Fructose-glucose isomerase

Isomerases Various Isomerizations, C=C bond migration, cis-trans racemizations, etc. fructose-glucose isomerase... 

Glucose isomerase catalyzes the conversion of D-glucose to D-fructose and has also been used extensively on an industrial scale.1184 Some, but not all, enzymes of this family require Co specifically, while others can function with other divalent ions. Environmental and health issues limit the concentrations of Co in culture media during D-fructose production and other metal ions are being sought as substitutes. Although the active site structure remains unknown, EXAFS, optical and EPR spectroscopy has suggest a low-spin divalent Co ion, bound by N and O-donors only (no S-donors). 

A critical consideration in the development of biocatalytic systems is the form in which the enzyme or enzyme system is going to be used. There are two general approaches. One is to use isolated enzymes. If these are inexpensive, they can be used as disposable biocatalysts, as is the case for glucose isomerase, ° which is the key biocatalyst in the production of high-fructose corn syrups from starch, or the lipases and proteases that are present in detergents. Alternatively, if enzymes are expensive to produce, they can be immobilized and used repeatedly by recovering the enzyme particles after each use. 

Figure 4. Time course of glucose conversion into fructose using heat-treated glucose isomerase obtained from recombinant B. subtilis determined at 70 C and pH 7.0 using 35% (w/w) glucose solution and 10.8 units of enzyme/g of dry substance. Reprinted with permission from ref. 21. Copyright 1990 American Society for Microbiology.

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. 

In the production of the sweetener high fructose com symp (HFCS), glucose isomerase is used to convert glucose into fmctose (equilibrium mixture appr. 50 50) since fmctose is nearly three times as sweet as glucose. The enzyme belongs to the subclass intramolecular oxidoreductases interconverting aldoses and ketoses (EC 5.3.1). About 8 million tonnes pr. year of glucose is treated in this way. For further details, see Chapter 4. 

The discovery of a non-cofactor deperrdent errzyme (glucose isomerase) that wottld isomerise glucose into fructose (see Figrrre 4.1). 

Poulsen, P.B. and Zittan, L. (1976) Continuous production of high-fructose syrup by cross-linked homogenates containing glucose isomerase. Meth. Enzymol., 44,809-821. 

Conversion of Glucose 6-Phosphate to Fructose 6-Phosphate The enzyme phosphohexose isomerase (phospho-glucose isomerase) catalyzes the reversible isomerization of glucose 6-phosphate, an aldose, to fructose 6-phosphate, a ketose ... 

Enzymes are specific, however. For example, starch is depolymerized using enzymes to D-glucose (dextrose). The solution of glucose is then treated with glucose isomerase [9055-00-9] to give D-fructose in about 42% yield. No D-mannose is formed. Addition of isolated D-fructose to this solution gives the common 55% high fructose com syrup (HFCS) so widely used in soft drinks in the United States. HFCS is about 1.5 times as sweet as sucrose. 

You may have noticed that most soft drinks contain "high-fructose corn syrup." Corn starch is hydrolyzed to glucose, which is then treated with glucose isomerase to produce a fructose-rich mixture. The... 

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