Aminoalcohols as probes of the two-subsite active site of beta-D-xylosidase from Selenomonas ruminantium

Catalysis and inhibitor binding by the GH43 beta-xylosidase are governed by the protonation states of catalytic base (D14, pK(a) 5.0) and catalytic acid (E186, pK(a) 7.2) which reside in subsite -1 of the two-subsite active site. Cationic aminoalcohols are shown to bind exclusively to subsite -1 of the catalytically-inactive, dianionic enzyme (D14(-)E186(-)). Enzyme (E) and aminoalcohols (A) form E-A with the affinity progression: triethanolamine>diethanolamine>ethanolamine. E186A mutation raises the K(i)(triethanolamine) 1000-fold. By occupying subsite -1 with aminoalcohols, affinity of monosaccharide inhibitors (I) for subsite +1 is demonstrated. The single access route for ligands into the active site dictates ordered formation of E-A followed by E-A-I. E-A-I forms with the affinity progression: ethanolamine>diethanolamine>triethanolamine. The latter affinity progression is seen in formation of E-A-substrate complexes with substrate 4-nitrophenyl-beta-d-xylopyranoside (4NPX). Inhibition patterns of aminoalcohols versus 4NPX appear competitive, noncompetitive, and uncompetitive depending on the strength of E-A-4NPX. E-A-substrate complexes form weakly with substrate 4-nitrophenyl-alpha-l-arabinofuranoside (4NPA), and inhibition patterns appear competitive. Biphasic inhibition by triethanolamine reveals minor (<0.03%) contamination of E186A preparations (including a His-Tagged form) by wild-type-like enzyme, likely originating from translational misreading. Aminoalcohols are useful in probing glycoside hydrolases; they cause artifacts when used unwarily as buffer components.