title: Structural analysis of xylanase inhibitor protein I (XIP-I), a proteinaceous xylanase inhibitor from wheat (Triticum aestivum, var. Soisson).
abstract: A novel class of proteinaceous inhibitors exhibiting specificity towards microbial xylanases has recently been discovered in cereals. The three-dimensional structure of xylanase inhibitor protein I (XIP-I) from wheat (Triticum aestivum, var. Soisson) was determined by X-ray crystallography at 1.8 A (1 A=0.1 nm) resolution. The inhibitor possesses a (beta/alpha)(8) barrel fold and has structural features typical of glycoside hydrolase family 18, namely two consensus regions, approximately corresponding to the third and fourth barrel strands, and two non-proline cis -peptide bonds, Ser(36)-Phe and Trp(256)-Asp (in XIP-I numbering). However, detailed structural analysis of XIP-I revealed several differences in the region homologous with the active site of chitinases. The catalytic glutamic acid residue of family 18 chitinases [Glu(127) in hevamine, a chitinase/lysozyme from the rubber tree (Hevea brasiliensis)] is conserved in the structure of the inhibitor (Glu(128)), but its side chain is fully engaged in salt bridges with two neighbouring arginine residues. Gly(81), located in subsite -1 of hevamine, where the reaction intermediate is formed, is replaced by Tyr(80) in XIP-I. The tyrosine side chain fills the subsite area and makes a strong hydrogen bond with the side chain of Glu(190) located at the opposite side of the cleft, preventing access of the substrate to the catalytic glutamic acid. The structural differences in the inhibitor cleft structure probably account for the lack of activity of XIP-I towards chitin.
authors.affiliation: Architecture et Fonction des Macromolécules Biologiques, UMR6098, CNRS and Universities Aix-Marseille I and II, 31 chemin Joseph Aiguier, France. fran@afmb.cnrs-mrs.fr
authors.forename: Françoise
authors.initials: F
authors.lastname: Payan
authors.forename: Ruth
authors.initials: R
authors.lastname: Flatman
authors.forename: Sophie
authors.initials: S
authors.lastname: Porciero
authors.forename: Gary
authors.initials: G
authors.lastname: Williamson
authors.forename: Nathalie
authors.initials: N
authors.lastname: Juge
authors.forename: Alain
authors.initials: A
authors.lastname: Roussel
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002688:Chitinases / Q000737:chemistry / Q000378:metabolism*; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D004791:Enzyme Inhibitors / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009113:Muramidase / Q000737:chemistry; D010940:Plant Proteins; D011487:Protein Conformation; D017510:Protein Folding; D017386:Sequence Homology, Amino Acid; D014908:Triticum / Q000201:enzymology*; D043325:Xylan Endo-1,3-beta-Xylosidase; D014995:Xylosidases / Q000037:antagonists & inhibitors* / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; D010940:Plant Proteins; D014995:Xylosidases; D002688:Chitinases; C063764:hevamine; D009113:Muramidase; D043325:Xylan Endo-1,3-beta-Xylosidase