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1: J Biol Chem. 2005 Aug 5;280(31):28492-7. Epub 2005 Jun 1.
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The non-catalytic chitin-binding protein CBP21 from Serratia marcescens is essential for chitin degradation.

Vaaje-Kolstad G, Horn SJ, van Aalten DM, Synstad B, Eijsink VG.

Department of Chemistry, Biotechnology, and Food Science, Norwegian University of Life Sciences, P. O. Box 5003, 1432 As, Norway.

The Gram-negative soil bacterium Serratia marcescens uses three different family 18 chitinases to degrade chitin, an abundant insoluble carbohydrate polymer composed of beta(1,4)-linked units of N-acetylglucosamine. We show that efficient chitin degradation additionally depends on the action of a small non-catalytic protein, CBP21, which binds to the insoluble crystalline substrate, leading to structural changes in the substrate and increased substrate accessibility. CBP21 strongly promoted hydrolysis of crystalline beta-chitin by chitinases A and C, while it was essential for full degradation by chitinase B. CBP21 variants with single mutations on the largely polar binding surface lost their ability to promote chitin degradation, while retaining considerable affinity for the polymer. Thus, binding alone is not sufficient for CBP21 functionality, which seems to depend on specific, mostly polar interactions between the protein and crystalline chitin. This is the first time a secreted binding protein is shown to assist in the enzymatic degradation of an insoluble carbohydrate via non-hydrolytic disruption of the substrate. Interestingly, homologues of CBP21 occur in most chitin-degrading microorganisms, suggesting a general mechanism by which chitin-binding proteins enhance chitinolytic activity. Homologues also occur in chitinase-containing insect viruses, whose infectiousness is known to depend on chitinase efficiency.

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PMID: 15929981 [PubMed - indexed for MEDLINE]