Isomerization of glucose to fructose

The majority of current industrial-scale applications of immobilized microbial cells in continuous biolechnical processes are based on single-enzyme-catalyzed transformations. Potential applications involve carbohydrate conversions, of which 

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S (2)-hydroxy-3-butenenitrile from acrolein and HCN trans hydrocyanation using, for instance, acetone cyanohydrin Hydrolysis of nitriles to amides, e.g. acrylonitrile to acrylamide Isomerization of glucose to fructose Esterifications and transesterifications Interesterify positions 1 and 3 of natural glycerides Oxidation of glucose to gluconic acid, glycolic acid to glyoxalic acid... 

Unlike reactions involving microorganisms, in enzyme reactions the catalytic agent (the enzyme) does not reproduce itself. An example in the use of enzymes is the isomerization of glucose to fructose ... 

Production of HMF from glucose was also evaluated using titanium and zirconium oxides as solid acid catalysts in hot compressed water (473 K) [78-80], Interestingly, it was found that the Zr02 catalyst acts as an acido-basic catalyst. Therefore, Zr02 was able to promote a tandem reaction involving (1) the isomerization of glucose to fructose and (2) the dehydratation to fructose to HMF (yield of HMF = 20% Scheme 8). 

Glucose Isomerization. Enzymatic isomerization of glucose to fructose provides a real alternative to sugar (sucrose) derived from sugarcane or sugarbeets. The commercial product obtained is known as high fructose com syrup (HFCS). Two grades of the syrup have become established on the world market, HFCS-42 and HFCS-55, which contain 42 and 55% fructose on dry substance basis. These products account for over one-third of the caloric sweetener market in the United States. 

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 isomerization of glucose to fructose has been extensively studied and the mechanism is well-documented.35-37 The reaction is essentially first order and reversible, following Michalis-Menten characteristics shown in Figure 21.9. 

Glucose Isomerase (xylose isomera.se) Isomerization of glucose to fructose, and xylose to xylulose. 

The isomerization of glucose to fructose opened the way for starch hydrolyzates to replace cane or beet sugar (Dziezak 1987). This process is done with glucose isomerase in immobilized enzyme reactors. The conversion is reversible and the equilibrium is at 50 percent conversion. High-fructose com syrups are produced with 42 or 55 percent fructose. These sweeteners have taken over one-third of the sugar market in the United States (Olsen 1995). 

It may be noted that simple alkaline-catalyzed isomerization of glucose to fructose is possible, but gives rise to serious lactic acid and coloured by-product formation. Alkaline catalysis, however, is still applied for the conversion of lactose to lactulose, used in treatment of constipation and PSE. The reason is that no enzyme has been found that is able to isomerize the glucose unit of lactose into a fructose moiety. As a consequence, a low conversion is applied or borate is used as a protecting group. In the latter case extra separation and recycle steps are required. 

The chromatographic SMB reactor has been examined for various reaction systems, with the main focus on reactions of the type A + B C + D. Examples are esterifications of acetic acid with methanol (Lode et al., 2003b), ethanol (Mazotti et al., 1996a) and (5-phenethyl alcohol (Kawase et al., 1996) as well as the production of bisphenol A (Kawase et al., 1999). The same reaction type can also be found for various hydrocarbons, such as the transfer reaction of sucrose with lactose to lactosuc-rose (Kawase et al., 2001) and the hydrolysis of lactose (Shieh and Barker, 1996). Barker et al. (1992) focused on reactions of the type A B + C, such as enzyme-catalyzed sucrose inversion and the production of dextran. Also, reactions of the type A tB have been investigated, e.g. isomerization of glucose to fructose by Fricke (2005) as well as Tuomi and Engell (2004). Michel et al. (2003) have examined the application of electrochemical SMB reactors for consecutive reactions and used as an example the production of arabinose. 

So far, the concept has been used for the isomerization of glucose to fructose (Hashimoto et al., 1983 and Borren and Schmidt-Traub, 2004), but further applications such as thermal racemizations are conceivable. An application for reactive systems with more than two components is also possible if special catalysts or operating conditions are required. However, an even more complex design, as well as control of the process, has to be taken into account. 

The reactive simulated moving bed process considered here is the isomerization of glucose to fructose. The plant consists of six reactive chromatographic fixed beds that are interconnected to form a closed-loop arrangement (Fricke and Schmidt-Traub, 2002). As shown in Fig. 9.3, a pure glucose solution is injected to the system at the feed line. At the extract line, a mixture of glucose and fructose, called high fructose... 

Fig. 18. Enzymatic hydolysis of sucrose following isomerization of glucose to fructose...

Fig. 2 Isomerization of glucose to fructose over an immobilized glucose isomerase.

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. ... 

Reactions of the type A B have been investigated, for example, the isomerization of glucose to fructose, by Fricke (2005) as well as Toumi and Engell (2004). However, for isomerizations conventional SMB reactors are suitable only if the purity requirements are moderate. In other cases, a spatial distribution of the reaction and separation is necessary as discussed below. 

All reactions, catalyzed or otherwise, are reversible at the molecular level. This fact is most important in kinetics when overall conversion is limited thermodynamically. For example, the isomerization of glucose to fructose by glucose isomerase... 

Illanes A, Zufiiga ME, Contreras S et al. (1992) Reactor design for the enzymatic isomerization of glucose to fructose. Bioproc Eng 7 199-204... 

The invention of framework tin-containing zeotype materials created new opportunities for reactions requiring weak Lewis acid sites. An example is Sn-beta, which has exhibited extraordinary selectivity in partial oxidation reactions like the Baeyer—ViUiger (BV) oxidation of ketones to lactones, and also has been shown to be highly selective in the isomerization of glucose to fructose. 

Figure 1.12 Pathways characterizing the isomerization of glucose to fructose catalyzed by base (A) or by Sn-beta (B). The base-catalyzed process is initiated by proton abstraction, whereas in Sn-beta, the glucose forms a chelating complex with the tin. Adapted from Ref (244b), with permission from Wiley.

Selective isomerization of glucose to fructose in aqueous medium Sn-p zeolite Fructose is easier to transform in suitable platform molecules than the abundant glucose. Hence, to find selective catalysts for this transformation is a critical key in the lignocellulosic biomass conversion [74]... 

Step 2 is the isomerization of glucose to fructose. This reaction involves the conversion of the aldohexose into the 2-ketohexose. Retro-aldol reaction of the aldohexose leads to a C4 and C2 sugar, whereas the ketohexose leads to the two trioses, dihydroxyacetone (DHA) and glyceraldehyde (GLY). As the pathway to LA involves the trioses, selective glucose isomerization is essential, its conversion being limited by equilibrium in the operational temperature window. The isomerization of aldo- to ketoses can proceed via an acid-catalyzed hydride shift, a base-catalyzed mechanism with a proton shift (and intermediate enol), or via a concerted 1,2-hydride shift in neutral media [96, 97]. The latter isomerization mechanism occurs at mild temperatures (100°C) in the presence of Lewis acid catalysts, first... 

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