Fructoses, isomerization

VieiUe et al. studied the activity of the enzyme xylose isomerase, which was derived from the thermophilic organism T. neapolitana, for the isomerization of fructose (F) to glucose. This is the reverse of the reaction that is used to produce high-fructose com syrup for the beverage industry. The reverse reaction was studied to help understand the biochemistry and the behavior of the enzyme. 

The table above shows the rate of disappearance of fructose as a function of its concentration. 

An isothermal batch reactor was used at 70 °C and pH 7, with a sodium phosphate buffer. The initial concentration of glucose was zero in all cases. The rates in this table arc initial rates, i.e., the rate at essentially zero fructose conversion. 

As an aside, the rate of disappearance, —rp, would usually be designated V (for reaction velocity) in the biochemistry and biochemical engineering literature. In addition, the reactant, fructose in this case, would be referred to as the substrate. 

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Mechanism of D-fructose isomerization by Arthrobacter D-xylose isomerase,... 

The kinetics of the D-glucose-D-fructose isomerization reaction catalyzed by soluble D-glucose isomerase at a specified concentration of substrate can be related to a simple concept of an enzyme-catalyzed, reversible reaction. 

Glucose to fructose isomerization (HFCS production) Glucose isomerase... 

The latter isomerization via an intramolecular hydride shift is catalyzed by Lewis acids. Tin-containing P-zeoHte exhibits remarkable activity for the glucose-fructose isomerization in water. and NMR studies have elucidated this Lewis acid-catalyzed isomerization mechanism, and show a clear difference between Sn-fl (Lewis acid catalysis) and NaOH (Bronsted base catalysis). 

To begin, let s determine wheOier an irreversible, first-order rate equation, —rp = A [F], fits fire data. At 70 °C, fructose isomerization is quite reversible. However, in this study, fire conversions of fructose were close to zero, and there was no glucose in file initial mixture. Therefore, the reverse reaction was not kineticaUy important for these particular experiments. 

Figure 6-9a Test of first-order rate equation for fructose isomerization at 70 °C using xylose isomerase derived from T. neapolitana.

The Michaelis-Menten model appears to provide a much better description of the fructose isomerization data than the simple, first-order model. The points in the above graph fall very close to the best fit straight line, and the scatter seems to be random. The values of the slope and the intercept ate shown on the graph we will return to them shortly. These values were used to calculate the values of the residuals. 

Figure 6-9f Another form of residual plot to test Michaelis-Menten rate equation for fructose isomerization using xylose isomerase derived fiom T. neapolitana. Residuals in reaction rate plotted against experimental values of reaction rate.

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