Isomerization, xylose isomerase

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

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. 

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. ... 

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... 

Glucose isomerase was initially isolated and identified in 1953 as a xylose isomerase (Hochester and Watson). Four years later the glucose isomerizing capability of xylose isomerase from Pseudomonas liydrophilki was discovered although arsenate was required to enhance the reaction. This enzyme was thus impractical for food production (Marshall and Kooi). 

Mechanism of D-fructose isomerization by Arthrobacter D-xylose isomerase,... 

B xylose isomerase Mg2+ hydride transfer between adjacent carbons during aldose/ketose isomerization... 

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,... 

The key to the successful scale-up was the immobilization of the enzyme, which increased stability and reduced the enzyme costs to an acceptable level. The isomerization step is typically carried out in a parallel series of packed-bed reactors, where the enzyme is immobilized on silica or inert cellulose carriers (Figure 5.22). A ton of immobilized xylose isomerase can catalyze the production of 5000 tons of HFCS [35]. Current research is concentrating on developing a more thermostable enzyme variant which would reach the 5 5 45 ratio directly in the reactor column. 

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

L-Arabinose isomerase has been purified from Escherichia coli. The enzyme catalyzes the isomerization of L-arabinose to L-ribulose by a proton transfer mechanism in contrast to xylose isomerase which uses a hydride transfer mechanism to perform a similar isomerization [20], This enzyme has high substrate specificity for L-arabinose, and its catalytic activity is dependent on manganese(II). It has been shown, however, that the enzyme could catalyze the exchange of the proton attached to C2 of arabinose with the solvent even in the absence of metal ion [20],... 

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... 

D-Xylose isomerase catalyzes the interconversion between D-xylose and D-xylulose (Fig. 17-21). Since this enzyme acts on D-glucose to produce D-fructose, it is often referred to as glucose isomerase (Fig. 17-21). The isomerization of glucose to fructose by this enzyme is a very important process for the industrial production of high fructose com syrup. This enzyme is also applicable to the synthesis of many aldoses and ketoses because of its wide substrate specificity. The enzyme gene has been cloned from various microorganisms, and the enzyme has been overexpressed, purified, and characterized. Their three dimensional structures have also been determined I203-206. ... 

Xylose isomerase has a wide substrate specificity, and 3-, 5-, or 6- substituted glucose and fructose are isomerized by this enzyme. Since this enzyme requires the 4-OH group for hexoses to be substrates, phosphoglucose isomerase instead of xylose isomerase is used for the synthesis of 4-substituted fructose as described below. 

Xylose isomerase is also used for the synthesis of modified sucroses, which is important in the study of the topographical aspects of the binding of sucrose to a sucrose carrier protein 236. 6-Deoxy- and 6-deoxy-6-fluoroglucose chemically synthesized are isomerized to the corresponding 6-substituted fructose by xylose isomerase. The resultant substrates are subsequently condensed with UDP-glucose by sucrose synthase. Although the equilibrium of the first step lies towards the glucose... 

To overcome the disparity in the optimal pH s for the isomerization and fermentation, our group [29, 35, 36] proposed a novel scheme of isomerization that incoiporates urease co-immobilized with xylose isomerase. This technique uses XI immobilized in a porous pellet for isomerization and the immobilized urease enzyme for pH control (Fig. 1). These co-immobilized enzyme pellets are dispersed in a fermentation broth, which contains urea in addition to the other necessary ingredients for fermentation. Theoretically, it is possible to sustain a significant pH gradient between the bulk liquid and the core region of the pellet... 

Simultaneous isomerization and fermentation (SIF) is expected to drive the isomerization forward and lead to higher xylose utilization. Unfortunately, the pH optima for the isomerization ( 7.5) and the fermentation ( 4.5) steps are vastly different. The approach we have taken is to develop a novel technique for SIF that is capable of sustaining two different pH-microenvironments in a single vessel—one optimal for xylose isomerization and the other optimal for fermentation of xylulose. The technique involves co-immobilization of urease with xylose isomerase. We have shown that it is possible to sustain a significant pH gradient between the bulk liquid and the core region of the pellet by adding urea to the fermentation broth. 

Liu, H. H., Shi, Y. (1997). The reaction pathway of the isomerization of D-xylose catalysed by the enzyme D-xylose isomerase A theoretical study. Proteins Structure, Function and Genetics, 27, 545-55. 

Fournier, R. L., Varanasi, S., Byers, J. P. Bilayer pellet containing immobilized xylose isomerase and urease for the simultaneous isomerization and fermentation of xylose to ethanol, US Patent 5,254, 468. 

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. 

Glucose isomerase (actually xylose isomerase) is undoubtedly the most important and successful application of enzyme technology. Glucose isomerization by glucose isomerase was developed in the late 1960s but it was not until the mid-1970s when the process acquired industrial significance as a consequence of the... 

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