Conversion to fructose

Fructose—Dextrose Separation. Fructose—dextrose separation is an example of the application of adsorption to nonhydrocarbon systems. An aqueous solution of the isomeric monosaccharide sugars, C(5H120(5, fructose and dextrose (glucose), accompanied by minor quantities of polysaccharides, is produced commercially under the designation of "high11 fructose com syrup by the enzymatic conversion of cornstarch. Because fructose has about double the sweetness index of dextrose, the separation of fructose from this mixture and the recycling of dextrose for further enzymatic conversion to fructose is of commercial interest (see Sugar Sweeteners). 

Adopting the values of the kinetic constants given by Fratzke et al. it can be shown that for the conditions prevailing in the fixed bed conversion process, Cg - Cg is small as compared to K g. At an initial syrup glucose concentration of 3000 moles/m and a relative conversion to fructose of 45%, K g is of the order of 5000 moles/m, and Cg - Cg varies between about 1000 moles/m and 20 moles/m (column inlet and column outlet respectively). 

One pathway for the metabolism of glucose 6-phos-phate is its enzyme-catalyzed conversion to fructose 6-phosphate ... 

In the previous section we considered three possible pathways for glucose-6-phosphate dephosphorylation by glucose-6-phosphatase, conversion to fructose-6-phosphate followed by oxidation through the Embden-Meyerhof pathway, and conversion to glucose-1-phosphate followed by glycogen synthesis. A fourth pathway exists—the hexose monophosphate shunt [61-63]. 

Early applications of crystalline fructose focused on foods for special dietary applications, primarily calorie reduction and diabetes control. The latter application sought to capitalize on a signiftcandy lower serum glucose level and insulin response in subjects with noninsulin-dependent diabetes melUtus (21,22) and insulin-dependent diabetes (23). However, because fmctose is a nutritive sweetener and because dietary fmctose conversion to glucose in the hver requires insulin in the same way as dietary glucose or sucrose, recommendations for its use are the same as for other nutritive sugars (24). Review of the health effects of dietary fmctose is available (25). 

Figure 25.8 Fructose, a ketose, is a reducing sugar because it undergoes two base-catalyzed keto-enol tautomerizations that result in conversion to an aldose.

Steroids, e.g. cholesterol, triolein, androsterone sugars, e.g. fructose, glucose, ribose amino acids, pyrimidines, purines, alkaloids 110-150°C, 2-12 h Conversion to fluorescent derivatives by heating. [5]... 

Phosphofructokinase (PFK) is a key regulatory enzyme of glycolysis that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-diphosphate. The active PFK enzyme is a homo- or heterotetrameric enzyme with a molecular weight of 340,000. Three types of subunits, muscle type (M), liver type (L), and fibroblast (F) or platelet (P) type, exist in human tissues. Human muscle and liver PFKs consist of homotetramers (M4 and L4), whereas red blood cell PFK consists of five tetramers (M4, M3L, M2L2, ML3, and L4). Each isoform is unique with respect to affinity for the substrate fructose-6-phosphate and ATP and modulation by effectors such as citrate, ATP, cAMP, and fructose-2,6-diphosphate. M-type PFK has greater affinity for fructose-6-phosphate than the other isozymes. AMP and fructose-2,6-diphosphate facilitate fructose-6-phosphate binding mainly of L-type PFK, whereas P-type PFK has intermediate properties. 

Methylation was accomplished by direct conversion of the acetyl derivative as previously described. The final product contained 45% OCHs. Molecular weight determinations in benzene gave values representing 18 to 19 D-fructose residues. Hydrolytic products of the methyl derivative were separated by conversion to benzoyl derivatives. The trimethylfructose was identified as 3,4,6-trimethyl-D-fructofuranose by its phenylosazone. The authors conclude that the ratios 1 3 1 exist among the di-, tri-, and tetramethyl-D -fructoses produced by the hydrolysis. 

Methylgraminin was obtained as a colorless sirup by the method of Haworth and Streight.24 Hydrolysis of the methyl derivative gave a mixture of di-, tri-, and tetramethyl-D-fructoses that were separated after conversion to benzoyl derivatives. The authors conclude that these partially methylated D-fructoses are present in the ratios 1 1 1 or 1 2 1. 

Upon hydrolysis an equal amount of tetramethyl-D-fructose and dimethyl-D-fructose was obtained. The first of these was identified as 1,3,4,6-tetramethyl-D-fructose by conversion to the characteristic crystalline acid amide obtained by Haworth, Hirst and Nicholson.66 The dimethyl-D-fructose was obtained as a thick brown sirup having [oT]d20 = + 20.0° (c = 1.82, chloroform). 

The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate in a reaction catalysed by the enzyme fructose-bisphosphatase ... 

Figure 11.2 Pathway for conversion of fructose to acetyl-CoA. The enzyme fructokinase phosphorylates fructose to form fructose 1-phosphate. (The enzyme is present only in the liver.) Fructose 1-phosphate is cleaved by aldolase to form glyceraldehyde and dihydroxyacetone phosphate. Glyceraldehyde is phos-phorylated to form glyceraldehyde 3-phosphate, catalysed by the enzyme triokinase. Dihydroxyacetone phosphate is converted to glyceraldehyde 3-phosphate, catalysed by the isomerase. Glyceraldehyde 3-phosphate is converted to pyruvate by the glycolytic reactions (Chapter 6).

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. 

Glucose 6-phosphate is then isomerized to fructose 6-phosphate. This conversion of an aldose sugar to a ketose sugar is easy to rationalize in terms of keto-enol tautomerism (see Box 10.1). 

In 1999, Carlini et al. investigated the ability of niobium-based phosphate to catalyze the selective dehydration of fructose, sucrose, and inulin to HMF (Scheme 7) [74]. Starting from fructose and using a column reactor packed with niobium phosphate catalyst, 67% selectivity to HMF was obtained at 38% conversion. This catalyst was stable in the presence of water and was successfully reused without notable change of activity. Interestingly, from sucrose and inulin, the niobium-based catalysts afforded HMF with 66% selectivity at 47% conversion. A significant improvement of both the catalyst activity and the HMF selectivity was achieved when the HMF was continuously extracted from the water phase with methylisobutylketone (MIBK). Indeed, under these conditions, HMF was produced with 98% selectivity at 60% conversion of fructose. Using the same procedure, but from inulin, HMF was obtained with 72% selectivity at 70% conversion. 

Vanadyl phosphate (VOPO4 2H2O), a layered material containing Lewis and Brpnsted acid sites, was also tested as an acid catalyst in the dehydration of fructose to HMF (Scheme 7) [76]. These catalysts afforded HMF with a selectivity of 80% at 46% conversion of fructose. Other catalytic systems, obtained by partial substitution of groups with trivalent metals (Fe ", Cr, Ga, Mn, Al, were also investigated. It has been shown that when Fe-substituted vanadyl phosphate was employed a selectivity of 90% to HMF was obtained at 38% conversion without the formation of secondary products such as levuUnic and formic acid. This catalyst was also proved to be efficient for the conversion of inuUn into HMF with the same selectivity as with fructose. 

As observed above, in order to quench HMF produced in situ, dealuminated H-form mordenites were investigated in a water/MIBK mixture (1/5) [84, 85]. In this case, a maximum conversion of fructose of 54% (along with 90% selectivity to HMF) was obtained over an H-mordenite with a Si/Al ratio of 11. HMF was continuously extracted with a flow of MIBK circulating in a countercurrent way through a catalytic heterogeneous reactor containing the H-mordenite zeolite. On the continuation of their efforts, the same authors then set up a new continuous solid-liquid-liquid reactor where the zeolite was now in suspension in the aqueous phase while the HMF was continuously extracted with MIBK in a countercurrent way to the aqueous phase and catalyst feed. 

---