Site-Resolved Backbone and Side-Chain Intermediate Dynamics in a Carbohydrate-Binding Module Protein Studied by...

Magic-angle spinning solid-state NMR spectroscopy has been applied to study the dynamics of CBM3b-Cbh9A from Clostridium thermocellum (ctCBM3b), a cellulose binding module protein. This 146-residue protein has a nine-stranded β-sandwich fold, in which 35 % of the residues are in the β-sheet and the remainder are composed of loops and turns. Dynamically averaged (1) H-(13) C dipolar coupling order parameters were extracted in a site-specific manner by using a pseudo-three-dimensional constant-time recoupled separated-local-field experiment (dipolar-chemical shift correlation experiment; DIPSHIFT). The backbone-Cα and Cβ order parameters indicate that the majority of the protein, including turns, is rigid despite having a high content of loops; this suggests that restricted motions of the turns stabilize the loops and create a rigid structure. Water molecules, located in the crystalline interface between protein units, induce an increased dynamics of the interface residues thereby lubricating crystal water-mediated contacts, whereas other crystal contacts remain rigid.