The refined crystal structure of an endochitinasefrom Hordeum vulgare L. seeds at 1.8 Å resolution

P. John Hart, Heather D. Pfluger, Arthur F. Monzingo, Thomas Hollis, Jon D. Robertus

Research output: Contribution to journalArticlepeer-review

138 Scopus citations

Abstract

Class II chitinases (EC 3.2.1.14) are plant defense proteins. They hydrolyzechitin, an insoluble 0-1,4-linked polymer of N-acetylglucosamine (NAG), which is a major cell-wall component of many fungal hyphae. We previously reported the three-dimensional structure of the 26 kDa class II endochitinase from barley seeds at 2.8 Å resolution, determined using multiple isomorphous replacement (MIR) methods. Here, we report the crystallographic refinement of this chitinase structure against data to 1.8 Å resolution using rounds of hand rebuilding coupled with molecular dynamics (X-PLOR). The final model has an R-value of 18.1 % for the 5.0 to 1.8 Å. data shell and 19.8% for the 10.0 to 1.8 Å shell, and root-mean-square deviations from standard bond lengths and angles of 0.017Å and 2.88°, respectively. The 243 residue molecule has one β-sheet, ten α-helices and three disulfide bonds; 129 water molecules are included in the final model. We show structural comparisons confirming that chitinase secondary structure resembles lysozyme at the active site region. Based on substrate binding to lysozyme, we have built a hypothetical model for the binding of a hexasaccharide into the pronounced active site cleft of chitinase. This provides the first view of likely substrate interactions from this family of enzymes; the model is consistent with a lysozyme-like mechanism of action in which Glu67 acts as proton donor and Glu89 is likely to stabilize the transition state oxycarbonium ion. These binding site residues, and many hydrophobic residues are conserved in a range of plant chitinases. This endochitinase structure will serve as a model for other plant chitinases, and that catalytic models based on this structure will be applicable to the entire enzyme family.

Original languageEnglish (US)
Pages (from-to)402-413
Number of pages12
JournalJournal of Molecular Biology
Volume248
Issue number2
DOIs
StatePublished - 1995

Keywords

  • Crystal structure
  • Endochitinase
  • Refinement
  • Substrate binding
  • X-ray analysis

ASJC Scopus subject areas

  • Structural Biology
  • Molecular Biology

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