Glucose-to-fructose conversion at high temperatures with xylose (glucose) isomerases from Streptomyces murinus and two hyperthermophilic Thermotoga species

The conversion of glucose to fructose at elevated temperatures, as catalyzed by soluble and immobilized xylose (glucose) isomerases from the hyperthermophiles Thermotoga maritima (TMGI) and Thermotoga neapolitana 5068 (TNGI) and from the mesophile Streptomyces murinus (SMGI), was examined. At pH 7.0 in the presence of Mg2+, the temperature optima for the three soluble enzymes were 85degreesC (SMGI), 95degrees to 100degreesC (TNGI), and >100degreesC (TMGI). Under certain conditions, soluble forms of the three enzymes exhibited an unusual, multiphasic inactivation behavior in which the decay rate slowed considerably after an initial rapid decline. However, the inactivation of the enzymes covalently immobilized to glass beads, monophasic in most cases, was characterized by a first-order decay rate intermediate between those of the initial rapid and slower phases for the soluble enzymes. Enzyme productivities for the three immobilized GIs were determined experimentally in the presence of Mg2+. The highest productivities measured were 750 and 760 kg fructose per kilogram SMGI at 60degreesC and 70degreesC, respectively. The highest productivity for both TMGI and TNGI in the presence of Mg2+ occurred at 70degreesC, pH 7.0, with approximately 230 and 200 kg fructose per kilogram enzymeforTNGI and TMGI, respectively. At 80degreesC and in the presence of Mg2+, productivities for the three enzymes ranged from 31 to 273. A simple mathematical model, which accounted for thermal effects on kinetics, glucose-fructose equilibrium, and enzyme inactivation, was used to examine the potential for high-fructose corn syrup (HFCS) production at 80degreesC and above using TNGI and SMGI under optimal conditions, which included the presence of both Co2+ and Mg2+. In the presence of both cations, these enzymes showed the potential to catalyze glucose-to-fructose conversion at 80degreesC with estimated lifetime productivities on the order of 2000 kg fructose per kilogram enzyme, a value competitive with enzymes currently used at 55degrees to 65degreesC, but with the additional advantage of higher fructose concentrations. At 90degreesC, the estimated productivity for SMGI dropped to 200, whereas, for TNGI, lifetime productivities on the order of 1000 were estimated. Assuming that the most favorable biocatalytic and thermostability features of these enzymes can be captured in immobilized form and the chemical lability of substrates and products can be minimized, HFCS production at high temperatures could be used to achieve higher fructose concentrations as well as create alternative processing strategies. (C) 2002 Wiley Periodicals, Inc.