Aspergillus niger xylanase is a target enzyme of the two wheat proteinaceous inhibitors, XIP-1 and TAXI-1. We previously suggested that the xylanase "thumb" region was XIP-1 binding site. Here, we expressed the Asp37Ala mutant in Pichia pastoris and showed that the mutation abolished the enzyme capacity to interact with both inhibitors, suggesting a direct contact at the active site. The mutant pH profile was altered, confirming the key role of Asp37 in determining the pH optima of glycoside hydrolase family 11. The results are consistent with a competitive inhibition mode and underline the strategic importance of Asp37 in the inhibition mechanism. (C) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.