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J. Biol. Chem., Vol. 283, Issue 18, 12415-12425, May 2, 2008

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Molecular Basis for the Selectivity and Specificity of Ligand Recognition by the Family 16 Carbohydrate-binding Modules from Thermoanaerobacterium polysaccharolyticum ManA^*

Brian Bae, Samuel Ohene-Adjei, Svetlana Kocherginskaya, Roderick I. Mackie^¶, M. Ashley Spies^¶, Isaac K. O. Cann^¶||, and Satish K. Nair^¶**1

From the Departments of Biochemistry, ^||Microbiology, and Animal Sciences, the ^**Center for Biophysics and Computational Biology, and the ^¶Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

Enzymes that hydrolyze complex polysaccharides into simple sugars are modular in architecture and consist of single or multiple catalytic domains fused to targeting modules called carbohydrate-binding modules (CBMs). CBMs bind to their ligands with high affinity and increase the efficiency of the catalytic components by targeting the enzymes to its substrate. Here we utilized a multidisciplinary approach to characterize each of the two family 16 carbohydrate-binding domain components of the highly active mannanase from the thermophile Thermoanaerobacterium polysaccharolyticum. These represent the first crystal structures of family 16 CBMs. Calorimetric analysis showed that although these CBMs demonstrate high specificity toward β-1,4-linked sugars, they can engage both cello- and mannopolysaccharides. To elucidate the molecular basis for this specificity and selectivity, we have determined high resolution crystal structures of each of the two CBMs, as well as of binary complexes of CBM16-1 bound to either mannopentaose or cellopentaose. These results provide detailed molecular insights into ligand recognition and yield a framework for rational engineering experiments designed to expand the natural repertoire of these targeting modules.

Received for publication, August 6, 2007 , and in revised form, November 13, 2007.

The atomic coordinates and structure factors (codes 2ZEW, 2ZEX, 2ZEY, and 2ZEZ) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported in part by American Cancer Society Grant 04-37 (to S. K. N.) and in part by the Agricultural Experimental Station Grant ILLU 538-364 (to I. K. O. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biochemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Ave., Urbana, IL 61801. Tel.: 217-333-0641; Fax: 217-244-5858; E-mail: snair{at}uiuc.edu.

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