The basidiomycete Coprinopsis cinerea produces the glycoside hydrolase family 6 enzyme CcCel6C at low and constitutive levels. CcCel6C exhibits unusual cellobiohydrolase activity; it hydrolyses carboxymethyl cellulose, which is a poor substrate for typical cellobiohydrolases. Here, we determined the crystal structures of CcCel6C unbound and in complex with p‐nitrophenyl β‐d‐cellotrioside and cellobiose. CcCel6C consists of a distorted seven‐stranded β/α barrel and has an enclosed tunnel, which is observed in other cellobiohydrolases from ascomecetes Hypocrea jecorina (HjeCel6A) and Humicola insolens (HinCel6A). In HjeCel6A and HinCel6A, ligand binding produces a conformational change that narrows this tunnel. In contrast, the tunnel remains wide in CcCel6C and the conformational change appears to be less favourable than in HjeCel6A and HinCel6A. The ligand binding cleft for subsite −3 of CcCel6C is also wide and is rather similar to that of endoglucanase. These results suggest that the open tunnel and the wide cleft are suitable for the hydrolysis of carboxymethyl cellulose.

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The crystal structures of Thermoactinomyces vulgaris cyclo ⁄ maltodextrinbinding protein (TvuCMBP) complexed with a-cyclodextrin (a-CD), b-cyclodextrin (b-CD) and maltotetraose (G4) have been determined. A common functional conformational change among all solute-binding proteins involves switching from an open form to a closed form, which facilitates transporter binding. Escherichia coli maltodextrin-binding protein (EcoMBP), which is structurally homologous to TvuCMBP, has been determined to adopt the open form when complexed with b-CD and the closed form when bound to G4. Here, we show that, unlike EcoMBP, TvuCMBPa-CD and TvuCMBP-b-CD adopt the closed form when complexed, whereas TvuCMBP-G4 adopts the open form. Only two glucose residues are evident in the TvuCMBP-G4 structure, and these bind to the C-domain of TvuCMBP in a manner similar to the way in which maltose binds to the C-domain of EcoMBP. The superposition of TvuCMBP-a-CD, TvuCMBP-b-CD and TvuCMBP-c-CD shows that the positions and the orientations of three glucose residues in the cyclodextrin molecules overlay remarkably well. In addition, most of the amino acid residues interacting with these three glucose residues also participate in interactions with the two glucose residues in TvuCMBP-G4, regardless of whether the protein is in the closed or open form. Our results suggest that the mechanisms by which TvuCMBP changes from the open to the closed conformation and maintains the closed form appear to be different from those of EcoMBP, despite the fact that the amino acid residues responsible for the initial binding of the ligands are well conserved between TvuCMBP and EcoMBP.

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CcCel6C is a gene that encodes a glycoside hydrolase family 6 (GH6) enzyme in the Coprinopsis cinerea genome. In the evolutionary tree of GH6 enzymes, the encoded enzyme was closely related to Cel6B from Humicola insolens, previously called endoglucanase VI, while its amino-acid sequence revealed a region corresponding to the C-terminal active-site-enclosing loop typical of cellobiohydrolase II. Here, the crystallization of CcCel6C produced in Escherichia coli is reported. The square prismatic crystal belonged to the triclinic space group P1, with unit-cell parameters a = 44.04, b = 45.11, c = 48.90 Å , = 77.81, = 87.34, = 68.79 . Diffraction data were collected to 1.6 Å resolution.