Physico-chemical and enzymatic properties of purified glucose isomerases from Streptomyces olivochromogenes and Bacillus stearothermophilus.

Abstract

The physico-chemical properties of the purified glucose isomerases [D-xylose ketol iso-merase, EC 5. 3. 1. 5] of Streptomyces olivochrornogenes and Bacillus stearothermophilus were examined. The molecular size and shape of both enzymes were similar. The molecular weights, sedimentation coefficients, partial specific volumes, diffusion constants and Stokes' radii of the Streptomyces and Bacillus enzymes were determined to be 120, 000 and 130, 000, 7.55 S and 9.35 S, 0.725 and 0.736ml/g, 5.87×10-7 and 6.82×10-7cm2/sec, and 51 and 53 Å, respectively. The Streptomyces glucose isomerase was found to consist of two subunits, each having a molecular weight of 56, 000. Large differences were found in the amino acid composi-tions of these two enzymes, especially in their serine, proline, tyrosine, lysine and arginine contents. The enzymatic properties of both these purified glucose isomerases were also examined, and it was seen that they both displayed activity on D-xylose, D-xylulose, D-glucose, D-fructose, D-arabinose and D-ribose. The smaller Km values and the larger molecular ac-tivities for D-xylose and D-xylulose indicated that both enzymes are essentially D-xylose iso-merases. The optimum temperature was 80°C for both enzymes. The optimum pH was 8 to 10 for the Streptomyces enzymes and 7.5 to 8.0 for the Bacillus enzyme. The Bacillus enzyme was more thermostable than the Streptomyces enzyme, but required cobalt ions in addition to magnesium ions for the full expression of its activity.