Significance of a family-6 carbohydrate-binding module in a modular feruloyl esterase for removing ferulic acid from insoluble wheat arabinoxylan.

Ruminiclostridium josui Fae1A is a modular enzyme consisting of an N-terminal signal peptide, family-1 carbohydrate esterase module (CE1), family-6 carbohydrate-binding module (CBM6), and dockerin module in that order. Recombinant CE1 and CBM6 polypeptides were collectively and separately produced as RjFae1A, RjCE1, and RjCBM6. RjFae1A showed higher feruloyl esterase activity than RjCE1 towards insoluble wheat arabinoxylan, but the latter was more active towards small synthetic substrates than the former. This suggests that CBM6 in RjFae1A plays an important role in releasing ferulic acid from the native substrate. RjCBM6 showed a higher affinity for soluble wheat arabinoxylan than for rye arabinoxylan and beechwood xylan in native affinity polyacrylamide gel electrophoresis. Isothermal titration calorimetry analysis demonstrated that RjCBM6 recognized a xylopyranosyl residue at the nonreducing ends of xylooligosaccharides. Moreover, it showed exceptional affinity for 2-α-l-arabinofuranosyl-xylotriose (AXX) among the tested branched arabinoxylooligosaccharides. Fluorometric titration analysis demonstrated that xylobiose and AXX competitively bound to RjCBM6, and both bound to the same site in RjCBM6. RjCBM6's preference for the xylopyranosyl residue at the nonreducing end of xylan chains explains why the positive effect of CBM6 on RjFae1A activity was observed only during short incubation but not after extended incubation.