Glucuronoxylan xylanohydrolase. A unique xylanase with the requirement for appendant glucuronosyl units

A new category of beta-(1----4)-xylan xylanohydrolases that exhibit a specific capacity to hydrolyze glucuronoxylans was characterized using heteroxylans prepared from Vigna (Vigna angularis Ohwi et Ohashi cv. Takara) and maize (Zea mays L.) cell walls together with appropriate derivatives as substrates. Glucuronopyranosyl moieties, as side chains, were prerequisite for enzyme-mediated hydrolysis of the beta-(1----4)-xylosyl linkages. The enzyme degraded glucuronoxylans derived from Vigna cell walls to yield a major oligomeric species (formula; see text) where Xyl represents xylose and GlcA represents glucuronic acid. The enzyme also degraded glucuronoarabinoxylans derived from maize cell walls to yield a major oligomeric species containing a single glucuronosyl side chain and a single unsubstituted beta 1----4Xyl pendant terminal. These results indicate that this xylanohydrolase recognizes glucuronosyl moieties inserted as monomeric side chains along the xylan backbone and mediates the hydrolysis of the beta-(1----4)-xylosyl linkage of the adjacent unsubstituted xylosyl residue in heteroxylans. This enzyme is the first xylanohydrolase identified that recognizes distinctly different sugars constituting side chains. We propose to designate this new enzyme as a glucuronoxylan xylanohydrolase to be abbreviated as glucuronoxylanase. Use of this unique enzyme demonstrated the presence of repeating units in heteroxylans in cell walls of higher plants.