Biochemical and structural characterization of TLXI, the Triticum aestivum L. thaumatin-like xylanase inhibitor

Abstract

Thaumatin-like xylanase inhibitors (TLXI) are recently discovered wheat proteins. They belong to the family of the thaumatin-like proteins and inhibit glycoside hydrolase family 11 endoxylanases commonly used in different cereal based (bio)technological processes. We here report on the biochemical characterisation of TLXI. Its inhibition activity is temperature- and pH-dependent and shows a maximum at approximately 40°C and pH 5.0. The TLXI structure model, generated with the crystal structure of thaumatin as template, shows the occurrence of five disulfide bridges and three β-sheets. Much as in the structures of other short-chain thaumatin-like proteins, no α-helix is present. The circular dichroism spectrum of TLXI confirms the absence of α-helices and the presence of antiparallel β-sheets. All ten cysteine residues in TLXI are involved in disulfide bridges. TLXI is stable for at least 120 min between pH 1–12 and for at least 2 hours at 100°C, making it much more stable than the other two xylanase inhibitors from wheat, i.e. Triticum aestivum xylanase inhibitor (TAXI) and xylanase inhibitor protein (XIP). This high stability can probably be ascribed to the high number of disulfide bridges, much as seen for other thaumatin-like proteins.

Keywords::

• wheat
• xylanase inhibitor
• thaumatin-like protein
• biochemical characterisation
• disulfide bridges
• stability
• inhibition

Acknowledgements

The authors wish to acknowledge ‘Instituut voor de aanmoediging van Innovatie door wetenschap en technologie’ in Vlaanderen (IWT, Brussels, Belgium) for financial support. Kurt Gebruers is a postdoctoral fellow of the ‘Fonds voor Wetenschappelijk Onderzoek-Vlaanderen’ (FWO-Vlaanderen, Brussels, Belgium). This study was in part carried out in framework of research project GOA/03/10 financed by the Research Fund K. U. Leuven. We thank Prof. Paul Proost for the help with the MS analysis.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.