Xylose isomerase from Piromyces sp. E2 is a promiscuous enzyme with epimerase activity

Xylose isomerase catalyzes the isomerization of D-xylose to D-xylulose with promiscuous activity for other saccharides including D-glucose, D-allose, and L-arabinose. The xylose isomerase from the fungus Piromyces sp. E2 (PirE2_XI) is used to engineer xylose usage by the fermenting yeast Saccharomyces cerevisiae, but its biochemical characterization is poorly understood with divergent catalytic parameters reported. We have measured the kinetic parameters of the PirE2_XI and analyzed its thermostability and pH-dependence towards different substrates. The PirE2_XI shows promiscuous activity towards D-xylose, D-glucose, D-ribose and L-arabinose with variable effects depending on different divalent ions and epimerizes D-xylose at C3 to produce D-ribulose in a substrate/product dependent ratio. The enzyme follows Michaelis-Menten kinetics for the substrates used and although K_M values for D-xylose are comparable at 30 and 60 °C, the k_cat/K_M is three-fold greater at 60 °C. The purified PirE2_XI shows maximal activity at 65 °C in the pH range of 6.5-7.5 and is a thermostable enzyme, maintaining full activity over 48 h at 30 °C or 12 h at 60 °C. This is the first report demonstrating epimerase activity of the PirE2_XI and its ability to isomerize D-ribose and L-arabinose, and provides a comprehensive in vitro study of substrate specificity, effect of metal ions and temperature on enzyme activity and these findings advance the knowledge of the mechanism of action of this enzyme.