Isomerization, of glucose

Among sucrose glucose and fructose fructose is the sweetest Honey is sweeter than table sugar because It contains fructose formed by the isomerization of glucose as shown in the equation... 

Liquid sucrose and Hquid invert, generally made by redissolving white sugar and inverting with invertase enzyme, are refinery products in Europe and outside the United States. In the United States they have been almost completely replaced by cheaper com symps made by enzymatic hydrolysis of starch and isomerization of glucose. 

Increasingly, biochemical transformations are used to modify renewable resources into useful materials (see Microbial transformations). Fermentation (qv) to ethanol is the oldest of such conversions. Another example is the ceU-free enzyme catalyzed isomerization of glucose to fmctose for use as sweeteners (qv). The enzymatic hydrolysis of cellulose is a biochemical competitor for the acid catalyzed reaction. 

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

Reaction 2 Phosphoglucoisomerase Catalyzes the Isomerization of Glucose-6-Phosphate... 

Glycolysis is a ten-step process that begins with isomerization of glucose from its cyclic hemiacetal form to its open-chain aldehyde form—a reverse nucleophilic addition reaction. The aldehyde then undergoes tautomerixa-tion to yield an enol, which undergoes yet another tautomerization to give the ketone fructose. 

Figure 29.8 Mechanism of step 2 in glycolysis, the isomerization of glucose 6-phosphate to fructose 6-phosphate.

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 ... 

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. ... 

This is achieved by two keto-enol tautomerism reactions and a common enol (see Box 10.1). Mechanistically, it is identical to the isomerization of glucose 6-phosphate to fructose 6-phosphate seen earlier in the sequence, so we can move on to the next step of the pathway. 

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). 

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 ... 

In 1895, Emil Ficher proposed an enediol intermediate for this isomerization. As would be expected, the enzyme-catalyzed isomerization of glucose-6-phosphate in 2H20 is accompanied by incorporation of deuterium into the product fructose 6-phosphate at C-l. In the reverse reaction 2H-containing fructose 6-phosphate was found to react at only 45% of the rate of the 1H-containing compound. Thus, the primary deuterium isotope effect expected for a rate-limiting cleavage of the C-H bond was observed (see Chapter 12, Section B,3). 

Step 2. The Isomerization of Glucose-6-Phosphate into Fructose-6-Phosphate The reaction is... 

As very high degrees of conversion are achieved in a PFR with much smaller reactor volumes than in a CSTR (Lilly, 1976 Vieth, 1976) a PFR with immobilized enzymes is most suitable if complete conversion has to be achieved, as in the case of isomerization of glucose or the decarboxylation of D,L-aspartate with L-aspar-tate-/l-decarboxylase. 

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