Production of glucose isomerase by genus Streptomyces

Molecular and industrial aspects of glucose isomerase

Glucose isomerase (GI) (D-xylose ketol-isomerase; EC. 5.3.1.5) catalyzes the reversible isomerization of D-glucose and D-xylose to D-fructose and D-xylulose, respectively. The enzyme has the largest market in the food industry because of its application in the production of high-fructose corn syrup (HFCS). HFCS, an equilibrium mixture of glucose and fructose, is 1.3 times sweeter than sucrose and serves as a sweetener for use by diabetics. Interconversion of xylose to xylulose by GI serves a nutritional requirement in saprophytic bacteria and has a potential application in the bioconversion of hemicellulose to ethanol. The enzyme is widely distributed in prokaryotes. Intensive research efforts are directed toward improving its suitability for industrial application. Development of microbial strains capable of utilizing xylan-containing raw materials for growth or screening for constitutive mutants of GI is expected to lead to discontinuation of the use of xylose as an inducer for the production of the enzyme. Elimination of Co2+ from the fermentation medium is desirable for avoiding health problems arising from human consumption of HFCS. Immobilization of GI provides an efficient means for its easy recovery and reuse and lowers the cost of its use. X-ray crystallographic and genetic engineering studies support a hydride shift mechanism for the action of GI. Cloning of GI in homologous as well as heterologous hosts has been carried out, with the prime aim of overproducing the enzyme and deciphering the genetic organization of individual genes (xylA, xylB, and xylR) in the xyl operon of different microorganisms. The organization of xylA and xylB seems to be highly conserved in all bacteria. The two genes are transcribed from the same strand in Escherichia coli and Bacillus and Lactobacillus species, whereas they are transcribed divergently on different strands in Streptomyces species. A comparison of the xylA sequences from several bacterial sources revealed the presence of two signature sequences, VXW(GP)GREG(YSTAE)E and (LIVM)EPKPX(EQ)P. The use of an inexpensive inducer in the fermentation medium devoid of Co2+ and redesigning of a tailor-made GI with increased thermostability, higher affinity for glucose, and lower pH optimum will contribute significantly to the development of an economically feasible commercial process for enzymatic isomerization of glucose to fructose. Manipulation of the GI gene by site-directed mutagenesis holds promise that a GI suitable for biotechnological applications will be produced in the foreseeable future.

Safety evaluation of Streptomyces murinus glucose isomerase

A glucose isomerase enzyme, obtained from Streptomyces murinus, was produced by a fermentation process and subjected to a series of tests to investigate its safety in use and manufacture. It was not mutagenic (Ames test, using liquid culture) nor did it provoke chromosomal damage (rat bone marrow cytogenetics test). It did not contain (nor did the organism produce) antimicrobial activity or macrolidpolyene antibiotics. It had no teratogenic activity when administered to pregnant rats at 100,000 ppm in the diet. It was without effect upon rats when administered at this dietary concentration for 4 weeks. Dietary administration at 5000, 15,000 or 50,000 ppm to rats for 13 weeks resulted in nephrocalcinosis in females at all dosages (probably a physiological response to the altered calcium:phosphate ratio in the admixed diet) and status spongiosus in the brains of males receiving 50,000 ppm. As the finding of nephrocalcinosis in rats is generally agreed to be of no toxicological importance with regard to the use in man, the dietary concentration of 15,000 ppm was considered to be highest no-effect level. This level corresponds to an intake of some 1000 mg/kg/day, which represents approximately 8000 times the human intake based on a conservative estimation.