Carbohydrate-binding modules of ChiB and ChiC promote the chitinolytic system of Serratia marcescens BWL1001

Abstract

Carbohydrate-binding modules (CBMs) are commonly found within chitinases, but their contributions to chitinolytic systems are poorly understood. To address this knowledge gap, full-length chitin-acting enzymes (ChiA, ChiB, ChiC, and CBP21) of Serratia marcescens BWL1001 and CBM-truncated versions (ChiB-dCBM and ChiC-dCBM) were heterologously expressed for enzymological analysis. The CBM5 of ChiB and the CBM12 of ChiC both exhibited an affinity for α-chitin, while only CBM12 could bind colloidal chitin based on adsorption assays and affinity electrophoresis. Consistent with their ligand specificity, both CBMs were essential to α-chitin hydrolysis, while only CBM12 enhanced the hydrolytic efficiency of colloidal chitin by individual chitinases. Analysis of synergistic hydrolysis with separate full-length and CBM-truncated chitinases revealed that the two CBMs promoted the synergistic activity of chitinases on crystalline and amorphous chitin. The two CBMs also promoted the hydrolysis when chitin was mixed with non-substrate polysaccharides. This study reveals not only the effects of CBMs on enzymatic characteristics of individual chitinases, but also their contributions to the overall efficiency of chitinolytic systems during synergistic hydrolysis.