MOLECULAR AND INDUSTRIAL-ASPECTS OF GLUCOSE-ISOMERASE

Glucose isomerase (GI) (D-xylose ketol-isomerase; EC. 5.3.1.5) catalyz es the reversible isomerization of D-glucose and D-xylose to D-fructos e and D-xylulose, respectively The enzyme has the largest market in th e food industry because of its application in the production of high-f ructose 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 ser ves a nutritional requirement in saprophytic bacteria and has a potent ial application in the bioconversion of hemicellulose to ethanol. The enzyme is widely distributed in prokaryotes. Intensive research effort s are directed toward improving its suitability for industrial applica tion. Development of microbial strains capable of utilizing xylan-cont aining raw materials for growth or screening for constitutive mutants of GI is expected to lead to discontinuation of the use of xylose as a n inducer for the production of the enzyme. Elimination of Co2+ from t he fermentation medium is desirable for avoiding health problems arisi ng from human consumption of HFCS. Immobilization of GI provides an ef ficient means for its easy recovery and reuse and lowers the cost of i ts use. X-ray crystallographic and genetic engineering studies support a hydride shift mechanism for the action of GI. Cloning of GI in homo logous as well as heterologous hosts has been carried oat with the pri me aim of overproducing the enzyme and deciphering the genetic organiz ation of individual genes (xylA, xylB, and xylR) in the xyl operon of different microorganisms. The organization of xylA and xylB seems to b e highly conserved in all bacteria. The two genes ave transcribed from the same strand in Escherichia coli and Bacillus and Lactobacillus sp ecies, whereas they are transcribed divergently on different strands i n Streptomyces species. A comparison of the xylA sequences from severa l bacterial sources revealed the presence of two signature sequences, VXW(GP)GREG(YSTAE)E and (LIVM)EPKPX(EQ)P. The use of an inexpensive in duce in the fermentation medium devoid of Co2+ and redesigning of a ta ilor-made GI with increased thermostability, higher affinity for gluco se, and lower pH optimum will contribute significantly to the developm ent of an economically feasible commercial process for enzymatic isome rizaton of glucose to fructose. Manipulation of the GI gene by site-di rected mutagenesis holds promise that a GI suitable for biotechnologic al applications will be produced in the foreseeable future.