# Glucose Isomerase: Functions, Structures, and Applications
## 4 Application of GI  
### High-Fructose Corn Syrup (HFCS)  
Owing to its beneficial properties, including a high solubility at low temperature, a lower tendency to crystallize than sucrose, and a high freezing point depression, fructose is widely used in the manufacture of ice cream and frozen desserts to influence taste and texture [48]. Fructose is also used in the production of cake, biscuits, bread, and other confectionery products [49; 50]. Moreover, fructose increases the shelf-life of food products, with its high osmotic pressure in solution, making it a better preservative against microbial growth than sucrose syrup [51]. Additionally, fructose is used in the pharmaceutical industry to manufacture diabetes medicine, as it does not influence the blood levels of glucose and insulin [52]. HFCS is an equilibrium mixture of glucose and fructose, which has the advantages of sweetness, low cost, and high solubility [53]. Depending on the fructose content, HFCS can be classified as HFCS-42 (42% fructose, 53% glucose, and 5% polysaccharide), HFCS-55 (55% fructose, 42% glucose, and 3% polysaccharide), and HFCS-90 (90% fructose, 9% glucose and 1% polysaccharide) [54]. Among them, HFCS-55 is the most commonly used, with its commercial production involving several processes such as chromatography, purification, and concentration [55]. HFCS is widely applied in the food, detergent, and pharmaceutical industries [56]. Although glucose can be converted to fructose using a chemical process for HFCS production, this chemical reaction is non-specific and leads to the formation of non-metabolizable sugars [4]. In contrast, GI catalyzes the isomerization of glucose to fructose with excellent specificity, which is critical for the industrial production and application of HFCS [4]. As GI exhibits a reversible isomerization activity, a thermodynamic equilibrium exists in the isomerization reaction between glucose and fructose [57]. The rate of the enzymatic conversion of glucose to fructose can be increased by increasing the reaction temperature; therefore, using a highly thermostable GI capable of sustained operation at high temperatures is critical for one-step HFCS production [57]. [PERSON] et al. reported a GI from the thermophilic eubacterium _Thermotoga maritima_ (TmaGI) [58], which is produced when the bacterium is grown in the presence of xylose. TmaGI requires the metal cations Co\({}^{2+}\) and Mg\({}^{2+}\) for enzyme activity [58]. This enzyme prefers xylose as a substrate and is most active at pH 6.5-7.5. TmaGI displays the maximum activity at an optimum temperature of 105 to 110 \({}^{\circ}\)C, and its half-life is approximately 10 min at 120 \({}^{\circ}\)C and pH 7.0. TmaGI is a promising candidate for improving the efficiency of the industrial glucose isomerization process, as it exhibits optimal pH from neutral to slightly acidic, as well as a high thermostability [58]. [PERSON] et al. reported the characterization of GI from _Thermobifida fusca_ WSH03-11 (TfuGI) [59]. TfuGI displayed the maximum activity at an optimum temperature of 80 \({}^{\circ}\)C, with a half-life of approximately 2 h at 80 \({}^{\circ}\)C or 15 h at 70 \({}^{\circ}\)C. TfuGI was the most active at pH 10 and retained 95% of its initial activity after incubation at pH 5-10 and 4 \({}^{\circ}\)C for 24 h. Analysis of TfuGI enzyme kinetics revealed the K\({}_{\rm m}\) and k\({}_{\rm cat}\) values to be 197 mM and 1688 min\({}^{-1}\), respectively [59]. TfuGI was able to convert glucose (45% \(w/v\)) to fructose, with a maximum conversion yield of 53% at pH 7.5 and 70 \({}^{\circ}\)C.  
[PERSON] et al. have characterized GIs from _Thermanaeraobacterium xylanolyticum_ (TxyGI), _Thermus oshimi_ (TosGI), _Geobacillus thermocatenulatus_ (GthGI), and _Thermoanaerobacter siderophilus_ (TsiGI) [60]. These enzymes were identified using a genome mining approach for their potential application in manufacturing HFCS at elevated temperatures with a low cost of enriching syrups.