Solution Structure of a Cellulose-Binding Domain from Cellulomonas fimi by Nuclear Magnetic Resonance Spectroscopy

Multidimensional, multinuclear nuclear magnetic resonance spectroscopy combined with dynamical simulated annealing has been used to determine the structure of a 110 amino acid cellulose-binding domain (CBD) from Cex, a beta-1,4-glycanase from the bacterium Cellulomonas fimi (CBDcex). An experimental data set comprising 1795 interproton NOE-derived restraints, 50 phi, 34 chi 1, and 106 hydrogen bond restraints was used to calculate 20 final structures. The calculated structures have an average root-mean-square (rms) deviation about the mean structure of 0.41 A for backbone atoms and 0.67 A for all heavy atoms when fitted over the secondary structural elements. Chromatography, ultracentrifugation, and 15N NMR relaxation experiments demonstrate that CBDcex is a dimer in solution. While attempts to measure NOEs across the dimer interface were unsuccessful, a computational strategy was employed to generate dimer structures consistent with the derived data set. The results from the dimer calculations indicate that, while the monomer topologies produced in the context of the dimer can be variable, the relative positioning of secondary structural elements and side chains present in the monomer are restored upon dimer formation. CBDcex forms an extensive beta-sheet structure with a beta-barrel fold. Titration with cellohexaose, [beta-D-glucopyranosyl-(1,4)]5-D-glucose, establishes that Trp 54 and 72 participate in cellulose binding. Analysis of the structure shows that these residues are adjacent in space and exposed to solvent. Together with other proximate hydrophilic residues, these residues form a carbohydrate-binding cleft, which appears to be a feature common to all CBDs of the same family.