TIM barrel fold and glycan moieties in the structure of ICChI, a protein with chitinase and lysozyme activity

Abstract

The ICChI is a 35-kDa, glycosylated protein isolated from the latex of the weed Ipomoea carnea. It displays chitinase and lysozyme activity, which could be important for the defense against pathogenic fungi, insects and bacteria. The ICChI enzyme was crystallized, and a diffraction data set was collected from a single crystal to 1.42 Å resolution. The crystals belong to the primitive tetragonal space group P43212, with unit-cell parameters a = b = 57.9, c = 172.0 Å, and α = β = γ = 90°. The structure was elucidated by molecular replacement method using a mixed model of three homologous structures from the N-terminal sequence of ICChI. The refined model consists of 272 amino acid residues and has a Rfactor of 18.93% and Rfree of 22.42%. The protein consists of a single globular domain with a (α/β)8 triosephosphate isomerase barrel fold. Three of the consensus sites for N-glycosylation viz., Asn45, Asn172, and Asn194 containing carbohydrate moieties N-Acetylglucosamine (NAG), mannose, fucose, and xylose. The putative catalytic residues are Asp125, Glu127, and Tyr184. The crystal structure may provide fundamental information of GH18 family chitinases.