Analysis of the relationship between enzyme activity and its internal motion using nuclear magnetic resonance: ¹⁵N relaxation studies of wild-type and mutant lysozyme

A mutant lysozyme where R14 and H15 are deleted together has higher activity and a similar binding ability to an inhibitor, trimer of N-acetyl-glucosamine ((NAG)₃), compared with wild-type lysozyme. Since this has been attributed to intrinsic protein dynamic properties, we investigated the relationship between the activity and the internal motions of proteins. Backbone dynamics of the free and the complex forms with the (NAG)₃ have been studied by measurement of the ¹⁵N T₁ and T₂ relaxation rates and NOE determinations at 600 MHz. Analysis of the data using the model-free formalism showed that the generalized order parameters (S²) were almost the same in wild-type and mutant lysozyme in unbound state, indicating that the mutation had little effect on the global internal motions. On the other hand, in the presence of (NAG)₃, although some signals located around the active site were broadened or decreased in intensity because of strong perturbation by (NAG)₃, there were several residues that showed increased or decreased backbone S² in the complexed lysozymes. A comparison of the internal motions of the wild-type and mutant complexes showed a number of distinct dynamic differences between them. in particular, many residues located at or near active-site regions (turn 1, strand 2, turn 2 and long loop), displayed greater backbone dynamics reflecting the order parameter in mutant complex relative to mutant free. Furthermore, the R(ex) values at the loop C-D region, which was considered to be important for enzymatic activity, significantly increased. From these results, it was suggested that variations in the dynamics of these regions may play an important role in the enzyme activity.