Xylose reductase

Magnesium is bound in the active site of D-xylose isomerase (Carrell et al., 1984, 1989 Farber et al., 1987 Key etal., 1988 Henrick a/., 1989). Here, the site at which two metals [from among Mg(II), Mn(ll), and Co(II)] bind is similar to that found for Fe(ll) in ribonucleotide reductase (Nordlund et al., 1990). The active site of xylose isomerase is shown in Fig. 28. Magnesium ions are preferred in ATP-utilizing enzyme reactions (Mildvan, 1987). 

C Kinetic isotope effects (KIEs) of a xylose reductase-catalysed cinnamalde-hyde reduction have been determined by 13C NMR using competition reactions with reactants at natural 13C abundance. The primary KIEs indicated that the chemical reaction steps are only partly rate limiting during reduction of aromatic aldehydes and slow steps occur outside the catalytic sequence. The aldo-keto reductase-catalysed... 

Lack of congruence of structure and mechanism Common structure does not imply a common mechanism The /1-barrel structures triosephosphate isomerase and xylose isomerase function by hydride transfer through enol, whereas aldose reductase performs hydride transfer through a metal ion. 

H Lee. The structure and function of yeast xylose (aldose) reductases. Yeast 14 977-984, 1998. 

Fig. 7 Xylose reductase catalyzed xylitol synthesis. The coenzyme NADPH was regenerated by a NADP+-accepting mutant of phosphite dehydrogenase...

Xylitol is the probable connecting point between the D-xylose and L-arabi-nose metabolic pathways (Fig. 5). L-arabinose is the form found most abundantly in nature. Early work by Chaing and Knight showed that cell-free extracts of Penicillium chrysogenum convert L-arabinose to both L-ribose and L-xylulose through the intermediate, L-arabinitol (= L-arabitol) [80]. Only one enzyme, aldose reductase, appears to be responsible for the conversion of L-arabinose to L-arabinitol. Aldose reductase also acts on D-arabinose to produce D-arabitol. Witterveen et al. obtained a mutant of Aspergillus niger deficient in... 

Fig. 5. Assimilation of D-xylose, L-arabinose and D-arabinose. In yeasts and fungi, pentoses are assimilated by way of oxidoreductases. D-xylose. L-arabinose and D-arabinose are each reduced to their respective polyols by aldose reductases, designated here as Xor, Lar and Dar. Both D-xylose and L-xylose are reduced to xylitol, which is symmetrical. D-xylose and L-arabinose are the forms normally found in nature. D- and L-arabitol dehydrogenases (Dad and Lad) form D- and L-xylulose, respectively. D- and L-Xylitol dehydrogenase (Dxd and Lxd) mediate the formation of D- and L-xylulose from xylitol...

Yeasts and bacteria metabolize xylose by following sHghtly different pathways as showing in Fig. 1. Yeasts rely on xylose reductase and xyHtol dehydrogenase, but bacteria rely on xylose isomerase, to convert xylose to xylulose [2, 3]. Although the Saccharomyces yeasts as well as other fermentative yeasts are not able to ferment xylose, Saccharomyces yeasts are able to ferment xylulose to ethanol [4]. Furthermore, they are also able to ferment xylose when a bacterial xylose isomerase is present in the medium [5]. This indicates that Saccharomyces yeasts lack only the enzymes for the conversion of xylose to xylulose. 

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). 

Xylose reductase and xylitol dehydrogenase have different cofactor requirements. 

Saccharomyces cerevisiae. Kotter and Ciriacy [97] studied xylose utilization of an S. cerevisiae transformant that expressed two key enzymes (xylose reductase... 

Another enzyme in the same family is xylose reductase of Candida tenuis, for which a structure is available,and which can use either NAD" or NADP" . Site-directed mutagenesis of this enzyme suggests an important, but not critical, role for an active site histidine.The Hisll3Ala mutant had 10 -fold lower activity than wt, but like wt had a sigmoid pH-rate profile, with the decrease in to higher pH governed by a of 8.8 (wt) and... 

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