Enzyme xylose isomerase

The information that there is a high probability that the metal lies in the carboxyl plane, but with possible deviations for specific metals, provides a mechanism for searching for metal-binding positions in proteins. This method was used in a study of the enzyme xylose isomerase from Streptomyces rubiginosus (Carrell et al., 1989). Two metal sites were located. One metal-binding site involves three carboxylates (aspartate and glutamate), histidine, and water, and the other involves four carboxylate... 

The isomerization of glucose to fructose, catalyzed by the enzyme xylose isomerase, is by far the largest-scale biocatalytic process. Already known for several decades,... 

R. M. Nicoll, S. A. Hindle, G. MacKenzie, I. H. Hillier and N. A. Burton, Quantum mechan-ical/molecular mechanical methods and the study of kinetic isotope effects modelling the covalent junction region and application to the enzyme xylose isomerase, Theor. Chem. Acc., 106 (2000) 105-112. 

Xylose can be metabolized by bacteria, fungi or yeast. In bacteria, the initial step of xylose metabolism involves inducible enzymes (i.e., xylose transport enzymes, xylose isomerase and xylulokinase). The direct isomerization of xylose... 

The CO-factor imbalance generated by the first two steps in xylose metabolism could be entirely circumvented if the conversion of xylose to xylulose were to be catalyzed by the prokaryotic enzyme xylose isomerase (XI, Fig. 1). o-Xylose (glucose) isomerase EC 5.3.1.5 catalyses the reversible isomerization of o-xylose and D-glucose to D-xylulose and D-fructose, respectively. XI does not require redox cofactors and cannot generate cofactor imbalance during anaerobic xylose utilization. 

Fig. 7 Hexose and pentose pathways for Eukarya and Bacteria. Mannose, glucose, and galactose are quickly phosphorylated after uptake in the cell. Pentoses are assimilated by yeast (solid lines) through an oxidoreductase pathway, whose bottleneck to xylose assimilation is the imbalance redox generated by xylose reductase (XR) and xylitol dehydrogenase (XDH) distinct cofactor preference. The same does not occur in bacteria, once single step xylose assimilation is done by enzyme xylose isomerase (XI). Blue dotted lines represent pentose assimilation pathways for Bacteria...

VieiUe et al. studied the activity of the enzyme xylose isomerase, which was derived from the thermophilic organism T. neapolitana, for the isomerization of fructose (F) to glucose. This is the reverse of the reaction that is used to produce high-fructose com syrup for the beverage industry. The reverse reaction was studied to help understand the biochemistry and the behavior of the enzyme. 

The enzymic synthesis of D-plant polysaccharides. Xylose isomerase has been found in Lactobacillus pento-... 

Figure 6. Plot of relative log of apparent versus pH for Ghiioi mutant (filled circles) and wild-type (open circles) xylose isomerases. Apparent values at different pH were determined from a Lineweaver-Burk plot. The scale of relative log Vmax pp indicates the fraction of each experimental value at different pH relative to the maximal value. Both enzymes were stable under the assay conditions used. Reprinted with permission from ref. 22. Copyright 1990 American Society for Biochemistry and Molecular Biology.

Interest in the bacterial ens me xylose/glucose isomerase has been driven by its use in the isomerization of ucose to fructose to produce high>fructose corn syrups, and in the isomerization of xylose to xylulose for the conversion of the more fermentable xylulose to ethanol In this work, a brief historical perspective is presented, followed by a summary of the current understanding of the enzyme s major features. Also, a useful compilation of available xylose isomerase DNA sequences is presented with annotation of some of the major areas identified as being of functional significance. The extent of homology between the xylose isomerases is discussed with reference to differences in their function. 

The enzyme is also being studied for use in converting of biomass to ethanol for fuel usage. Prospects for the conversion of cellulolytic biomass to ethanol for fuel or as a fuel additive have improved within the last decade because of the development of methods for the fermentation of xylose, which can comprise as much as 50% of the fermentable sugars in these feedstocks. One of these methods uses xylose isomerase to convert xylose, which is difficult to ferment by ethanol-tolerant yeasts, to the fermentable sugar xylulose (12,13). 

Initial Discoveries. Xylose isomerase activity was initially found in 1953 in extracts of Lactobacillus pentosus (14), followed by similar activities in extracts of Pseudomonas hydrophila and Pasteurella pestis in the mid-1950s (15-17). An enzyme activity that was found to convert glucose to fructose was discovered in 1957 (18). This activity, found in sonicated extracts from Pseudomonas hydrophila, was enhanced in the presence of... 

Since these early discoveries, xylose isomerases have been isolated from many bacterial species, and these enzymes have been intense investigated, especially those of the genera Streptomyces, Lactobacillus, and Bacillus. The characteristics of substrate specificity (xylose glucose > ribose), divalent metal cation activation (Mg, Mn or Co ), and activity at alkaline pH are properties that most of the enzymes share to a certain extent, but significant variations exist. Some of these em es have been immobilized and patented for commercial use. There are many good reviews in the literature that describe the enzymatic characteristics of the xylose isomerases 9,28,29). 

Xylose isomerases with higher thermostability were found in the strains of Streptomyces and relaxed Actinoplanaceae (which includes the generdLAmpullariella and Actinopianes). High thermo-tolerance is desirable for production of HFCS because at equilibrium, as the temperature of the enzyme reaction is increased, the ketose/aldose ratio increases proportionately 30). In addition, reactors running at higher temperatures have less risk of microbial contamination, allowing for less frequent and less costly enzyme replacement. 

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). 

Carrell, H. L., dusker, J. P., Burger, V., Manfre, F., Tritsch, D., and Biellmann, J.-F. (1989). X-Ray analysis of o-xylose isomerase at 1.9 A Native enzyme in complex with substrate and with a mechanism-designed inactivator. Proc. Natl. Acad. Sci. U.S.A. 86, 4440-4444. 

Knowledge of the three-dimensional structure of an enzyme is the basis for understanding its substrate specificity. Thus, the E. coli Rhal was crystallized and its X-ray crystal structure was solved in collaboration with I. Korndorfer [22]. The enzyme is a tight tetramer of four (P/a)g barrels, with structural similarity to xylose isomerase (Eig. 2.2.5.3). The structures of complexes of rhamnose isomerase with the inhibitor L-rhamnitol and the natural substrate L-rhamnose were... 

These enzymes vary widely in secondary and tertiary structure.1273 Mannose-6-phosphate isomerase is a 45 kDa Zn2+-containing monomer. The larger 65 kDa L-fucose isomerase, which also acts on D-arabinose, is a hexameric Mn2+-dependent enzyme.1273 L-Arabinose isomerase of E. coli, which interconverts arabinose and L-ribulose, is a hexamer of 60-kDa subunits128 while the D-xylose isomerase of Streptomyces is a tetramer of 43-kDa subunits.129 The nonenzymatic counterpart of the isomerization catalyzed by the enzyme is the base-catalyzed Lobry deBruyn-Alberda van Ekenstein transformation (Eq. 13-25).130... 

---