS comes from the hydrolysis of sucrose. This means that either the glucose or the fructose of sucrose can be used to make the EPS. Both moieties can not be used from the same sucrose molecule because there is not sufficient energy in the sucrose glycosidic bond to form two new glycosidic bonds.

This means that in the formation of a glucan, fructose is released and in the formation of fructan, glucose is released. The free glucose and fructose produced in the formation of EPS can be taken up by the cell and used in glycolysis.

The glycosidic bond in maltose links C1 and C4 of two glucose units and the free energy associated with the hydrolysis of the bond is 4 kcal/mol. This structure leaves the anomeric carbon atom, C1, of the second glucose unit un-bonded and available for reactions such as reduction. Maltose is, therefore, a reducing sugar. Lactose and many other disaccharides are also reducing sugars.

Sucrose is a disaccharide of glucose and fructose. In contrast to most disaccharides and oligosaccharides, sucrose contains no free anomeric carbon atoms because they are linked to each other in the glycosidic bond. (In ketoses, the anomeric carbon atom is C2)

Sucrose is not, therefore, a reducing sugar and is more readily hydrolysed than other disaccharides releasing 7 kcal/mol.

Glucan, which contains a mixture of 1,3 and 1,6 glycosidic bonds, is less water-soluble than fructan

True. When it was first discovered that members of the mutans streptococci could synthesise extracellular polymers of glucose and fructose it was mistakenly believed that the polymers formed were dextrans and levans respectively.

However, we now know that the glucan polymers formed by mutans streptococci and some other oral streptococci do not consist of just single linkage types.

Glucans contain more or less amounts of 1,6 and 1,3 links.
A glucan comprising predominantly 1,6 links with a minority of 1,3 links is water-soluble.

Glucans containing predominantly 1,3 links with a minority of 1,6 links are highly branched and are water-insoluble.

Both types are synthesised by mutans streptococci, Streptococus sanguis and Streptococcus gordonii. However, at first it was thought that the highly branched water-insiluble form was only made by mutans streptococci and thus it was called "mutan".

In addition, mutans streptococci synthesise a 2-1-linked fructan (Inulin) and Streptococcus salivarius synthesise a 2-6-linked fructan (levan) from the fructose moiety of sucrose. Both are water soluble.

Fructan is synthesised by Actinomyces viscosus

True. The oral streptococci are not the only oral bacteria capable of synthesising EPS.

Sucrose is more cariogenic than maltose

True. Sucrose (glucose + fructose) and maltose (glucose + glucose) are both disaccharides so the amount of acid produced from each by fermentation should be the same.

Metabolism of maltose
Maltose is taken up by the bacterial cell and hydrolysed into 2 glucose which then enter glycolysis in the normal manner. Each mole of maltose will yield 4 moles of 3-carbon organic acid (lactic, formic, propionic, butyric etc).

Metabolism of sucrose
Sucrose has four possible fates:

1. It can be taken up directly by the bacterial cell, hydrolysed by an intracellular invertase to give 1 mole of glucose and 1 mole of fructose. Each will then enter glycolysis to yield 2 moles of 3-carbon organic acid (4 moles acid per mole sucrose)

2. It can be hydrolysed outside the bacterial cells by an extracellular invertase and the resulting glucose and fructose taken up by the cell prior to glycolysis. Again each mole of sucrose will yield 4 moles of 3-carbon organic acid.

3. It can be used by GTF to synthesise glucan. In this case the fructose moiety which is not used by the GTF may be taken up by the cell and enter glycolysis to yield 2 moles of 3-carbon organic acid.

4. It can be used by FTF to synthesise fructan. In this case the glucose moiety which is not used by the FTF may be taken up by the cell and enter glycolysis to yield 2 moles of 3-carbon organic acid.

Note that when the sucrose is used to synthesise EPS, only one half of the sucrose is used. This is because there is only sufficient energy in the sucrose glycosidic bond to form the new glycosidic bond of the extending EPS chain.

Cariogenicity
It would seem, therefore, that less acid is likely to be produced from sucrose because some will be used to synthesise EPS whereas all the sugar in maltose will be used for acid production.

However, theoretical considerations of the amount of acid likely to be produced by dental plaque bacteria can be misleading. Cariogenicity is a complex concept and can only really be measured in vivo and animal experiments show that sucrose is more cariogenic than other sugars. Diets rich in sucrose favour EPS production which makes a significant contribution to the structural integrity of dental plaque. There is also evidence that EPS bulk-out plaque and improve the diffusion of metabolic substrates thereby speeding-up acid production. Finally, EPS promote the accumulation of mutans streptococci within plaque which are capable of producing more acid at lower pH values than other plaque bacteria.

So, in vivo, diets rich in sucrose produce a more cariogenic plaque than diets rich in maltose even though the amount of acid ultimately produced by sucrose and maltose may be, at best, the same.

Glucosyltransferase has been shown to be present in the tooth pellicle

True. Some GTF is not associated with the cell surfaces but is free within the dental plaque. Some of this free-GTF finds its way into pellicle where it can be quite easily demonstrated because it is still functional and can synthesise glucans. These glucans can interact with glucan-binding-proteins present on the cell surfaces of oral streptococci thereby helping them to adhere to the pellicle of the tooth.

This type of GTF is known as "surrogate GTF" and is even found on the surfaces of unrelated bacterial species in plaque.

Surrogate GTF can, therefore, make a significant contribution to plaque matrix and help a variety of bacteria adhere to the tooth and to each other.

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