A novel class of protein from wheat which inhibits xylanases1

We have purified a novel class of protein that can inhibit the activity of endo-β-1,4-xylanases. The inhibitor from wheat (Triticum aestivum, var. Soisson) is a glycosylated, monomeric, basic protein with a pI of 8.7–8.9, a molecular mass of 29 kDa and a unique N-terminal sequence of AGGKTGQVTVFWGRN. We have shown that the protein can inhibit the activity of two family-11 endo-β-1,4-xylanases, a recombinant enzyme from Aspergillus niger and an enzyme from Trichoderma viride.The inhibitory activity is heat and protease sensitive. The kinetics of the inhibition have been characterized with the A. niger enzyme using soluble wheat arabinoxylan as a substrate. The K_m for soluble arabinoxylan in the absence of inhibitor is 20±2 mg/ml with a k_cat of 103±6 s^-1. The kinetics of the inhibition of this reaction are competitive, with a K_i value of 0.35 µM, showing that the inhibitor binds at or close to the active site of free xylanase. This report describes the first isolation of a xylanase inhibitor from any organism.