Isomerization to fructose

Glucose 6-phosphate is isomerized to fructose 6-phosphate by ring opening followed by a keto-enol tautomerization. 

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). 

The glucose moiety can be isomerized to fructose by the well-established glucose isomerization process to yield a galactose-glucose-fructose syrup with increased sweetness. Another possible variation would involve the isomerization of lactose to lactulose (galactose-fructose) which can be hydrolysed to galactose and fructose by some / -galactosidases. 

Lactulose. Lactulose is an epimer of lactose in which the glucose moiety is isomerized to fructose (Figure 2.18). The sugar does not occur naturally and... 

Mannose, the C-2 epimer of glucose (see p. 84), is an important component of glycoproteins (see p. 164). Hexokinase phosphorylates mannose, producing mannose 6-phosphate, which, in turn, is (reversibly) isomerized to fructose 6-phosphate by phosphoman-nose isomerase. [Note There is little mannose in dietary carbohydrates. Most intracellular mannose is synthesized from fructose, or is preexisting mannose produced by the degradation of structural carbohydrates and salvaged by hexokinase.]... 

Pathway for entry of mannose into intermediary metabolism Mannose, an important component of glycoproteins, is phosphorylated by hexokinase to mannose 6-phosphate, which is reversibly isomerized to fructose 6-phosphate by phos-phomannose isomerase. 

It has been postulated (37) that lactulose is formed from lactose by the Lobry de Bruyn and Alberda van Ekenstein transformation, whereby glucose is isomerized to fructose via an enol intermediate. In turn, two mechanisms have been proposed for the degradation of this intermediate (38)- One involves the addition of a proton to the enediol resulting in epimeric aldoses and the original ketose, while the other involves 8-elimination to yield galactose and saccharinic acids. The authors experimental data would tend to better support the second pathway. 

High fructose syrups are purified aqueous solutions of nutritive saccharides obtained from edible starch in which a portion (at least 42%) of the dextrose (D-glucose) has been isomerized to fructose. 

Step 2 is an isomerization, in which the pyranose ring of glucose 6-phosphate opens, tautomerism causes isomerization to fructose 6-phosphate, and a furanose ring is formed. 

Enzymes, in both soluble and immobilized forms, are also increasingly being employed in the food processing industry to convert other sugars with low sweetening power into more potent sweeteners. As examples, glucose is isomerized to fructose, which is 2.5 times as sweet, by an immobilized glucose... 

Mannose-6-phosphate is an intermediate in metabolism of mannose formed in a phosphorylation reaction catalyzed by hexokinase. Mannose-6-phosphate is readily isomerized to fructose-6-phosphate, which participates in the glycolysis, gluconeogenesis, pentose phosphate pathway, or Calvin cycle pathways. 

In the remainder of the preparative phase of glycolysis, glucose-6-P is isomerized to fructose 6-phosphate (fructose-6-P), again phosphorylated, and subsequently cleaved into two 3-carbon fragments (Fig 22.5). The isomerization, which positions a keto group next to carbon 3, is essential for the subsequent cleavage of the bond between carbons 3 and 4. 

In gluconeogenesis, glucose-6-phosphate is dephosphoiylated to glucose (the last step of the pathway) in glycolysis, it isomerizes to fructose-6-phosphate (an early step in the pathway). 

The end result of either of these reaction schemes is glucose-6-phosphate, which is now isomerized to fructose-6-phosphate by the enzyme phosphoglucose isomerase ... 

The model was applied to estimate the theoretical relationship between the fixed bed initial flow velocity resulting in 45% of glucose being isomerized to fructose and the operating temperature for an immobilized enzyme defined by the characteristics given in table I. The results of this evaluation, in terms of the initial flow velocity relative to that at 65°C, are shown in an Arrhenius plot in figure 4. The experimental results used in the parameter estimation are also shown. The theoretical relationship is shown to be definitely non linear in an Arrhenius plot. 

Ospina S, Lopez-Munguia A, Gonzalez R et al. (1992) Characterization and use of a penicillin acylase biocatalyst. J Chem Technol Biotechnol 53 205-214 Ospina S, Barzana E, Ramirez O et al. (1996) Effect of pH in the synthesis of ampicilfinby penicillin acylase. Enzyme Microb Technol 19 462-469 Palazzi E, Converti A (2001) Evaluation of diffusional resistances in the process of glucose isomerization to fructose by immobilized glucose isomerase. Enzyme Microb Technol 28 246-252 Pan JL, Syu MJ (2004) A thermal study on the use of immobilized penidllin G acylase in the formation of 7-amino-3-deacetoxy cephalosporanic acid from cephalosporin G. J Chem Technol Biotechnol 79(10) 1050-1056... 

Unlike fructose, glucose, one of the most common aldohexoses, is difficult to transform directly to HMF. It is the most abundant monosaccharide obtained from the depolymerization of cellulose. To achieve successful HMF production from glucose, heterogeneous catalyzed isomerization to fructose therefore appears as a key reaction. There are two mechanisms for glucose isomerization via proton transfer and intramolecular hydride shift. These are generally catalyzed by a base and Lewis acid, respectively. 

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