The multi-modular non-cellulosomal endo-1,3(4)-b-glucanase Lic16A from Clostridium thermocellum contains a so-called X module (denoted as CBMX) near the N terminus of the catalytic module (191-426 aa). Melting of X-module-containing recombinant proteins revealed an independent folding of the module. CBMX was isolated and studied as a separate fragment. It was shown to bind to various insoluble polysaccharides, including xylan, pustulan, chitin, chitosan, yeast cell wall glucan, Avicel and bacterial crystalline cellulose. CBMX thus contains a hitherto unknown carbohydrate-binding module (CBM54). It did not bind soluble polysaccharides on which Lic16A is highly active. Ca 2+ ions had effects on the binding, e.g. stimulated complex formation with chitosan, which was observed only in the presence of Ca 2+ . The highest affinity to CBMX was shown for xylan (binding constant K53.1¾10 4 M "1 ), yeast cell wall glucan (K51.4¾10 5 M "1 ) and chitin (K53.3.10 5 M "1 in the presence of Ca 2+ ). Lic16A deletion derivatives lacking CBMX had lower affinity to lichenan and laminarin and a slight decrease in optimum temperature and thermostability. However, the specific activity was not significantly affected.

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