Role of tryptophan residues in a class V chitinase from Nicotiana tabacum

Tryptophan residues located in the substrate-binding cleft of a class V chitinase from Nicotiana tabacum (NtChiV) were mutated to alanine and phenylalanine (W190F, W326F, W190F/W326F, W190A, W326A, and W190A/W326A), and the mutant enzymes were characterized to define the role of the tryptophans. The mutations of Trp326 lowered thermal stability by 5-7 °C, while the mutations of Trp190 lowered stability only by 2-4 °C. The Trp326 mutations strongly impaired enzymatic activity, while the effects of the Trp190 mutations were moderate. The experimental data were rationalized based on the crystal structure of NtChiV in a complex with (GlcNAc)(4), in which Trp190 is exposed to the solvent and involved in face-to-face stacking interaction with the +2 sugar, while Trp326 is buried inside but interacts with the -2 sugar through hydrophobicity. HPLC analysis of anomers of the enzymatic products suggested that Trp190 specifically recognizes the β-anomer of the +2 sugar. The strong effects of the Trp326 mutations on activity and stability suggest multiple roles of the residue in stabilizing the protein structure, in sugar residue binding at subsite -2, and probably in maintaining catalytic efficiency by providing a hydrophobic environment for proton donor Glu115.