Glucose-6-isomerase

This is an enzyme that catalyzes the conversion of glncose into fructose and, therefore, the key for the production of fructose-containing syrups. Glucose isomerase is produced intracellularly by certain species of microorganisms. Most commercial types are obtained from Actinoplanes and Streptomyces (Hobbs 2003). Glucose isomerase is used almost eutirely in immobilized form. The enzyme is usually attached to an insoluble support, such as diethylaminoethyl (DEAL), cellulose, or alumina, via adsorption. In most conunercial applications, the inunotriUzed enzyme 

In the United States, more than 90% of the starch is transformed into syrups. The production and utilization of maize syrups has increased during the past decades because the soft drink industries prefer to use sweeteners instead of crystallized sugars. Maize sweeteners are preferred over sucrose because they readily dissolve in water and are easier to incorporate into soft drinks, are easily flavored, and impart a fruit flavor to beverages and foods. The high-fructose com syrup (HFCS), containing 90% fructose, imparts 1.4 times more sweetness than sucrose at equivalent concentrations (Table 13.2). 

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Bacillus sp. These bacteria are gram-positive soil microbes. Members of the Bacillus species supply 58% of iadustrial enzymes sold (19). Eor example, proteases from B. amjloliquefaciens and amylases from B. licheniformis glucose isomerase from B. coagulans are used ia a variety of iadustrial processes (see Enzyme applications-industrial). The proteiaaceous iaclusioas produced by B. thuringiensis are useful as iasect toxias. Thus exteasive fermentation technology has been developed for Bacillus species and low cost media are available (19). 

Bulk Enzymes. Enzymes such as proteases, amylases, glucose isomerases, and rennin are used in food processing. Similarly proteases and Hpases are used in detergents. CeUulases and xylanases are used in the paper pulp industry. The genes for most of the enzymes used in the various commercial processes have been cloned and overexpressed. Rennin (chymosin) produced from E. coli and A. nigerhas been approved by FDA for use in the dairy industry. 

P-amylase, and debranching enzymes. Conversion of D-glucose to D-fmctose is mediated by glucose isomerase, mosdy in its immobilized form in columns. Enzymic degradation of starch to symps has been well reviewed (116—118), and enzymic isomerization, especially by immobilized glucose isomerase, has been fiiUy described (119) (see Syrups). 

Manufacture. HFS containing 42% fmctose is produced commercially by column isomerization of clarified and refined dextrose hydrolyzate using an immobilized glucose isomerase. Enriched symp containing 90% fmctose is prepared by chromatographic separation and blended with 42% HES... 

Two types of immobilization are used for immobilizing glucose isomerase. The intracellular enzyme is either immobilized within the bacterial cells to produce a whole-ceU product, or the enzyme is released from the cells, recovered, and immobilized onto an inert carrier. An example of the whole-ceU process is one in which cells are dismpted by homogenization, cross-linked with glutaraldehyde, flocculated using a cationic flocculent, and extmded (42). 

Enzymes are specific, however. For example, starch is depolymerized using enzymes to D-glucose (dextrose). The solution of glucose is then treated with glucose isomerase [9055-00-9] to give D-fmctose in about 42% yield. No D-mannose is formed. Addition of isolated D-fmctose to this solution gives the common 55% high fmctose com symp (HFCS) so widely used in soft drinks in the United States. HFCS is about 1.5 times as sweet as sucrose. 

Table 2. Typical Operating Parameters for Immobilized Glucose Isomerase and Penicillin V Acylase...

Immobilized enzyme Glucose isomerase Penicillin V acylase... 

The Immobili dEn me System. The glucose isomerases used are immobilized and granulated to a particle size between 0.3 and 1.0 mm. The enzyme granulates must be rigid enough to withstand compaction when they are packed iato the column. Ca " acts as an inhibitor in the system, and therefore calcium salts need to be removed from the feed symp. Conversely, Mg " acts as an activator, and magnesium salts are added to the feed symp. 

During operation, the immobilized enzyme loses activity. Most commercial enzymes show decay as a function of time (Eig. 12). The glucose isomerase ia a reactor is usually replaced after three half-Hves, ie, when the activity has dropped to around 12.5% of the initial value. The most stable commercial glucose isomerases have half-Hves of around 200 days ia practical use. To maintain the same fmctose content ia the finished symp, the feed-flow rate is adjusted according to the actual activity of the enzyme. With only one isomerization reactor ia operation, the result would be excessive variations ia the rate of symp production. To avoid this, several reactors at different stages ia the cycle of enzyme decay are operated ia combiaation. 

Hager M, Currie F, Holmberg K (2003) Organic Reactions in Microemulsions. 227 53-74 Hausler H, Stiitz AE (2001) d-Xylose (d-Glucose) Isomerase and Related Enzymes in Carbohydrate Synthesis. 215 77-114... 

Some of the industrial biocatalysts are nitrile hydralase (Nitto Chemicals), which has a productivity of 50 g acrylamide per litre per hour penicillin G amidase (Smith Kline Beechem and others), which has a productivity of 1 - 2 tonnes 6-APA per kg of the immobilized enzyme glucose isomerase (Novo Nordisk, etc.), which has a productivity of 20 tonnes of high fmctose syrup per kg of immobilized enzyme (Cheetham, 1998). Wandrey et al. (2000) have given an account of industrial biocatalysis past, present, and future. It appears that more than 100 different biotransformations are carried out in industry. In the case of isolated enzymes the cost of enzyme is expected to drop due to an efficient production with genetically engineered microorganisms or higher cells. Rozzell (1999) has discussed myths and realities... 

Glucose isomerase catalyzes the conversion of D-glucose to D-fructose and has also been used extensively on an industrial scale.1184 Some, but not all, enzymes of this family require Co specifically, while others can function with other divalent ions. Environmental and health issues limit the concentrations of Co in culture media during D-fructose production and other metal ions are being sought as substitutes. Although the active site structure remains unknown, EXAFS, optical and EPR spectroscopy has suggest a low-spin divalent Co ion, bound by N and O-donors only (no S-donors). 

As mentioned above, certain metal ions may be necessary for activity or stability. Thus calcium is needed for bacterial a-amylase. Magnesium or cobalt is needed with glucose isomerase. Calcium stabilises the starch-liquifying bacterial a-amylases but inactivates the glucose isomerase that may be used subsequently. Many enzymes contain an additional non-... 

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