Isomerases xylose isomerase

Glucose Isomerase (Xylose isomerase) Isomerization of glucose to fructose, and xylose to xylulose. 

Glucose isomerase (Xylose isomerase) EC 5.3.1.5 Bacillus coagulans Arthrobacter spec. Streptomyces olivaceus Streptomyces phaeochromogenes Glucose High fructose com syrup (HFCS)... 

Carrel, H.L., et al. X-ray structure of D-xylose isomerase from Streptomyces nibiginosus at 4 A resolution. 

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

Figure 5. Summary of amino acid sequence homology between different xylose isomerases. The percent of homology was calculated by using the University of Wisconsin Genetics Computer Group, version 5, program (Devereux, L, Haeberli, P., and Smithies, O. Nucleic Acids Res. 12, 387-395, 1984). 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. 

Xylose isomerases (EC 5.3.1.5), often referred to as glucose isomerase, have been studied extensively, in large part because of their use in the conversion of glucose to fructose for high-fructose corn syrup (HFCS). The world market for HFCS is expected to reach a total of 7.9 million metric tons in 1990 which, at a cost of 0.20/LB, would amount to 3.2 billion (i), and sales of xylose isomerase is expected to be about 15 million (T. Wallace, International Biosynthetics, personal communication). Research on xylose isomerase has produced DNA sequences of the gene from a number of bacterial strains, including the detailed structure of the xylose operon (2-7). In addition, x-ray crystallographic studies (8), kinetic measurements (9), and the use of inhibitors (10,11) have led to descriptions of the location of the active site and mechanistic models of its activity. 

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... 

A true xylose isomerase, that did not require arsenate for its activity, was found in strains of Lactobacillus, especially L. brevis (25). In this study, it was found that the activity of isomerization of glucose and xylose were essentialfy equal, and that ribose was isomerized to ribulose at a reduced rate 24). The xylose isomerases from this strain, like those from other species, requires divalent cations for activity. In this case, Mn and Co were found to be required for activity. In other studies, Co has been found to increase xylose isomerase stability 25,26,21. ... 

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. 

This mechanism has been supported by x-ray studies on crystallized xylose isomerase that suggest that the "active-site histidine" (presumed to be His 53, or 54 in some sequences) is located correctly so as to properly remove a proton from either Cl or C2 of the substrate, and that two threonine residues (Thr90 and Thr91, or 89,90 in some sequences) are positioned to provide polar oxygens to be either the acceptor or the donor proton in the cu-enediol configuration (57). 

The xylose isomerase DNA sequences from Bacillus subtilis (5), Escherichia coli (2,5), Streptomyces violaceoniger (4), and mAmpullariella sp., (6) were computer down-loaded from GenBank (Mountain View, CA). The sequence from Actinoplanes missouriensis (7) was typed into the computer twice. Discrepancies between the sequences were then compared to the original to produce a verified copy. Analysis of these sequences were performed using software developed by D. Mount and B. Conrad at the University of Arizona (Department of Molecular and Cellular Biology, Biosciences West, Tucson AZ). 

Figure 2. Gene-coding regions of xylose isomerases isolated from 5 bacterial strains. Homologous regions (x) are mapped from the computer-generated amino acid sequences minimum = 4aa e x sam = AT0S b x sam = ATCG sam x sam or b X e = ATCO...

Ring opening of the cryptand derived from condensation of the branched tetraamine tren with 2,6-diacetylpyridine (in a 2 3 molar ratio) in the presence of manganese(II) acetate, NaBF4," " and NEts yielded the dinuclear complex Mn2L(CH3COO)](BF4)3 (where L = XH5) which was proposed as a structural model for active sites in natural systems. The Mn—Mn separation is 4.82 A compared with that of 4.9 A found in the D-xylose isomerase from Streptomyces rubiginosus. 

Figure 37 Schematic view of the active site of xylose isomerase in the absence of substrate.

MAPPING SUBSTRATE INTERACTIONS USING KINETIC DATA XYLOSE ISOMERASE GLUGOSE OXIDASE GLUOOSE-6-PHOSPHATASE... 

LAMBDA (or A-j ISOMERS OF METAL ION-NUCLEOTIDE COMPLEXES METAL IONS IN NUCLEOTIDE-DEPENDENT REACTIONS XYLOSE ISOMERASE... 

XYLOGLUCAN SYNTHASE d-XYLONATE DEHYDRATASE XYLOSE ISOMERASE D-Xylose 5-phosphate,... 

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