Tutorial structure and links
This is a bigger topic than any other in this series. So, in keeping with the "Bite-sized" ethic, it's been divided up into what I hope you will find to be logical sections. By following these links you will discover more about:
Part 1 Some basic sugar chemistry
Part 2 The structure of the polymers
Part 3 The reactions catalysed by FTFs and GTFs
Part 4 The enzymes themselves
Part 5 The role of EPS in dental plaque and caries
If you don't understand sugar names.
Any discussion of EPS can not avoid some of the basics of sugar chemistry such as how carbons are numbered and a little about isomers and sterochemistry. If you do not understand what a β-2,1 glycosidic bond looks like then you may wish to revise some sugar chemistry in Part 1.
Ackowledgement
The author is grateful to Prof Roy Russell, a real expert on EPS, for proof reading these pages and providing valuable comment and suggestions. Any mistakes, however, are all mine.
Overview
Fructans and glucans from sucrose
Several of the bacterial species normally resident in the mouth can synthesise polysaccharides from sucrose. This is accomplished using enzymes which hydrolyse sucrose and polymerise either the fructose or glucose moieties. Polymers of fructose are known as fructans and the enzymes responsible are called fructosyltransferases (FTFs). Polymers of glucose are known as glucans and the enzymes responsible are called glucosyltransferases (GTFs).
FTFs & GTFs are extracellular enzymes
These enzymes are secreted by bacterial cells and may stay attached to the outer surface or released from the cell. Either way they function extracellulary and the products are known collectively as Extracellular Polysaccharides (EPS). Although EPS formation is a feature of several oral bacteria a great deal of research suggests a stronger and more fundamental connection with the mutans streptococci and, because of the link between these bacteria and caries, a lot of research has focused on their role in tooth decay.
EPS have a complex structure
There are numerous types of each enzyme producing polymers differing in the type of glycosidic links between the constitutive monosaccharides and the type and degree of cross-linking.
These enzymes work in what is known as a semi-processive manner which will be explained later but one of the upshots of this is that polysaccharides in the process of being formed can be modified by other enzymes. This, as you might expect, can result in quite complex polymers which may be further modified by degradative enzymes such as dextranase.
Function
Fructans and glucans have very different functions as far as we currently know.
Fructans
The evidence so far suggests that fructans function as short-term extracellular energy reserves. It has been suggested that they can promote caries by extending the period of acid production by plaque bacteria by providing a source of fermentable carbohydrate when other sources are reduced. There is a lack of direct evidence for this at the present time so this remains an interesting but plausible theory only.
Glucans
These are more complex and appear much more interesting because their role in plaque formation and the ecology of mutans streptococci seems clearer. Glucans promote cell-cell and cell-surface interactions. That is they help mutans streptococci stick to each other and to surfaces such as teeth. They are also thought to bulk out dental plaque which aids diffusion of metabolites thereby speeding up acid production.
Bibliography
Not a comprehensive list of the published work on this subject but the main sources used to write these pages.
Banas JA, Vickerman MM (2003). Glucan-binding proteins of the oral streptococci. Crit Rev Oral Biol Med 14:89-99.
Colby SM, Russell RR (1997). Sugar metabolism by mutans streptococci. Soc Appl Bacteriol Symp Ser 26:80S-88S.
Ebisu S, Misaki A, Kato K, Kotani S (1974). The structure of water-insoluble glucans of cariogenic Streptococcus mutans, formed in the absence and presence of dextranase. Carbohydr Res 38:374-381.
Hanada N, Isobe Y, Aizawa Y, Katayama T, Sato S, Inoue M (1993). Nucleotide sequence analysis of the gtfT gene from Streptococcus sobrinus OMZ176. Infect Immun 61:2096-2103.
Koepsell HJ, Tsuchiya HM (1952). Enzymatic synthesis of dextran. J Bacteriol 63:293-295.
Kralj S, van Geel-Schutten IG, Faber EJ, van der Maarel MJ, Dijkhuizen L (2005). Rational transformation of Lactobacillus reuteri 121 reuteransucrase into a dextransucrase. Biochemistry 44:9206-9216.
Loesche WJ (1986). Role of Streptococcus mutans in human dental decay. Microbiol Rev 50:353-380.
Monchois V, Willemot RM, Monsan P (1999). Glucansucrases: mechanism of action and structure-function relationships. FEMS Microbiol Rev 23:131-151.
Moulis C, Joucla G, Harrison D, Fabre E, Potocki-Veronese G, Monsan P, et al. (2006). Understanding the polymerization mechanism of glycoside-hydrolase family 70 glucansucrases. J Biol Chem 281:31254-31267.
Robyt JF, Yoon SH, Mukerjea R (2008). Dextransucrase and the mechanism for dextran biosynthesis. Carbohydr Res 343:3039-3048.
Tanzer JM, Thompson AM, Grant LP, Vickerman MM, Scannapieco FA (2008). Streptococcus gordonii's sequenced strain CH1 glucosyltransferase determines persistent but not initial colonization of teeth of rats. Arch Oral Biol 53:133-140.
van Hijum SA, Kralj S, Ozimek LK, Dijkhuizen L, van Geel-Schutten IG (2006). Structure-function relationships of glucansucrase and fructansucrase enzymes from lactic acid bacteria. Microbiol Mol Biol Rev 70:157-176.
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