Team:TU Darmstadt/Project/Bio/Monomeres/Xylitol

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Biotechnological production of xylitol in Escherichia coli

Figure 1: xylitol pathway including NADPH-dependent aldose reductase

 

Due to its various hydroxyl-groups, the sugar substitute xylitol can be used as comonomer for the construction of heteropolymers. When the aldopentose xylose is reduced a pentahydroxy sugar alcohol, named xylitol, is received. Our focus lies on the attributes of xylitol as a monomer-precursor for plastics. It is possible due to the variety in hydroxyl groups that there are more potential ways of cross-linking between the chemicals. This would lead to different textures of the emerging polymer.

In technical process xylitol production is implemented by a hydrogenation reaction of the sugar over nickel catalysts under high temperature and enormous pressure. Here the biosynthetic production is of advantage because of the lower risks for the environment even if the costs for enzyme-based reactions in vitro are still quite high.

Figure 1 aldose reductase (coded by GRE3)

Xylitol, also known as xylit, is set out in slight amounts in several vegetables and in the bark of trees. Because of the dry mass of just 1%, conventional extraction out of these plants has no high economic value.(1) As a sugar alcohol, xylitol is a relevant substance for pharmaceuticals, oral and personal care product and as a precursor for chemical synthesis. Because of these applications a microbial based industrial scale production gets more and more interesting. The market size of xylitol amounts currently to $340 million with a cost of around $4-5 per kilo. (2)

Biotechnological production of xylitol in a host of choice requires one additional gene that is extracted from the conventional host Saccharomyces cerevisiae. The gene GRE3 (K1602004) codes for a NADPH-dependent heterologous aldose reductase and is induced under stress conditions in S. cerevisiae. Within our chosen model organism E.coli the conversion of the aldopentose D-xylose to xylitol is accomplished by reduction of the in D-xylose occurring oxygen double bond. (3)

 

1. Akinterinwa O, Cirino PC. Anaerobic obligatory xylitol production in Escherichia coli strains devoid of native fermentation pathways. Appl Environ Microbiol. 2011;77(2):706-9.
2. Akinterinwa O, Khankal R, Cirino PC. Metabolic engineering for bioproduction of sugar alcohols. Curr Opin Biotechnol. 2008;19(5):461-7.
3. Su B, Wu M, Zhang Z, Lin J, Yang L. Efficient production of xylitol from hemicellulosic hydrolysate using engineered Escherichia coli. Metab Eng. 2015;31:112-22.