Isolation and divalent-metal activation of a β-xylosidase, RUM630-BX

• GH43 β-xylosidase was isolated from a rumen metagenomic library.
• Enzyme is highly activated by Mg2+ and Mn2+ cations.
• Enzyme has the highest reported kcat/Km acting on the natural substrate xylotetraose.

The gene encoding RUM630-BX, a β-xylosidase/arabinofuranosidase, was identified from activity-based screening of a cow rumen metagenomic library. The recombinant enzyme is activated as much as 14-fold (kcat) by divalent metals Mg2+, Mn2+ and Co2+ but not by Ca2+, Ni2+, and Zn2+. Activation of RUM630-BX by Mg2+ (t0.5 144 s) is slowed two-fold by prior incubation with substrate, consistent with the X-ray structure of closely related xylosidase RS223-BX that shows the divalent-metal activator is at the back of the active-site pocket so that bound substrate could block its entrance. The enzyme is considerably more active on natural substrates than artificial substrates, with activity (kcat/Km) of 299 s−1 mM−1 on xylotetraose being the highest reported.