Cold adaptation of xylose isomerase from Thermus thermophilus through random PCR mutagenesis - Gene cloning and protein characterization

Random PCR mutagenesis was applied to the Thermus thermophilus xylA gene encoding xylose isomerase. Three cold-adapted mutants were isolated with the following amino-acid substitutions; E372G, V379A (M-1021), E372G, F163L (M-1024) and E372G (M-1026). The wild-type and mutated xyl4 genes were cloned and expressed in Escherichia coli HB101 using the vector pGEM(R)-T Easy, and their physicochemical and catalytic properties were determined. The optimum pH for xylose isomerization activity for the mutants was approximate to 7.0, which is similar to the wild-type enzyme. Compared with the wild-type, the mutants were active over a broader pH range. The mutants exhibited up to nine times higher catalytic rate constants (k(cat)) for D-xylose compared with the wild-type enzyme at 60 degreesC, but they did not show any increase in catalytic efficiency (k(cat)/K-m). For D-glucose, both the k(cat) and the k(cat)/K-m values for the mutants were increased compared with the wild-type enzyme. Furthermore, the mutant enzymes exhibited up to 255 times higher inhibition constants (K-i) for xylitol than the wild-type, indicating that they are less inhibited by xylitol. The thermal stability of the mutated enzymes was poorer than that or the wild-type enzyme. The results are discussed in terms of increased molecular flexibility of the mutant enzymes at low temperatures.