The bifunctional glycoside hydrolase enzyme, CelM2, is able to hydrolyze glucan and xylan effectively. The crystal structure of this protein has been determined, providing useful sequential and structural information [K.H. Nam, S.J. Kim, K.Y. Hwang, Crystal structure of CelM2, a bifunctional glucanase-xylanase protein from a metagenome library, Biochem. Biophys. Res. Commun. 383 (2009) 183-186]. In addition, this protein is a good model for understanding bifunctional enzymes, and it will provide information relevant for genetic engineering that will be useful in the design of bifunctional proteins. However, previous structural characterization was not sufficient to develop an understanding of the metal ion and substrate-binding moiety. Herein, we determined the metal-binding site of CelM2 using zinc ions. Our results revealed that the zinc ions participate in the crystallographic packing and enzyme folding of the external region of the TIM-like barrel domain. Based on our structure, zinc ions induce the passive form of the CAP region at the catalytic cleft of the CelM2 protein. Moreover, glucose was bound to the CelM2 structure at the catalytic site. This structure provides the binding moiety that binds to the hydroxyl group of substrates such as cellulose. In addition, a structural comparison of celM2 with Cel44 provides a good model of the binding mode of CelM2. Thus, our study represents a novel structural characterization of the metal-binding site and the structure of the complex formed between CelM2 and its substrate.
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