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Carbohydrate-binding properties of a separately folding protein module from β-1,3-glucanase Lic16A of Clostridium thermocellum

¹ Institute of Molecular Genetics RAS, 2 Kurchatov Square, 123182 Moscow, Russian Federation
² Department of Microbiology, Technische Universität München, Am Hochanger 4, D-85350 Freising-Weihenstephan, Germany

The multi-modular non-cellulosomal endo-1,3(4)-β-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. Ca²⁺ ions had effects on the binding, e.g. stimulated complex formation with chitosan, which was observed only in the presence of Ca²⁺. The highest affinity to CBMX was shown for xylan (binding constant K=3.1x10⁴ M^–1), yeast cell wall glucan (K=1.4x10⁵ M^–1) and chitin (K=3.3.10⁵ M^–1 in the presence of Ca²⁺). 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.

Correspondence
Igor A. Dvortsov
dvortsov{at}img.ras.ru

A supplementary table listing homologues of the CBMX module of Lic16A of C. thermocellum is available with the online version of this paper.