Characterization of thermoanaerobacter glucose isomerase in relation to saccharidase synthesis and development of single-step processes for sweetener production.

Regulation of glucose isomerase synthesis was studied in Thermoanaerobacter strain B6A, which fermented a wide variety of carbohydrates including glucose, xylose, lactose, starch, and xylan. Glucogenic amylase activities and beta-galactosidase were produced constitutively, whereas the synthesis of glucose isomerase was induced by either xylose or xylan. Production of these saccharidase activities was not significantly repressed by the presence of glucose or 2-deoxyglucose in the growth media. Glucose isomerase production was optimized by controlling the culture pH at 5.5 during xylose fermentation. The apparent temperature and pH optima for these cell-bound saccharidase activities were as follows: glucose isomerase, 80 degrees C, pH 7.0 to 7.5; glucogenic amylase, 70 degrees C, pH 5.0 to 5.5; and beta-galactosidase, 60 degrees C, pH 6.0 to 6.5 Glucose isomerase, glucogenic amylase, and beta-galactosidase were produced in xylose-grown cells that were active and stable at 60 to 70 degrees C and pH 6.0 to 6.5. Under single-step process conditions, these saccharidase activities in whole cells or cell extracts converted starch or lactose directly into fructose mixtures. A total of 96% of initial liquefied starch was converted into a 49:51 mixture of glucose and fructose, whereas 85% of initial lactose was converted into a 40:31:29 mixture of galactose, glucose, and fructose.