Properties of four C-terminal carbohydrate-binding modules (CBM4) of laminarinase Lic16A of Clostridium thermocellum

Abstract

At the C-terminus of multimodular laminarinase Lic16A Clostridium thermocellum four carbohydrate-binding modules (CBM), belonging to family 4, were found. The isolated CBM - CBM4_1, CBM4_2, CBM4_3, CBM4_4 and the tandem CBM4_(1-4) were obtained. None of the recombinant proteins did have the affinity to soluble beta-1,3-1,4-glucans--laminarin and lihenan--the main specific substrates of Licl6A. All modules, except CBM4_4, had the ability to bind bacterial crystalline cellulose, that was atypical for the family 4 CBMs. We found that all CBMs 4 of Licl6A had affinity for xylan, chitin, beta-glucan from yeast cell wall and Avicel, while CBM4_3 and CBM4_4 had additional affinity to chitosan. The tandem CBM4_(1-4) had the highest affinity to yeast cell wall beta-glucan, avicel and pustulan. The binding constants for these substrates were about 100 times higher than that of the individual modules, suggesting a synergy in the process of absorption to these polysaccharides. This finding helps to explain the evolutionary process of CBM multiplication.