A novel class of protein from wheat which inhibits xylanases

We have purified a novel class of protein that can inhibit the activity of endo-beta-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 AGGKTGQVTVFW GRN. We have shown that the protein can inhibit the activity of two family- 11 endo-beta-1,4-xylanases, a recombinant enzyme from Aspergillus niger and an enzyme from Trichoderma viride. The inhibitory activity is heat and pro tease sensitive. The kinetics of the inhibition have been characterized wit h 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 reac tion are competitive, with a K-i value of 0.35 mu M, showing that the inhib itor binds at or close to the active site of free xylanase. This report des cribes the first isolation of a xylanase inhibitor from any organism.