Reduction of xylose

The aldehyde group can be readily reduced by catalytic hydrogenation or with reagents such as sodium borohydride. The reduction of xylose produces xylitol, which is used as a sweetener in sugarless gum ... 

The most important pentose is xylose which can be produced from hardwoods by mild acid hydrolysis. Reduction of xylose gives xylitol, an interesting sweetener because of its ability to prevent dental caries. Industrial production of xylitol from birch wood hydrolyzates started in the 1970s in Finland. An interesting process was developed in which an ion exclusion separation technique is applied for purification and separation of xylose and xylitol from other impurities. Among the corresponding reduction products of hexoses, mannitol, which is also a natural product, has found some use. It can be separated from other alditols by crystallization. Under more drastic... 

Although abundant in nature, and although several opportunities have been identified [64], heteropolysaccharides have largely escaped industrial exploitation. Exceptions are the conversion of pentoses into furfural (for temperature-resistant foundry resins) by acid hydrolysis the fermentation of pentoses into single cell protein Torula yeast) the catalytic reduction of xylose to xylitol, a dietary sweetener and the use of larch [Larix sp.)-specific arabinogalactan extracts as dispersants in printing inks. 

In yeast and mycelial fungi, xylose is metabolized via coupled oxidation-reduction reactions . Xylose reductase is the enzyme involved in the reduction of xylose to xylitol. Sequential enzymatic events, through the oxidation of xylitol to xylulose, lead to the utilization of xylose. Many yeast species utilize xylose readily, but the ethanol production capability is very limited. Only a few yeast species effectively produce ethanol from xylose these include Pachysolen tan-nophilus, Candida shihatae and Pichia stipitis [80]. The production of ethanol from xylose by these three yeast strains has been studied extensively in recent years. Recently, genetically engineered yeast strains have been constructed for more effective conversion of xylose to ethanol. 

Ethanol fermentation from xylose by yeasts can be divided into four distinctive steps. The first step is the reduction of xylose to xylitol mediated by NADPH/ NADH-linked xylose reductase (XR). This is followed by the oxidation of xylitol to xylulose, mediated by NAD-linked xylitol dehydrogenase (XDH). Xylulose-5-phosphate, the key intermediate, is generated from the phosphorylation of xylulose by xylulose kinase. Xylulose-5-phosphate is then channeled into the pentose phosphate pathway for further metabolism (Fig. 9). 

XylitoL jcyfo-Pentane-l,2,3,4,5-pentol xylite Eutrit Kannit Klinit Kylit Newtol Torch Xyliton. C5-Hl20 mol wt 152.15. C 39,47%, H 7.95%, O 52.38%. Intermediate in metabolism of d-glucose through glucuronate cycle in livers. Prepd by reduction of xylose G. Bertrand Bull Soc Chim. France [3] 5, 555 (1891) E, Fischer R-Stahel, Ber 24, 538 (1891). Prepn of metastable crystals M. L, Wolfrom, E, J, Kotin, /, Am. Chem, Soc, 64, 1739... 

The xylose reductase (XR) catalyzes the first step of a fungal pathway that allows certain organisms to metabolize xylose, such as Candida boidinii [6], Candida guilliermondii [7], Candida tmpicalis [8], Candida parapsilosis [9], and Debaryomyces hansenii [10]. After the reduction of xylose to xylitol by XR in a manner that can utilize nicotinamide adenine dinucleotide (reduced form NADH) or nicotinamide adenine dinucleotide phosphate (reduced form NADPH), xylitol is re-oxidized to xylulose by xyUtol dehydrogenase, which is often specific for nicotinamide adenine dinucleotide (NAD) [11]. Xylulose is then phosphorylated. An efficient, pathway should recycle the cosubstrate such that there is no... 

Continuous enzymatic production of xylitol with simultaneous coenzyme regeneration in a charged membrane reactor was studied (210). An NADH dependent xylose reductase from C. tenuis catalyzed the reduction of xylose. This was coupled to enzymatic oxidation of glucose by glucose dehydrogenase from Bacillus cereus to make achievable an up to 10,000-fold regeneration of NADH per cycle of discontinuous conversion. Under suitoble conditions, 300 g/L of substrate could be converted in yields above 96% in one single batch reaction. 

NoUeau V, Preziosi-Belloy L, Navarro JM. (1995). The reduction of xylose to xyhtol by Candida guilliermondii and Candidaparapsilosis incidence of oxygen and pH. Biotechnol Lett, 17, 417 22. 

Scheme 8.7 Phosphite dehydrogenase (PDH)-catalyzed regeneration of NADPH to promote the reduction of xylose catalyzed by xylose reductase.

One example for the first scenario was published recently by Zhang et al. who coupled the reduction of xylose to xylitol to the oxidation of glycerol to dihydroxy acetone (Scheme 8.13) [76]. 

---