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Microbiol. Rev., 06 1996, 280-300, Vol 60, No. 2
SH Bhosale, MB Rao and VV Deshpande
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.
Copyright © 1996, American Society for Microbiology
Molecular and industrial aspects of glucose isomerase
Division of Biochemical Sciences, National Chemical Laboratory, Pune, India.
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