Isolation and Characterization of a Glucose/Xylose Isomerase from a New Thermophilic "Bacillus thermoantarcticus"
Licia LAMA*, Barbara NICOLAUS, Valeria CALANDRELLI, Enrico ESPOSITO, and Agata GAMBACORTA
Instituto per la Chimica di Molecole di Interesse Giologico, CNR via Toiano 6, 80072 Arco Felice, Napoli Italy
Various microorganisms are able to grow on hemicelluloses, which amount to 40% of the plant biomass, as sole carbon source (1,2). Exoenzymes degrade the polymer to D-xylose, which is transported into the cell, isomerized to D-xylulose and phosphorylated to xylulose 5-phosphate which enters either the pentose phosphate pathway or the phosphoketolase pathway. The isomerization of D-xylose to xylulose is carried out by D-xylose isomerase. This enzyme has been used commercially because of its capacity to produce a fructose-enriched syrup by converting D-glucose into fructose (3). D-xylose isomerase is also of industrial interest for the fermentation of hemicelluloses to ethanol together with yeasts (4).
A novel thermophilic Gram-positive bacterium, "Bacillus thermoantarcticus", isolated from geothermal soil in Antarctica (5), produces an endocellular xylose/glucose isomerase. We report the isolation and its characterization with particular reference to a comparison between the two types of isomerase reaction. The enzyme is efficiently induced by its substrate D-xylose. Both the activity and stability of this enzyme depend strongly on divalent metal ions. For xylose isomerase Mn2+ or Mg2+ ions were required and combination of Co2+ and Mg2+ showed a synergistic activation effect on the activity. The enzyme required Co2+ and/or Mg2+ for high thermal stability.
Moreover the glucose isomerase required the presence of arsenate to form glucose-arsenate complex which could act as a substrate analogue for this enzyme.
No significant differences in the pH and temperature behaviour were observed when D-xylose was compared with D-glucose as substrate.
The optimum temperature was 95℃ and it maintained more than 90% activity after incubation at 80℃ for 1h in the presence of metal ions. The optimum pH was 7.5 and the enzyme was stable in the broad pH range 5.5-11.
Moreover, preliminary data on immobilized xylose isomerase in sodium alginate beads will be reported.
This work was partially financed by PNRA.
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3. Antrim, R.L., Colilla, W., and Schnyder, B.J. (1979) Appl. Biochem. Bioeng., 2, 97-155.
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5. Nicolaus, B., Lama, L. Esposito, E., Manca, MC., di Prisco, G., and Gambacorta, A. (1996) Polar Biology, 16, 101-104.
