Carboxy-terminal glycosyl hydrolase 18 domain of a carbohydrate active protein of Chitinophaga pinensis is a non-processive exochitinase

Abstract

The recombinant C-terminal domain of chitinase C of Chitinophaga pinensis (CpChiC-GH18C) exhibits the highest activity at pH 6.0 and 35 °C, with a Km of 76.13 (mg−1 ml), a kcat of 10.16 (s−1), and a kcat/Km of 0.133 (mg−1 ml s−1) on colloidal chitin. Analysis of degradation of (GlcNAc)3–6 oligomers shows that CpChiC-GH18C releases (GlcNAc)2 as the main product, indicating an exo-type cleavage pattern. CpChiC-GH18C hydrolyzes the chitin polymers yielding GlcNAc, (GlcNAc)2, and (GlcNAc)3 as end products with no sign of processivity. Circular dichroism spectra indicate that the secondary and tertiary structures of CpChiC-GH18C are unaltered up to 45 °C and the protein denatures without an intermediate state. The urea-induced unfolding is a two-state process and the unfolding of native CpChiC-GH18C occurs in a single step. Among the metal ions tested, Hg2+ completely inhibits the enzyme activity. The chemical modulators, p-hydroxymercuribenzoic acid and N-bromosuccinimide considerably decrease the enzyme activity. Sequence analysis and homology modeling suggest that CpChiC-GH18C lacks a tryptophan residue at the aglycon site. Further, the CpChiC-GH18C has a shallow and open groove, suggesting that CpChiC-GH18C is non-processive exo-type chitinase with properties suitable for the bioconversion of chitin waste.